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NOAA Technical Memorandum NWS WR-277 

FORECASTERS HANDBOOK FOR EXTREME SOUTHWESTERN 

CALIFORNIA BASED ON SHORT TERM CLIMATOLOGICAL 

APPROXIMATIONS 

PART I – THE MARINE LAYER AND ITS EFFECT ON PRECIPITATION AND 

HEATING 

Ivory J. Small 

NATIONAL WEATHER SERVICE SAN DIEGO CA. 

October 2006 

United States National Oceanic and National Weather Service 

Department of Commerce Atmospheric Administration David L. Johnson, Assistant Administrator 

Carlos M. Gutierrez, Secretary VADM C. Lautenbacher for Weather Services 

 Under Secretary 
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and is approved for publication by 

Scientific Services Division, 

Western Region 

Delain A. Edman, Chief 

Scientific Services Division 

Salt Lake City, Utah

 
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TABLE OF CONTENTS

 PAGE

1. Introduction . ............. ............. ............................ ............. ............. .............. ............. ............. .............1 

2. Marine Layer ............. ............. ............................ ............. ............. .............. ............. ............. .............3 

2.1. General Marine Layer Characteristics ......... ............. ............. .............. ............. ............. .............3 

2.2. Example Of a Basic Marine Layer Event .... ............. ............. .............. ............. ............. .............6 

2.3. More On Variations in Marine Layer Clearing Phase ............. .............. ............. ............. .............9 

2.4. More on Low Level/Marine Layer Inverted V Soundings....... .............. ............. ............. ............10 

2.5. More on Reverse Clearing ........................... ............. ............. .............. ............. ............. ............11 

2.6. Variations in the Typical Mid Summer Marine Layer Cycle... .............. ............. ............. ............12 

2.7. Marine Layer Effects on Temperature and Precipitation ......... .............. ............. ............. ............12 

2.8. Low Level Redevelopment over the Coastal Waters. ............. .............. ............. ............. ............15 

2.9. Island Effects ... ............. ............................ ............. ............. .............. ............. ............. ............15 

2.10. Eddy Flow........ ............. ............................ ............. ............. .............. ............. ............. ............15 

2.11. Dense Fog Surrounding Santa Ana Wind Events ..... ............. .............. ............. ............. ............17 

2.12. Forecaster Hints – Marine Layer ................ ............. ............. .............. ............. ............. ............18 

3. Heat Waves................ ............. ............................ ............. ............. .............. ............. ............. ............22 

3.1. Forecaster Hints – Heat Waves.................... ............. ............. .............. ............. ............. ............25 

3.1.1. High Afternoon Temperatures reaching west into the mid-coastal Plain ............. ............25 

3.1.2. Other areas ......... ............................ ............. ............. .............. ............. ............. ............27 

3.1.3. Ingredients for high minimum temperatures. ............. .............. ............. ............. ............27 

3.1.4. Characteristics for ending a warm spell (large 24 hour changes in high temperatures)......28 

4. Summary and Conclusion........ ............................ ............. ............. .............. ............. ............. ............28 

5. Bibliography.............. ............. ............................ ............. ............. .............. ............. ............. ............31 

6. Appendix A – Marine Layer Matrix..................... ............. ............. .............. ............. ............. ............55 

7. Appendix B – Marine Layer Cases....................... ............. ............. .............. ............. ............. ............58 

8. Appendix C – Heat Wave Cases........................... ............. ............. .............. ............. ............. ............70 
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LIST OF FIGURES AND TABLES

FIGURES 

1. Terrain Map of southern California 

2. 1200 UTC KNKX mean sounding values for the warm seasons of 1998-2000 (June – September) 

3. Summer trends for selected parameters based on 1200 UTC KNKX sounding values for 1998-2000 

4. Mean surface pressure gradient from the coast to the Lower Colorado River Valley for 2000 – 2003 

5. Graph of summer 850 mb temperature versus inversion base, strength, and inversion base height 

6. Enhanced marine layer event cycle 

7. Example of schematic soundings for an enhanced marine layer cycle. 

8. Marine layer inversion height versus 850 mb temperature and 850 mb height gradient between KNKX and KDRA 

9. Approximate “Marine Layer Based” Low Level Storm Probability Of Precipitation (LLSPOP) based on areal 

coverage of precipitation in the coastal and valley areas for marine layer depths 4000-7999 feet deep [For the “cool” 

months of 1998-2000 (October – May)] 

10. Schematic showing the general relationship between the 700 mb subtropical moisture, 700 mb flow direction, and 

their locations in relation to the 700 mb ridge axis position. 

11. 500 mb map types of patterns commonly found during heat waves in extreme southwestern California 

12. Typical “highest” high temperature in each region for the coastal, mountain, valley, and desert regions base on 850 

mb temperature during the warm season 

13. Examples of diurnal temperature curves near the coast 

14. Example of the movement of the transition zones associated with the marine layer air and inversion layer 

15. Summer days with weak gradients only (less than 3 mb onshore from the coast to the Lower Colorado River 

Valley), end to end, for the period June – September 2000-2003 

16. Relationship between maximum and minimum temperatures at a lower mountain site for October 2002 – September 

2003 
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17. Schematic of the relationship between maximum temperatures at a valley site just below a mountain pass and a 

coastal site during the transition seasons (spring and fall) 

TABLES

1. Approximate warm season inversion base height based on 850 mb temperature 

2. Estimated Storm Probability of Precipitation (SPOP) based on marine layer depth and 850-700 mb relative humidity 

3. Approximate upper bounds of selected surface pressure gradient categories for above normal temperatures during 

the warm season 

4. Approximate high temperatures based on morning 850 mb temperatures 

5. Approximate high temperatures based on morning inversion top temperature 

6. Afternoon high temperatures based on afternoon 850 and 700 mb winds as well as 850 mb and inversion top 

temperatures 

7. Highest overnight minimum temperatures based on morning 850 mb wind and 850 mb dewpoint 
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FORECASTERS HANDBOOK FOR EXTREME SOUTHWESTERN CALIFORNIA 

(BASED ON SHORT TERM CLIMATOLOGICAL APPROXIMATIONS) 

PART I – THE MARINE LAYER AND ITS EFFECT ON PRECIPITATION AND HEATING 

IVORY J. SMALL 

ABSTRACT 

The varied terrain of southern California and its proximity to the Pacific Ocean results in many unique 

forecast challenges in southern California, therefore unique approaches for diagnosing and forecasting in 

this area are necessary. The purpose of this Forecasters Handbook is to present a variety of approaches 

to not only understand the processes involved in the area, but to somehow connect them to methods of 

forecasting for the area. A key component of this study is to employ short term climatological 

approximations (STCA). Data sets of generally less than 10 years are found to at least give a reasonable 

approximation of conditions expected with some meteorological parameters. Forecaster hints are “quick 

and dirty” guidance for the forecaster to get an initial idea of what to expect for certain parameters 

(although, not guaranteed to work in all cases), and more information is embedded in the discussions. 

Also for more detailed information the references can be consulted. It is hoped that this information 

helps to increase the understanding of the phenomena that occur in the extreme southwestern corner of 

our nation and also help to improve the ability to forecast them. 

1. INTRODUCTION 

The coastal marine layer is one of the most persistent characteristics of southern California weather (fig. 

1) and affects the entire Southern California Bight Region. [The Southern California Bight Region is 

essentially the area west of a 3000 foot contour on the higher mountains surrounding the coastal plain, 

and extends from Point Conception (just west of KSBA) south to the Mexican Border near KSAN. The 

coastal waters, out to about 60 miles, are also included]. The marine layer advects off the ocean to 

moderate the weather, with the amount of moderation based on the distance from the ocean and terrain 

height. It has the effect of cooling down the coastal areas during warm season afternoons, as well as 

keeping coastal areas warmer overnight during the cool season. The clouds do present a hazard, with 

dense fog becoming an issue. Dense fog develops any time of year, and is frequently associated with the 

cloudy air near/below the semi-permanent marine layer inversion. Southern California’s terrain varies 

from lower than 100 feet below sea level to over 11,500 feet. (All heights are in reference to MSL, 

unless otherwise noted). If there are low clouds below the inversion, there is almost always at least 

some patchy dense fog somewhere in the forecast area where the marine layer clouds and terrain meet. 

Hot days with strong subsidence (sinking motion) or moist monsoonal flow typically results in low 

(shallow) inversions, and confines the low clouds to near the immediate coast. Days with weaker 

subsidence tend to be cooler, with deeper marine layers (higher inversions) that allow the marine layer 

to extend inland to the coastal slopes, and locally through the mountain passes to the desert’s western 

edge. On nights with relatively high dewpoints (such as those after a rainstorm), and clear, moist nights 

with good radiational cooling, radiation fog develops as the cool air collects in the low spots such as 

river valleys, and possibly extend westward to near the coast. Even without rainfall, visibilities fall to 4 

miles or lower from just before sunrise until 11 AM or so in the inland valleys (for example KCNO) 

almost every night when winds are light. During the summer, there is almost always a marine layer 

inversion and coastal low clouds, and most likely in the southern areas. During patterns with cooling 

aloft, the marine layer deepens as the inversion rises and weakens. The marine layer can become so deep 

that rain can develop, even without a perceptible cold front or with atmospheric dynamics. On the other 

hand, during periods when high pressure aloft is strong enough to “squash the marine layer”, or push the 

coastal marine layer westward, and even totally out of the coastal areas, heat waves result (most 

effective when coupled with offshore flow). The burning question during these heat waves is how far 
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west will the marine layer influence be pushed, and how high will temperatures rise as a result. The 

factors surrounding these phenomena, in addition to others, will be explored in upcoming sections. 

Numerous figures and tables are used to help better visualize the different aspects and effects of the 

marine layer. One figure shows graphs of the June, August, and the June-September mean inversions 

(353 of a possible 366 soundings were available) for the period 1998-2000. Also included are 

temperature markers at 850 mb and at the inversion top values of 27 C and 30 C respectively. Those 

values are typically associated with very hot days during the warm season (June - September) when 

highly populated areas with east winds reach 110 F or higher. A comparison of warm season 500 mb 

heights, 1000-500 mb thickness, 650 mb relative humidity (approximate level of the monsoonal jet) , 

and KNKX to KDRA 850 mb height gradient for the 1998-2000 warm season periods follow. The data 

consists of 714 of a possible 732 soundings, and the data for each "half month" period for June through 

September is used for the graphic. The 650 mb relative humidity is also a good approximation of the 700 

mb relative humidity, and warm season thunderstorms can erupt if the 50 % relative humidity in the 

mid levels falls as low as 650 mb. To look at the relationship between surface pressure gradients and 

high temperatures, a graph of the mean 2000-2003 surface pressure gradient (in mb) from Las Flores at 

the coast to the Rice Valley sensor in the lower Colorado River Valley area near the Arizona border 

(LSF-RIC) was made. There were 1071 4 am local standard time observations out of a total of 1461 

possible values available for the time period. The marine layer inversion was analyzed using a graph of 

the 1200 UTC inversion strength and marine layer depth versus 850 mb temperature, inversion strength, 

and inversion strength running total. There were 460 of a possible 488 values for June through 

September 2000-2003 available. A graph of the marine layer depth based on 850 mb temperature gives 

some idea of how the two values compare. Events with onshore (offshore) flow at 850 mb are shown as 

shaded (clear) bubbles for June-September 1998-2000. There were 360 of a possible 366 values for 

June-September 1998-2000 included in that graph. A table showing a comparison between warm season 

inversion heights and 850 mb temperature (360 of a possible 366 values were available) is included. A 

“Storm Probability of Precipitation” (SPOP) is introduced. It was developed using the marine layer 

depths of 51 available storms for the 1998-2000 period, and 850-700 mb (lower mid-level) relative 

humidity from 52 available storms for the 1998-2002 period. Areal coverage of precipitation for 48 

hour periods was used to determine the SPOP. Cool season storms (October-May) were used to develop 

SPOP, but the values are likely to be applicable for the entire year. (An upper level low is frequently 

necessary in order to squeeze rainfall out of lower mid level relative humidities below approximately 75 

percent during the summer west of the mountains). Afternoon high temperatures were compared for 359 

of a total of 366 days for the coast, valleys, mountains, and deserts for June-September 1998-2000. In 

order to look at heat waves, a graph of heat wave days was compiled for weak gradient days (a total of 

46 days were included). As a preliminary look at the relationship between the maximum and minimum 

temperatures in the lower mountains, a table was compiled for the Santa Ana Radar Site (KSOX) at 

3092 feet MSL (288 of a possible 365 days were available). Finally, tables showing various 

relationships among certain key parameters such as 850 and 700 mb winds, inversion height, inversion 

top temperature, 850 mb and 700 mb temperature, and 850 mb dewpoint (along with the number of days 

satisfying certain conditions) are included. 

Mesoscale circulations make marine layer forecasting even more difficult, causing significant deviations 

from tables and charts developed to forecast atmospheric conditions. For example, a Catalina Eddy, a 

vortex that develops over the coastal waters, will sometimes strengthen rapidly. The strengthening is 

frequently during the late morning/early afternoon portion of the day when sea breeze augmentation 

occurs, and the strongest south to southwest eddy winds typically develop. This results in a rather rapid 

rise in the marine layer depth (inversion base height). On other occasions (however less frequently) the 

opposite occurs, when the eddy totally dissipates during the midday period, and the inversion base 

collapses to the surface, resulting in an unexpected warming. Even imbedded waves in the marine layer 
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result in unexpected local clearing, along with some warming. It is my hope that this Technical 

Memorandum helps to explain some of the weather phenomena that occur in the extreme southwest 

corner of our nation as far as the marine layer and its effect on precipitation and heating are concerned. 

To assist this, sections with forecaster hints are included. 

2. MARINE LAYER 

2.1 GENERAL MARINE LAYER CHARACTERISTICS 

Although it seems like clockwork at times, the marine layer and its effects are quite variable. The 

effects may be in the form of a cold gravity wave associated with the afternoon sea breeze (and a 

temperature drop with no clouds), or possibly filled with clouds. Strong onshore flow, strong onshore 

pressure gradient trends, downstream blocking terrain and PVA (positive vorticity advection from a low 

pressure area or trough aloft) all serve to locally deepen the marine layer, and even bring low clouds 

inland early (near or before sundown). Even the pressure gradient trend (the 24 hour change in the 

surface pressure gradient) is surprisingly significant in the overall forecast process. Strong high pressure 

aloft, weak onshore surface pressure gradients, weak onshore flow, offshore pressure gradient trends, as 

well as downslope flow all serve to lower the inversion. Mid level cloudiness (slows nighttime cooling), 

a California gulf surge of relatively cool, moist air into the deserts (reduces the onshore surface pressure 

gradient), a monsoonal thunderstorm outflow boundary (raises the pressure inland to reduce the onshore 

surface pressure gradient), as well as an offshore surface pressure gradient or surface pressure gradient 

trend tends to slow/disrupt the development or inland progression of the marine layer and associated low 

clouds. These are some of the challenges for the forecaster. 

Sometimes the factors that frequently help make the marine layer become more shallow can set off 

mesoscale events that actually deepen the marine layer. Ridging to the north, rising geopotential heights, 

and offshore flow aloft from the north serves to bring warming via subsidence aloft, which helps to 

lower the inversion. At the same time the associated low level northwest flow over the outer waters (30-

60 miles offshore) jumps to 15-20 knots. This frequently generates a Catalina Eddy, which tends to 

deepen the marine layer, counteract coastal warming, and make the clouds more persistent near the 

coast. Even while the clouds over land are burning off, the advection of low clouds due to the eddy will 

frequently keep the clouds from totally clearing near the coast. If upper level heights and thicknesses 

are increasing, inland areas and areas at higher elevations see 24 hour temperature increases, while at the 

same time coastal areas are seeing lower afternoon high temperatures because of a more persistent 

marine layer cloud deck. Although the marine layer depth typically slopes downward to the north, and 

slopes upward to the east toward the mountains, a rather strong eddy will sometimes make the depth 

nearly the same everywhere. 

Seasonal changes such as the sea surface temperature increases and mid level (and possibly upper level) 

cloudiness make for a very erratic, confused deck of overnight low clouds during the mid summer 

through early fall. Perhaps the strongest fight that causes the inversion to oscillate is the fight between 

the Pacific high, its subsidence, its low level northwesterly jet just off the coast, and the Monsoonal 

high, with its moist east to southeast mid level jet and moist low level surges of monsoon moisture into 

the deserts from the southeast. Some surges, possibly in conjunction with thunderstorm outflow 

boundaries, can send damaging winds and low visibilities in blowing dust as they weaken the coast-todesert pressure gradient. In addition, there are pockets of upward motion from disturbances in the mid 

level flow which tend to disrupt the subsidence and weaken the inversion at times. 

During the spring through early summer, and again in the early fall the subsiding air of the Pacific high 

raises the temperature at the top of the inversion and strengthens the inversion, with strong heating of the 

deserts, and the higher valley, foothill and mountain areas near and above the inversion. Desert 
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temperatures climbing to around 115-120 F are a signal of this pattern. This enhances the onshore flow 

at the coast as the deepening thermal low in the deserts increases the coast-to-desert pressure gradient. 

This results in relatively cool afternoon temperatures in the coastal areas and a more persistent low cloud 

layer as the strong onshore flow pulls the marine air in beneath the inversion (which strengthens the 

inversion even more). This is most prevalent from May through early July. 

Further east, the development of the Monsoonal High over the “4 corners area” of the states of 

Colorado, New Mexico, Arizona, and Utah counteracts some of the effects of the Pacific high during the 

mid summer through early fall. This monsoonal high develops over the interior, and generates a 

southeasterly monsoonal jet that initially develops around 700-500 mb along with the highest relative 

humidity aloft outside of the winter rainy season. This cloudiness reduces overnight thermal radiation 

along the coast. It also reduces the heating in the desert resulting in a weaker east-west (onshore) 

pressure gradient, a weaker inversion, and better afternoon clearing of the marine layer. This pattern is 

easily seen with 700 mb dewpoints reaching 0 C and 850 - 800 mb dewpoints of around 10 C or more 

for the “lopsided inversion” type of pattern. [The huge dewpoint depression at the inversion level 

associated with the rapid fall in dewpoint (commonly found with subsidence from the Pacific high, and 

is apparent in figure 2) becomes nearly absent, resulting in a “lopsided inversion”]. Although this 

profile favors monsoonal convection, it is prone to supporting a very erratic low cloud pattern. Perhaps a 

stronger effect on the marine layer is the southerly surge of low level moisture into the deserts. This 

weakens the thermal low in the deserts via a surge of cooler, moist air from decaying large thunderstorm 

or Mesoscale Convective System (MCS) outflow boundaries that develop over Mexico or Arizona. 

Another source is a surge of cool, moist low level air generated by a huge north-south pressure gradient 

from the deserts to the southern tip of Baja. The combination of the surface flow and the moisture aloft, 

(and in particular with easterly flow) works to keep the low clouds out completely or only allow them to 

develop during the late night and morning hours. 

Near the end of summer the first few fronts result in surface high pressure areas building over the Great 

Basin plateau and locally into the deserts, which weakens the onshore pressure gradient (thus weakens 

or delays the sea breeze). This allows heat waves at the coast that are commonly seen in September. At 

the peak of our “tropical storm remnants” season (late August into September), the south and 

southwesterly flow from transition season cut-off lows and troughs finally dip far enough south at a 

higher frequency, and pull in moisture aloft from tropical storms while the low level remnants (typically 

the low level circulations) pass by to the south. This moisture flowing into southern California further 

disrupts the coastal marine layer under the moisture of the wave (including mid and high level clouds). 

It is more likely for low clouds to develop in a more solid fashion around the edges of the mid and high 

cloud shield as opposed to under the shield. One fly in the ointment is if the mid level moisture is 

minimal when the trough or low moves over, the marine layer might actually deepen. 

Fig. 2 shows the variation in the marine layer from June to August, along with the mean inversion. 

Figure 3 gives some of the reasons behind the inversion lowering from spring to summer. First, the 

upper level high over the Pacific (the dominant high during the spring and early summer) loses some of 

its influence as the upper level (monsoonal) high builds to the east. The 500 mb heights peak at 591 

decameters and 1000-500 mb thickness peaks around 579 decameters during the second half of July. 

The 850 mb height gradient from the coast to southern Nevada remains an “onshore” gradient, but 

continues to weaken all summer. The reflection at the surface is seen via the weakening of the onshore 

surface pressure gradient. Along with increasing temperatures aloft through mid summer, the inversion 

is forced downward from 2000-2500 feet msl in June to 1000-1500 feet msl in August. Aloft, the 650 

mb relative humidity rises, (a reasonable height of the monsoonal jet and an approximation of the 700 

mb moisture) which has the effect of slowing low cloud development. This disrupts the marine layer 

low cloud pattern, resulting in breaks in the cloud deck, along with erratic arrival/departure times for the 
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low clouds. This is in addition to the cool moist southerly surges into the deserts, which cools the 

deserts and weakens the coast-to-desert pressure gradient, as well as affects the sea breeze. These 

factors, in addition to higher sea surface temperatures, tend to result in decreased episodes of 

widespread, persistent low cloudiness during the mid summer period, and only patchy low clouds 

develop on some days. 

Also due to the change in heights, thickness, and temperatures aloft, the onshore surface pressure 

gradient from the coast to the Lower Colorado River [for example, from Las Flores (LSF) near Camp 

Pendleton (KNFG) to Rice Valley (RIC) near Blythe (KBLH)] changes. The gradient falls from near 6.5 

mb to around 4.5 mb between late spring (June) and mid summer (August), as seen in figure 4. This 

helps change the marine layer pattern from a deep persistent “all day overcast in some areas” pattern, to 

one where the land areas (including the beaches) routinely clear during the mid morning hours. 

(However, there are occasions when the clearing is as late as mid afternoon). During the spring and 

early summer desert heating far outpaces the coastal heating (desert afternoon high temperatures peak in 

July, with the coastal area high temperatures peaking as late as September). This strong inland heating 

creates the largest coast-to-desert temperature differentials of the year, approaching 60 degrees F at 

times (hence the annual maximum pressure gradient occurs during the late spring and early summer). 

This helps to pull in the low clouds. The low clouds, during deeper marine layer episodes, can push 

inland through the passes to the desert edge of the mountains, with strong cooling in the deserts to well 

below normal. Desert temperatures easily drop below 100 degrees with some episodes. The clouds, 

although visible from the desert, typically evaporate before they push into the deserts, which also results 

in higher relative humidity on the coastal side of the mountains than on the desert side at similar 

elevations. On average, higher, weaker inversions in the spring become lower, stronger inversions mid 

summer. However, inversion strength in the 1000-1500 foot msl range might actually be weaker during 

mid summer than spring. Part of the reason is the sea surface temperatures at the inner coastal waters 

buoys rise into the upper 60s by September, which could result in weaker inversions in the 1000-1500 

foot range and better clearing. Aloft, the height gradient at 850 mb is weaker during the mid summer 

period than it is during the spring. Along with the relatively cool, moist surges into the deserts from the 

south during the mid and late summer period, the combination results in a weakening of the surface 

pressure gradient and weakening of the onshore flow. Above the marine layer there is around 2000 feet 

of warm inversion layer air. The mid to late summer weakening of the onshore flow partially explains 

the September peak in afternoon high temperatures at some coastal sites. (This is in contrast to the July 

maximum temperature peak in the deserts, where the marine influence is less of a factor than the 

atmospheric thickness between pressure levels and “heights” of the mandatory pressure levels aloft such 

as those at 500 mb and 850 mb). 

Going into the mid summer period, day to day changes can be rather small. 

 

When 24 hour changes in conditions (or expected changes) are subtle, for example: 

1. Pressure gradients to local deserts (for example KLAX-KTRM) change less than 1.5 mb 

2. The 850 mb temperature changes less than 1.5 degrees C 

3. The inversion base height changes 300 feet or less 

conditions are very similar to those of the previous day in most cases, and persistence just might be the 

best solution. 

During mid summer the low clouds almost never remain in the valleys all day, and rarely stay in at the 

coastal airports all day (and it is generally best not to forecasts these events to occur). For the low clouds 

to remain at the beaches all day during the mid summer period, conditions should approximate the 

following (very strong inversion for the depth involved along with strong onshore gradients and trend), 
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1. Inversion depth (in thousands of feet) + inversion strength (in degrees C) 

should exceed 11. 

2. Approximately 6-8 mb or more of onshore gradient to the local deserts (coast to KDAG, 

KTRM, KPSP, or KIPL) where “coast” can be KLAX or KSAN. 

3. Approximately a 2 mb onshore trend to the local deserts and about a 4 mb onshore trend coast 

to Tonopah (KTPH), Nevada. 

An upper level trough or low moving over the area also helps. 

On the other hand, a 2 mb offshore gradient coast to KDAG (or KTRM) with a 2 mb offshore 24 hour 

trend to these local desert sites accompanies a lowered inversion base, and the low clouds clear 

everywhere much of the time, including the coastal waters, with above normal temperatures. This is 

true all year, but is particularly reliable during the summer. A “Marine Layer Matrix” outlining 

approximated morning cloud/fog coverage as well as clearing times can be found in Appendix A. 

Figure 5 shows the relationship between the 850 mb temperature, the inversion base height, and 

inversion strength for June-September 1998-2000. Highlighted are the June (deep marine layer period), 

August (summer marine layer depth minimum or “shallow marine layer period”) and the JuneSeptember summer mean. Lower 850 mb temperatures (basically cooling aloft) correlate well with 

deeper marine layers and weaker inversions. Higher 850 mb temperatures correlate well with shallower 

marine layers and stronger inversions, so typically, the inversion weakens as the marine layer deepens. 

Conversely, in many cases the inversion strengthens as the inversion lowers. 

2.2 EXAMPLE OF A BASIC MARINE LAYER EVENT 

Typically, during the winter, the polar jet migrates far enough south to bring strong lows and troughs 

through southern California. Occasionally, it combines with the subtropical jet for an enhanced thermal 

gradient and very strong rainfall/thunderstorm activity. On the edges of the cool season (mid fall and 

mid spring), subtropical high development pushes the polar jet northward, nearly eliminating the threat 

of storms moving along a low latitude track. During these periods the biggest storms are cutoff lows 

(hence the phrase “cut off seasons”). Most troughs pass by to the north, and simply modulate the 

deepening and lowering of the marine layer during the late spring through early fall. Since the fronts 

stay further north during the mid summer period, the fluctuations in the marine layer depth are much 

smaller during mid summer. 

Figures 6 and 7 show an “Enhanced Marine Layer Event” or possibly a “Hybrid Weak Frontal/Marine 

Layer Event”. The basic marine layer cycle will typically begin with a “transition day” possibly with 

little or no marine layer clouds or fog. This is sometimes the case after a period of offshore flow and 

offshore trend. If the winds at the beaches are easterly and about 10 knots or more the marine layer and 

associated marine inversion is effectively wiped out. When the wind stops or temporarily turns west 

with the sea breeze, the marine layer air deepens to about 500 feet deep. Conditions often transition to 

onshore flow and trend with dense fog developing at the major coastal airports. The fog generally 

resides in a layer just under the inversion base (and within 800 to 1000 feet of the inversion base) with 

the inversion base itself at or below 1000 feet MSL for coastal dense fog events. The fog can distribute 

upward into the inversion to about 800-1000 feet AGL for surface based inversions. The change to a 

shallow marine layer regime results in a capping inversion near 800-1000 feet MSL, and dense fog over 

the highly populated coastal and mesa areas if the onshore trend in the pressure gradient is small. When 

the inversion base approaches 1000 feet, the visibilities at the lower airports can improve significantly 

since the typical low inversion stratus deck is about 700 feet thick (sun or moon dimly visible) to about 
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1000 feet thick (opaque). If the winds aloft are rather strong [southwest at around 50-60 knots (30 knots) 

or more at 500 mb (700 mb)] with cyclonic flow aloft, the rapidly deepening marine layer, on occasion, 

will bypass this coastal dense fog stage (picking it up later in the valleys and/or mountains). These 

events are generally expected to result in a rather saturated airmass with bases down to below 1500 feet 

as the cloud tops rise and thicken into a deep marine layer. When the winds aloft are strong there is 

probably a weak front involved, [which helps keeps the cloud bases low (lower layers more saturated)] 

and the clouds thicken upward. Winds near 500 mb of 50-60 knots or more are notorious for squeezing 

measurable rain out. (A moderate acceleration of the onshore flow, such as that associated with a 

Catalina eddy, upper low/trough, or simply southerly flow) also help to make the coastal dense fog stage 

brief or non-existent, followed by a “stratus” stage as the cloud bases lift up). The passage of a strong 

trough to the north rapidly lowers pressures inland over the plateau region of Nevada/Utah, and rapidly 

deepens the marine layer cloud tops up to near 2500 feet. This could generate 2 cloud decks [possibly 

transition to a cumulus regime in the boundary layer, signified by a lower (convective) deck with or 

without solar heating]. Although convection does occur with only 1500 foot deep marine layers, some 

marine layers with cloud tops of around 2500 feet (along with clouds “convecting” beneath the inversion 

with cloud bases 1000-1500 feet or so) generate episodes with drizzle, and even have locally lower 

cloud bases to below 1000 feet. This drizzle scenario is common for rapidly deepening marine layers 

with cloud top heights of approximately 2500 feet deep or deeper, especially if the inversion is strong. 

Eventually, the cloud bases rise a bit as the inversion base rises. The marine layer dense fog region 

associated with the cloud deck then spreads inland and upward, generally at altitudes within 800 - 1000 

feet of the cloud tops (where cloud tops are approximately the inversion base). If the clouds reach 1500 

to 2000 feet thick, (which is actually rather thick for a marine layer cloud deck), drizzle might occur 

long enough to “measure” at non-ASOS sites, and even at standard NWS observing locations (ASOS) 

sites with clouds of over 2000 feet thick. Although measurable drizzle might occur with marine layers 

only 2500-3000 feet deep (cloud layer thicknesses approaching 2000 feet thick for a fairly saturated 

marine layer), these situations are quite rare as trace amounts are more common with marine layers 

depths of only 3000 feet or so. Drizzle from such thick marine layer clouds with cloud tops 2500-3000 

feet deep are typically followed by an afternoon with little if any coastal clearing. Drizzle is a good sign 

that clearing, if any, will be slow, and if there was no drizzle the previous day, it is a good bet that it will 

clear later than it did the previous day since the marine layer is typically deepening during such a 

pattern. 

Some light rain reports, with measurable amounts possible, occur with clouds tops reaching 

approximately 4000 feet with cloud thicknesses of 2000-2500 or more. At 5000-6000 feet deep, with 

cloud bases still down around 2000 feet or lower (lowest levels still relatively saturated, resulting in a 

thick cloud layer), scattered light rain develops. This is rather reliable if the inversion is fairly strong 

compared to the marine layer depth. A very strong inversion in comparison to cloud top height is when 

the approximate inversion height (or marine layer depth) in thousands of feet + inversion strength in 

degrees C is 11 or more. Locally moderate rain falls from the lower based convective clouds when their 

tops stretch upward to near the top of the marine layer, resulting in rather thick marine layer clouds. If 

the cloud distribution is mainly one of relatively thin stratiform layers, then much less rain occurs. 

At a cloud top height of a little over 7000 feet, which is rare for mid summer, it rains almost everywhere 

from the mountains westward (with rainfall heavier and more likely along the coastal slopes). This is 

more of an “enhanced” marine layer, or moist layer, since a front with thick cloud layers is possibly 

associated with such a deep moist layer. Some of these are more of a “hybrid marine layer/weak frontal 

event”. 

Below 800 mb (around 6200 feet MSL), the cooling at and just below the base of the inversion is mainly 

a reflection of the rising inversion base (deepening marine layer). This creates a very unstable marine 
, 

8

layer with a large temperature lapse rate from the surface to the inversion base in many cases. This 

easily results in boundary layer convection. On the other hand, while the deepening occurs, the 

inversion is weakening since the cooling flow aloft near the top of the inversion is probably going to 

exceed the cooling at the base of the inversion due to the lifting marine layer inversion base, (especially 

for deeper marine layers). In some cases, the inversion base will lift up to just higher than the mountain 

resorts near 6000 feet, and cool enough for snow to fall out of mainly marine layer moisture. This type 

of scenario has occurred as late as June. 

The time of day is also significant when it comes to drizzle and light rain scenarios. If these deep moist 

layers or “weak fronts" arrive midday, then the probability of rainfall decreases since there is a midday 

minimum in the boundary layer relative humidity. Mid day periods are the most unfavorable conditions 

for weak fronts to produce rain. The lowlands west of the mountains (and away from upslope flow 

influences) are more prone to this drying. The higher night and morning relative humidity favors night 

and morning drizzle and/or light rain since the drying in the lowest portion of the sounding becomes a 

“low level inverted V” profile during the day. If a front approaches a well developed marine layer, it 

could enhance and deepen the marine layer by becoming entrained into it. The front is likely to hold 

together better with higher precipitation probabilities during the night and morning hours (with the peak 

late night through mid morning). The clearing of the dense fog at around 8 to 9 am or so signals the 

thinning of the low clouds, a reduction in the drizzle, and possible development of a mid day inverted 

“V” low level profile. One variation on the daytime weak frontal theme is if the marine layer deepens up 

to near 8000 feet and is conditionally unstable (nearly isothermal above the moist layer near mountain 

top), there could be some afternoon convective showers in the mountains. These light rain scenarios are 

in contrast to much of the rest of the country away from the coast. The diurnal cycle in the marine layer 

moisture is so strong that it skews the light rainfall events into a late night and morning maximum in 

coastal southern California (for example, measurable amounts of less than a tenth of an inch in one 

hour). This is in contrast to the fact that many locations in the nation’s interior have an afternoon and 

evening maximum for rain events. Such light rainfall events have a big impact on travel, such as traffic 

accidents on slick roads, and airport runway landing pattern changes caused by wet runways (in 

conjunction with wind speed thresholds). 

Shallow, weaker fronts have trouble getting past Point Conception, and a break often develops in the 

front. Fronts with a northwest to southeast trajectory are most vulnerable. When the moist layer depth 

falls below 8000 feet deep or so, blocking by the mountains to the north (and some downslope flow to 

the lee of the mountains) shuts off the precipitation to many northern areas, and the northwest flow shifts 

to become more westerly south of Point Conception. This is also true for postfrontal airmasses in west 

to northwest flow), when the moist layer depth falls below 8000 feet deep. With the rain tapering off or 

ending in the north, the largely un-obstructed, shallow west to northwest flow into the southern areas 

(along with island effects) helps keep rain going well after the dynamics end there, producing more 

precipitation in the south than the north if the airmass is very moist and somewhat unstable. If the 

dynamics hold together past Point Conception, the residual weakening front results in a typical rainfall 

pattern (A typical weakening front has more rain in the north than in the south since northern areas are 

closer to the stronger dynamics and lifting of the front). This typical scenario occurs when the low level 

airmass is not terribly unstable. Very weak fronts in this flow might have only clouds that simply end 

up banked up against the mountains with little or no precipitation anywhere. A convergence zone often 

develops and parallels the coast during post frontal flow, locally enhancing any precipitation under the 

convergence zone. (This type of convergence zone is frequently seen in the visible satellite imagery 

during a Catalina Eddy). A Hybrid Coastal Convergence Zone/Land Breeze front develops when there 

is a weak land breeze that collides with westerly or southerly flow over the coastal waters. 
, 

9

At some point in the life cycle of a marine layer event (usually near the end of the cycle), a low level 

inverted V marine layer moisture profile can develop. In these cases, the clouds become quite thin as the 

cloud base heights rise faster than the marine layer deepens, and the drizzle and light rain ends. (Also the 

inversion weakens, so clouds become thin and/or break up, which helps end the rapid deepening/light 

rain period). Later, a second, lower inversion generally develops around the 1500-2500 foot level, and 

the marine layer re-establishes itself in the typical night and morning fashion. In these cases, clouds 

form under the strengthening, lowest inversion, and the clouds dissipate aloft at the old inversion. 

Eventually, the old, upper inversion dissipates. This is typically the “transition day” in the offshore trend 

direction, when the marine layer transitions from a day with clouds most of the day (if not all day) to a 

day that has better clearing most areas by afternoon. Before the marine layer becomes a solid, reestablished deck, the cloud pattern becomes somewhat erratic, (partly because the inversion is so weak) 

and the pull on the low clouds (via an onshore trend) is absent. With such a weak inversion, the marine 

layer clouds are prone to “reverse clearing” (low clouds clear in the coastal areas before the inland areas 

clear, partly due to boundary layer convection inland and stabilizing marine air moving in off the 

Pacific). In some cases it is actually cloudier during the day than at night while the re-adjustment of the 

marine layer occurs due to the daytime boundary layer convection. West of the inland edge of the 

marine layer, holes in the marine layer tend to fill in during the transition to a more convective marine 

layer regime, especially when the marine layer is rather deep. 

Near the end of the event, the trough moves further inland, and high pressure develops aloft and over the 

plateau areas of Nevada and Utah to the northeast. The surface pressure gradient decreases, and the 

heights and thicknesses rise. If the offshore flow event is strong enough, the inversion is wiped out and 

clear skies prevail until a new inversion strengthens enough to cap a significant marine layer cloud deck 

(5 C or more). If the inversion reaches 5 C, or stronger, there is almost always a morning low cloud 

deck in some areas during the summer. Conditions favor development in the south first, and if it only 

develops in one area, it is most likely in the south. (Typically, the inversion height is higher in the south, 

due partly to better exposure to the northwesterly flow over the coastal waters, so low clouds develop or 

advect in more frequently, and there is a good chance that the clouds will hang around longer in the 

south). 

2.3 MORE VARIATIONS IN THE MARINE LAYER CLEARING PHASE 

One of the big challenges is the clearing pattern. If the offshore gradient trend is very fast (for example, 

a 5 mb offshore trend from the coast to the local deserts), there are mostly clear skies since the inversion 

is lowered to the surface or is totally wiped out. The actual pressure gradients might become offshore. 

On some occasions the gradients become weak, but not offshore enough to push the marine layer 

offshore. In these cases, the marine layer inversion will lower and strengthen. With the cooler, 

transition season sea surface temperatures, after the low clouds burn back to near the coast during the 

day, the beaches may not clear since the stratus may linger at the beaches well into the afternoon. In the 

opposite flow direction, when a period of offshore flow ends, the result is basically the same. The 

onshore gradient trend results in an onshore flow of marine air, the re-establishment of a shallow marine 

layer and a low, strong marine layer inversion. This also results in the beaches remaining cloudy, or 

mostly cloudy skies lingering at the beaches well into the afternoon. During the evening, stratus that is 

still at the beaches tends to move inland faster than it does when it either has to form, or move in from a 

point further offshore. With strong surface and 850 mb onshore gradients in May and June and an even 

deeper marine layer than the later months, low clouds linger much of the day over the coastal plain. The 

low clouds remain much of the day in the valley and coastal slope regions on some days when the 

marine layer is very deep. If it is August or September, there is a good chance that all land areas will 

clear and the stratus will move offshore away from the beaches in the afternoon. “All day” low cloud 

patterns become rare by mid summer as the inland “pull on the low clouds” is reduced. This is possibly 
, 

10

due to the weaker onshore gradients, cooling gulf surges into the deserts, monsoonal moisture aloft, and 

possibly somewhat higher sea surface temperatures. This reduction in onshore pull results in the 

beaches becoming clear almost every afternoon mid July through September. The low clouds that do 

happen to occasionally linger are very patchy. If the gradients are weak [or better, offshore (with 

pressure values higher inland than over the coastal waters] during the mid summer period, the marine 

layer will typically be reduced or wiped out, resulting in heat waves. 

 

At times a trough in the flow aloft or an upper level low is marked by a band of high clouds while it 

moves though California. It is possible for the low or trough to provide enough lift to keep the marine 

layer in all day, or bring it in during the mid afternoon, earlier than the usual pattern of near or after 

sundown. (Basically, mid and high clouds associated with a trough stop the clearing of the low clouds, 

and enhance the low clouds enough for an early evening arrival. At other times it simply slows the 

daytime burn-off). 

Low, strong inversions (typically 10 degrees C or stronger with a depth of around 1500 feet MSL or 

lower) precede a transition from morning fog to thick afternoon haze for locations at and below the 

inversion base. This haze causes a mid afternoon dip in visibilities as a “haze front” on the leading edge 

of the sea breeze as it sweeps past the inland valley airports. For instance, a 3 mile visibility around 

noon at KSNA will typically cause a haze front in the Inland Empire as the winds jump to 8 to 15 knots 

from the west at KONT or KCNO, temporarily reducing the visibility. An early stratus arrival is a good 

bet when the visibility at KCRQ is restricted to 5 miles or less in afternoon haze, or stratus clouds 

surround Catalina Island (KAVX) during the afternoon. This is a characteristic of a pattern with marine 

layer low clouds that are only a few miles offshore or locally onshore in spots, and the inversion is 

strong. In these situations, coastal cities, including major coastal airports get the low clouds around 

sundown as the advection of low clouds overpowers the dissipating effect of solar heating early. If the 

KCRQ midday visibility jumps up to 6 miles or more, the low clouds and fog tend to wait till after 

midnight or so to move in at KCRQ and the other coastal airports. 

2.4 MORE ON LOW LEVEL (MARINE LAYER) INVERTED V SOUNDINGS 

The deterrent to all deep marine layers producing widespread drizzle and/or light rain is the low level 

inverted V sounding. The question is will the marine layer be saturated from the surface to the inversion 

base, or will there be a low level inverted V sounding throughout the entire event (not just at the end of 

an event)? These “unsaturated” events are characterized by a rather thin cloud deck just below the 

inversion base, with very little drizzle or light rain. For very deep events with cloud tops reaching 

upward to 7000-8000 feet or higher, (better if associated with a weak front), at least for a while the 

airmass becomes rather saturated with a period of rain from a thick layer of clouds with no low level 

inverted V problem. Also in this type of case, 500 mb winds typically reach 60 knots or more, which 

seem to accompany rain events primarily consisting of boundary layer moisture. Heights around 500 

mb approaching 564 decameters (best during cyclonic flow) accompany many of these weaker events. 

At around this 7-8 thousand foot depth are the “high POP low QPF” type events, where the probability 

of precipitation (POP) is high, but the quantitative precipitation forecast (QPF, or expected rainfall 

accumulation) is low. For example, the POP will sometimes be 60-80 percent, but less than 1/4 inch of 

accumulation is expected in most areas. Interestingly, with events consisting of mainly boundary layer 

moisture, if there is a light low level wind with an easterly component at or below 850 mb the moist 

layer just might stay rather saturated down to near the surface. This cooling, “drainage wind” type of 

effect lowers the surface temperature, reduces the dewpoint depression, and consequently keeps the 

lifting condensation level (LCL) low. However, if the winds are in excess of 10 knots or so, it helps to 

dry the boundary layer and delay the onset of any rain. 

 
, 

11

2.5 MORE ON REVERSE CLEARING 

Reverse clearing patterns occur during any season, and are a frequent occurrence behind winter storms. 

[Reverse clearing is when instead of the low clouds clearing inland areas first (before clearing westward 

back to the coast) the low clouds clear at the coast first, with the clearing typically following the 

stabilizing sea breeze air inland]. During the non-winter months it is most likely to occur during the 

transition seasons of spring and fall (most probable in the spring) and least likely to occur during mid 

summer. The pattern starts with low clouds already in place. As the cold advection settles in aloft 

(typically associated with a trough passage), the marine layer deepens while the inversion rises and 

weakens. This weakening inversion is seen in the low cloud pattern as clearing moving down the coast 

in the Point Conception area west of Santa Barbara (KSBA) over the offshore waters (where there is a 

tendency to clear first). Downslope flow into a weak inversion is a strong contributor to the clearing 

there. The deepening of the marine layer during these “boundary layer convection/reverse clearing 

episodes” can result in the clouds clearing all areas, except the low clouds that are still “slapped up 

against the mountains” during the afternoon since the convection continues there. 

At the end of the deepening episode there is frequently a “transition day” as 850 mb temperatures fall 

into the low to mid teens (when the 24 hour pressure gradient trends become 0 or reverse to become 

offshore). If the reverse is slow, then the moist layer slowly erodes from the top, and the inversion 

lowers. The inversion eventually caps the shallower moist layer at a more “seasonable” level. This 

“seasonable” level is around 1500 to 2500 feet. Initially a disorganized marine layer cloud deck (if at all) 

is expected with cloud bases somewhere in the 1000 to 2000 foot range if the marine layer inversion is 

rather weak. 

Occasionally, a more “wind enhanced” inversion aloft is created via a jet streak and associated jet 

dynamics [possibly in addition to or above any other inversions]. This is sometimes seen via 

strengthening of the winds aloft and possibly a wind shift, along with warming aloft (for example, at 850 

or 700 mb) resulting in a sounding with a “double inversion”. The inversion aloft, created via a jet streak 

and associated jet dynamics lowers and/or swallows up the old (lower) inversion and establish itself as 

the “new inversion”. During some events this lowering brings the inversion to within 1000 feet of the 

surface, resulting in dense fog in the highly populated coastal areas. If the reverse in the pressure 

gradient trend is fairly rapid from onshore to offshore, then the moist layer significantly dries as the 

marine layer is “wiped out”. It is possible to have a period with little (if any) inversion or marine layer 

before the re-establishment of a low inversion and the “new marine layer” develops. 

Marine layer inversions are occasionally very weak, (possibly even isothermal). In these cases solid low 

cloud development becomes very difficult since the inversion is too weak to “cap” the marine layer very 

well. The typical “night and morning low clouds and fog” cycle is reversed, with the best cloud cover 

during the midday period due to solar heating when the marine layer is deep, and if there are breaks in 

the clouds during the midmorning hours. (These marine layer “disruptions” are rather frequent in the 

spring and fall since troughs and fronts are stronger and more frequent, but they also occur during mid to 

late summer monsoonal moisture intrusions). During these events the best low cloud coverage during 

such a deep pattern might be in the inland areas for a day or so. The transition from a “standard clearing 

pattern” (clears inland first) to a reverse clearing pattern (clears at the coast first) generally occurs when 

cloud tops reach or exceed 3500 feet MSL and the inversion weakens enough to allow it. Embedded in 

the annual cycle is a diurnal cycle of the marine layer strength. A morning sounding typically contains 

the strongest inversion of the day, with the weakest inversion on the afternoon sounding. A quick 

estimate of the inversion strength for the following morning is obtained by lowering the temperature at 

the base of the inversion on the 0000 UTC sounding by about 1 or 2 degrees C. This adjustment is 

likely to be a smaller value for days with little clearing and a larger value for days with good clearing. 
, 

12

This is because the better drying and larger temperature/dewpoint spread on days that clear well 

necessitate a larger temperature drop and dewpoint temperature increase to reach saturation at the 

inversion base. 

Typically the inversion strength weakens in proportion to its depth, and is less than 7 C for deep marine 

layers. The 850 mb temperatures are also relatively low [low to mid teens (in degrees C)] for deep 

marine layers. Cloud decks are generally less solid with such weak inversions. These weak inversion 

scenarios, at times, are characterized by higher daytime than nighttime cloud coverage, with marine 

layer depths at or in excess of 3500 feet deep, with a significant impact on high temperatures. 

Under more extreme offshore trends, such as Santa Ana Winds, a very dry, relatively hot airmass 

advects in. This might be enough to totally clear out the low clouds for a day or so. This allows a 

shallow, “dense fog producing” inversion to develop after a day or two near the coast as the cool, low 

level oceanic moist layer returns under the hot airmass (transition day). At times this is a rather frequent 

pattern, with the most vulnerable periods being the fall and early winter. At around the same time the 

500 mb ridge axis moves inland to reduce some of the subsidence. The transition must be slow enough 

so that the layer deepens slowly, otherwise the coastal dense fog stage is bypassed completely. 

Interestingly, areas 15 miles or more inland are quite possibly as warm (or warmer) than the previous 

day while beach highs are actually lower than the previous day during such a transition. 

While the 500 mb ridge is overhead, and the 700 mb closed high is to the north [possibly supplemented 

by northeast winds at Fremont Canyon (FMC)] record high temperatures frequently occur. In these 

cases 850 mb potential temperatures surface all the way to the coast. Highs in the “inland coastal areas”, 

approximately 10 miles or so inland, are expected to be around T850 + 26 to T850 + 30 F. Hot days 

with a big temperature gradient, (for example, those with over 40 degrees of temperature difference 

from the coast to the inland valleys) may generate a pressure gradient that boosts the sea breeze as much 

as 5 mph from 10 to over 15 mph (and gusty), which is very noticeable in the highly populated coastal 

areas. This is common during the later periods of a heat wave event. If an eddy develops, gust to 15-25 

mph are even possible. A signal for eddy development is when the pressure gradients get into the 

favored eddy configuration (trending offshore to the north, and trending onshore to the south and east) 

as a trough moves inland. The isobars sometimes orient themselves parallel to the coast, which tends to 

be a signal for a significant increase in low clouds. 

2.6 VARIATIONS IN THE TYPICAL MID SUMMER MARINE LAYER CYCLE 

Enhanced marine layer events are rare during the mid summer period (much of July and August) and the 

cycle is somewhat different. It is very difficult to generate marine layers over 3500 feet deep during the 

mid summer period. Southerly surges of cool gulf of California air into the deserts weakens the coast to 

desert pressure gradient, and the mid level monsoonal moisture also helps to disrupt the low cloud 

pattern, making all day low cloud events in the inland areas very rare. This is in addition to high sea 

surface temperatures helping to weaken the inversion and assist with clearing. When the flow starts to 

trend onshore, (and during extreme cases, deepen up to 2500-3000 feet), it should still clear over all land 

areas, including the beaches, by mid afternoon during this mid summer period. 

2.7 MARINE LAYER EFFECTS ON TEMPERATURES AND PRECIPITATION 

As for the relationship between 850 mb temperatures and the marine layer depth changes, figure 8 and 

table 1 gives a better look at how the inversion height changes with 850 mb temperature. The top of 

most marine layers are below 4000 feet MSL, where the inversion base typically changes 200 feet for 

every degree C. (This rate roughly doubles in the 4000-6000 foot realm, and roughly doubles yet again 

at depths over 6000 feet). For a 2500 foot deep marine layer, a 4 degree C increase in the 850 mb 
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13

temperature should lower the marine layer 800 feet. The marine layer inversion base is very resistant to 

lowering below 800 feet during the summer, and will do so only under extreme conditions. As for 

precipitation, figure 9 and table 2 hint at the marine layer depths of just over 3000/5000/6000/7000 feet 

MSL as 10 %, 20 %, 60 %, and 80 % respectively, which are the slight chance, slight chance again, 

likely, and “categorical” rainfall categories. The deepening can really help to keep temperatures down. 

For relatively shallow fronts moving in from the northwest, it is a good idea t o check the moisture depth 

on the Vandenberg (KVBG) sounding and its surrounding observations (including the offshore islands) 

to see how much rain has fallen in order to gage how much precipitation sweeps south into the Bight 

region (the region from near KSBA down to near KSAN). This is because many fronts cannot survive 

the trip past Point Conception intact. It is a good reality check on how well the inversion depth matches 

the probability of rainfall. Although some of the moisture is swept out during shallow events by the 

Santa Ynez Mountains near KVBG, enough moisture still sweeps past the area just offshore and hits 

mainly our southern areas. The precipitation for marine layer events with mainly shallow southwest to 

northwest flow develops from the coastal slopes of the mountains westward. When the winds aloft “go 

south” (switch to become southerly) tropical airmasses advect into the area, and rain chances increase 

dramatically in the deserts as well. If a summer storm moving into the area is strong enough to generate 

deep southerly flow, or even easterly flow, precipitation becomes rather heavy (since there are almost 

always thunderstorms with these summer rain events), and fall heaviest on the eastern slopes of the 

mountains. Heavy rains with flooding are possible. During the summer, it is most likely that a tropical 

storm or a rather strong upper low will accompany this scenario. 

Pressure gradient trends (the 24 hour change in the pressure gradient) have more of an effect on heating 

and clouds than the actual pressure gradient at times. Pressure gradient trends result in their largest net 

change in local weather when the gradients are “weak onshore” (Table 3) as far as temperatures and 

clouds are concerned. 

When actual pressure gradients are already “strongly onshore” it is difficult for significant 24 hour 

changes in maximum temperatures to occur over the coastal and valley areas for deep (3000 feet) marine 

layers and weak offshore pressure gradient trends. A couple of mb of offshore trend (or even weak 

onshore trend at times), results in much smaller 24 hour changes in high temperatures than if the actual 

gradients were already weak, (in particular, for regions well below a strong inversion). This is because 

the thermal transition zone (responsible for many of the large 24 hour temperature changes) is so far 

above the coastal and valley locations that small gradient changes have very little effect. (All of the 

mixing occurs inside the cool air of the marine layer). This assumes that the cloud cover pattern remains 

basically unchanged. Of course, mountain locations near the inversion see much bigger changes as the 

inversion moves. 

The significant 24 hour changes in temperature and cloud cover typically occur if either; 

1. The pressure gradient trends are strong enough to pull in cold air, which weakens the inversion 

 enough to clear the clouds (in the onshore trend direction) 

2. The clouds remain all day (in the onshore trend direction) when the previous day saw some 

clearing. 

3. The “Thermal Transition Zone” sweeps by via changes in the flow direction or subsidence 

pattern (either the offshore or onshore trend direction). 

For example, a pressure gradient change from 10 mb onshore to 8 mb onshore from the coast to the local 

deserts in 24 hours during a deep marine layer will most likely have almost no affect on temperature 

(since the clouds become persistent, and the flow cannot dislodge the marine layer cold air). In 

comparison, a change in the gradient from near 0.0 mb to -2.0 mb from the coast to the deserts is still a 2 

mb offshore trend, but the difference is due to the marine layer inversion being so close to the surface 
, 

14

with such a weak gradient. The 850 mb potential temperature easily lowers to the surface in the highly 

populated coastal areas for a higher impact, and the inversion correspondingly lowers and sweeps the 

thermal transition zone and associated clouds westward. In these cases large 24 hour temperature 

changes (which tend to follow the inversion) occur. This is frequently the case with 850 mb 

temperatures and inversions that are around the August average of 1300 feet and 23 C, which places the 

warmest inland valley high temperature around 100 F. During these times there is a huge temperature 

(and humidity) change with distance from the coast to inland areas as well as with elevation. Jumps of 

10-20 degrees F with the “inversion base passage” can occur in some locations over the previous day. 

In the mountains and foothills, this zone is typically found near the 1500 to 4000 feet MSL region, seen 

as heating or cooling that follows the inversion when the inversion sweeps past. Lower mountain and 

foothill regions become very warm during a downward sweep, with high temperatures in the mid 90s 

resulting in low temperatures in the mid 70s. At times, highest mountain minimums match the 950 mb 

temperatures when they are in the 70s or higher. The inversion locally lowers to below passes and 

canyons when winds aloft lower to the surface there, resulting in huge one hour temperature changes, 

occasionally some 20 to 30 degrees F. Ramona (KRNM) in the inland valley area east of San Diego 

(KSAN) and the Devore (DEV)/Riverside (KRAL) in the inland valley area east of Santa Ana (KSNA) 

are notorious for rapid fluctuations. Large temperature variations of 20 to 30 degrees are common 

between cities, sometimes only 5 miles apart. 

When the inversion base falls below 1000 feet, the transition zone pushes westward into the coastal 

plain, resulting in huge temperature differences across the area, with dense morning fog and very hazy 

afternoon skies. On very hot days, even small amounts of surface and atmospheric moisture results in 

some clouds developing over the mountains by afternoon. Good visibility during the morning (most 

prominent as one looks toward the mountains) most often precedes a very hot day during the summer. 

This is a sign that the marine layer inversion has been lowered to the surface and the marine air has 

largely been pushed offshore, possibly by warming, descending easterly flow below the canyons and 

passes with much less haze. 

As for the onshore flow direction, a 2 mb onshore trend with weak gradients are prone to generating a 

sharp decrease, (sometimes 10-25 degree F 24 hour temperature falls) near the coast as the inversion 

rises and heat no longer mixes down easily to the levels of the previous day. When the inversion base 

rises, the transition zone pushes eastward across the coastal plain, resulting in these huge 24 hour 

temperature falls across the area. During the waning stages of a warm spell the sea breeze front 

(especially those that are accompanied by low clouds) can drop temperatures more in one hour than 

most synoptic scale cold fronts do. Large 24 hour temperature differentials easily occur with low, strong 

transition season inversions since the stratus might clear near the coast on day 1 and clear little if at all 

on day 2. This can result in lower afternoon high temperatures coastal areas (possibly to below normal). 

Temperatures have actually experienced 10 degree F falls in only 20 minutes as early in the day as late 

morning! At the same time increasing thickness values and 850 mb temperatures can simultaneously 

result in an increase in afternoon high temperatures to well above normal in the inland areas on some of 

those occasions. Temperature patterns with near to below normal values at the coast with record highs 

inland at the same time are not uncommon. 

On a day during the spring and early summer when the clouds do not clear west of the mountains (deep 

marine layer) high temperatures west of the mountains are almost always in the 60s to lower 70s, even 

in the valleys. Because of the adiabatic profile and lifting/cooling in the upper portion of the marine 

layer, locations like Julian (JUL, in the foothills around 4000 feet MSL) are the coolest locations in the 

forecast area (“reverse heating pattern” foothills westward, with higher temperatures and better clearing 

closer to the coast). The higher mountains get rare marine layer snow during some transition season 

enhanced marine layer events. In these cases the lowest temperatures in the surface to 10,000 foot MSL 
, 

15

region are found at the base of the inversion near mountain resort levels (approximately 6000-8000 feet 

MSL). 

Inversion height and cloud top height are frequently different. Since marine layers are generally 

“conditionally unstable” below the inversion, the boundary layer easily “convects” with or without solar 

heating and frequently create a second cloud deck, especially for marine layer depths of around 2500 

feet or more above ground level (AGL). If the inversion strength is less than 7 C, (even more likely for 

nearly isothermal inversions), cloud tops easily extend into the inversion, possibly to near the inversion 

top in inland areas and near mountains where stronger convection and upslope flow are present. For 

example, with little pressure gradient trend, clouds under a transition season 3500 foot deep inversion 

with an 8 degree C inversion will become scattered clouds in the Inland Empire in areas such as KONT 

and KRNM. However, the cloud deck remains solid over the coastal plain (best with a “typical cloud 

thickness” of 800 - 1000 feet thick or thicker). With a weaker inversion (for example, a 3 degree 

inversion capping the marine layer) it might clear early, or start out mostly clear in the morning. If there 

are clear spots at around 1600 UTC, then the moisture will often “convect” into a midday cloud ceiling 

in the clear areas west of the inland edge of the marine layer air (clouds “fill in” the gaps in the cloud 

cover). On these days dramatically slowed clearing of these midday decks is expected, with clouds that 

stay in most (if not all) day in most areas. (In the forecast models such as the time/height crossection, the 

marine layer is the portion below the 40 % contour on the GFS and the 50 % contour on the NAM/NGM 

models). Sometimes the models don’t “moisten up fast enough” after a dry period in the boundary 

layer, and shallow marine layers sneak into the coastal areas before the model contour thresholds listed 

above are reached. The marine layer is basically below the 60 % relative humidity level on the actual 

sounding. In the other direction, an offshore flow event with approximately 10 mb of offshore gradient 

from the coast to Nevada (for example, KLAX to KDRA) often results in 850 mb dewpoints and relative 

humidities dropping to the surface rather than the “wetter” surface values shown in the models. 

2.8 LOW LEVEL REDEVELOPMENT OVER THE COASTAL WATERS 

For very strong inversions (typically around 10 C or more), the cloud tops are confined to near the 

inversion base. With the “low strong inversions” of around 1500 feet or less with 10 degrees C or more, 

the transition season marine layer typically retreats temporarily off the coast mid morning. If it moves 

far enough offshore and the deck is less than around 800 – 1000 feet thick, it generally will not support 

much island effect stratus/blocking stratus, and little if any coastal re-development around noon. In 

contrast, the low clouds will re-develop near the coastline an hour or so after the initial clearing if the 

edge of the main low cloud deck is 10 miles or less from the coast during the afternoon. Sea breeze 

convergence or shoreline blocking helps the low clouds to re-develop. In some cases the low clouds are 

simply “advected back in” during the late morning hours, eliminating the complete late morning 

clearing. 

2.9 ISLAND EFFECTS 

With low inversions, sometimes the flow downwind of the islands creates alternating bands of enhanced 

stratus and reduced stratus in their wake, that move into the coastal areas. At other times the heating of 

the islands clears all of the stratus downwind (quite noticeable for very low inversions). To add more 

difficulty, some cloud patterns consist of just a single convergence band downwind of an island with 

clearing on either side of the band that moves into the coastal areas. Deciding whether the clear portion 

or the cloudy portion will be over a forecast site (and when) is very difficult. 

2.10 EDDY FLOWS 

Although smaller eddies occasionally form near Point Conception, or downwind of terrain features such 

as the islands, the most famous local eddy is the “Catalina Eddy”. It typically starts with a trough 

moving through to the north, followed by northwest flow of 15-20 knots out near San Nicolas Island 
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16

(KNSI) and outer water buoys. A good indicator of a developing eddy is when KSAN winds pick up a 

southerly component (wind directions from 140 degrees to 240 degrees are a good indicator of an eddy). 

The Catalina Eddy has a habit of showing up or strengthening during the mid morning hours. Eddies 

often spin up during the mid morning with very little notice. They occasionally become strong. 

(“Strong” is 7 knots sustained on 3 hourly observations between 0700 and 2100 UTC and between 140 

and 240 degrees, with at least one gust to 10 knots). This helps to keep the low clouds from clearing near 

the coast, typically from the KCRQ area south. A moderate eddy is essentially when the sustained 

winds between 140 degrees and 240 degrees reach 7 knots on at least 3 hourly reports at KSAN between 

0700 UTC and 2100 UTC. The eddy winds typically peak late morning and become more southwesterly 

as the day progresses. A strong eddy is in place if during a moderate eddy at least one 10 knot report is 

received during that time period. Pressure gradients that “trend offshore to the north and onshore in the 

south” are a good precursor for eddy development. The result is a deepening marine layer with “road 

wetting” drizzle possible. The marine layer deepens rapidly (1000 feet or more in 24 hours) to nearly 

2500 feet deep or more during such a pattern. The circulation pushes the stratus inland and northward, 

or just northward along the coast. During these southerly pushes, the low clouds reach KONT through 

the “back door” (from the southeast, hitting the southern inland valleys first) rather than the typical 

arrival from the west. Depending on the depth and position of the circulation, it even pulls the stratus 

westward out of the coastal and valley areas and speeds up the clearing at times. Another issue is the 

advection of deeper marine layer clouds toward the coast from the inland and southern areas, which 

results in higher cloud tops near the coast than is expected since tops are generally higher inland and in 

the south. This causes a later clearing time than previously thought. On some occasions an upper level 

low (large scale eddy) drifts over the area and mimics the flow and effect of a Catalina Eddy with an 

“eddy like flow”. An eddy like (southerly) flow along the coast, whether or not it is eddy related, just 

might keep the low clouds in all day under a low, strong inversion. 

Mesoscale flow directions also have serious effects on afternoon temperature and wind direction at 

smaller scales. With the southerly eddy flow, winds in the KFUL/Anaheim area are southerly, with only 

10 miles of trajectory over land. When eddy winds are weak or absent, the flow over the coastal areas is 

mainly west-northwesterly, and heated airflow from the northern coastal areas of the California Bight 

(from the KSBA area to the KLGB area) drifts south and southeast over the coastal waters north of the 

KCRQ area. This results in complete afternoon clearing in the coastal waters north of KCRQ. Also, 

without the overpowering southerly winds of an eddy, a west-northwest sea breeze flow develops 

around the northern side of the Palos Verdes peninsula (a hill approximately 1500 feet high just south of 

KLAX and north of KLGB). When this occurs, the overland trajectory of the sea breeze air into the 

KFUL/Anaheim area increases to 20 miles. Thus, afternoon high temperatures are expected to be higher 

at KFUL/Anaheim with west winds than south winds. Here, prediction of whether or not there is an 

eddy makes a big difference in the afternoon temperature and wind direction forecasts. 

 

As for inland extent, the clouds typically reach the coastal valleys, Inland Empire (KONT/KRIV area), 

and coastal slopes with cloud tops near 1300 feet, 1800 feet, and 3000 feet respectively during the 

transition season, but might need to deepen to around 2500 ft deep to reach Ontario (KONT) during 

mid-summer. (A deeper layer is more necessary if there is no eddy present to help “prop up" the marine 

layer and push the low clouds inland). With weak easterly flow aloft and/or weak offshore surface 

pressure gradient trends, the low clouds may be delayed till morning. For weak inversions with deep 

marine layers, (and only patchy or areas of low clouds overnight) there can be a dramatic increase in 

cloud coverage into a ceiling to fill in the gaps in coverage during the morning, (mainly through 

boundary layer convection). On these occasions it is less cloudy overnight than during the day. 

Mid level moisture moving in aloft from the south typically “disrupts” the marine layer, (making it very 

patchy), similar to what happens when the inversion weakens. These are hot humid summer days with 
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17

monsoonal moisture moving in aloft, raising 700 mb dewpoints to near 0 C or higher. Also, near the 

coast maximum and minimum temperatures as well as low cloud cover basically follow the sea surface 

temperatures during the year. Sea surface temperatures may peak in September, explaining why some 

coastal sites have their annual high temperature peak in September. Cooling surges of gulf of California 

moisture into the deserts also weaken the “coast to desert” pressure gradient and disrupt the marine 

layer. Surface dewpoints rise as well, and could reach 70 F, even at the coast, during such hot humid 

days. This can easily result in a “heat index” rise to near warning levels, especially for temperatures 

around 105 F. 

The low cloud deck also seems to react to dynamics such as those associated with waves in a subtropical 

jet from the southwest or monsoonal flow. Outflow boundaries from Mesoscale Convective System 

(MCS) development over Mexico are prone to generating waves of enhancement and dissipation in the 

low clouds, and are visible as waves traveling away from the coast through the cloud field. At times 

tropical cloud features locally clear the low clouds below as they pass by. 

2.11 DENSE FOG EPISODES SURROUNDING SANTA ANA WIND EVENTS 

 

Because of the way it hugs the bottom of the almost daily inversion, some dense fog is evident almost 

every day during the spring through fall period. For the typical 1000-3000 foot marine layer, at least 

patchy dense fog is found at an elevation halfway between sea level and the top of the marine layer [the 

“half-depth point”, or approximately 800-1000 feet below the top of the marine layer (whichever is 

lowest)]. The days surrounding Santa Ana Winds are notorious for setting the stage for dense fog. A 

typical cycle begins when the atmosphere has exhibited offshore flow and hot days, but the surface 

pressure gradients have begun to trend onshore along with the actual gradients (transition day). The 500 

mb upper level ridge axis moves inland as well, which reduces the subsidence and allows deepening of 

the marine layer. The upper level flow changes from anticyclonic to cyclonic, and the coastal marine 

layer low clouds return and deepen. The end of a Santa Ana wind event that slowly weakens results in 

dense fog in the highly populated coastal areas for 2 days or so as the marine layer depth re-adjusts to 

the normal height of around 1300-2400 feet deep. If the return to onshore flow progresses slowly, then a 

layer of dense fog follows the inversion from over the Pacific, into the coastal areas, into the valley 

areas, and possibly into the mountains over a period of several days. If the marine layer becomes very 

deep it extends through the passes and locally into the deserts. When the upper level high begins to 

build in again, and the flow becomes anticyclonic, subsidence lowers the inversion so even lower terrain 

is again within 800-1000 feet of the inversion base, and dense fog retreats back to the lower elevations 

such as the western portion of the valleys or into beaches and mesas of the coastal areas. Dense fog 

tends to lift somewhere between 8 and 9 am local time during the warm season (1500–1600 UTC is a 

good estimate). It is possible to have strong high pressure generating high winds below the passes on the 

inland side of the valley areas, while at the same time 20 miles away the western portions of the valleys 

remain covered with dense fog, resulting in both dense fog and high winds coexisting in opposite ends 

of the same valley forecast zone! (More commonly seen is clear skies, calm wind and cold pooling in 

one part of the valley, while strong winds and temperatures 15-25 degrees warmer occur in another part 

of the valley). In these cases, when strong winds and low clouds already exist, a Catalina Eddy is 

probably in progress. The Catalina Eddy and associated clouds can hang on tenaciously during the 

beginning of an offshore flow event until the offshore flow finally scours it out. After the offshore flow 

event has been around for a while, eddies sometimes develop midday near the end of the offshore flow 

event, and possibly drop temperatures 10 degrees near the coast in one hour or less, heralding the return 

of onshore flow. In some cases there is an upper level trough of low pressure moving into the west coast 

with a cold front sweeping through the plateau, rapidly lowering the surface pressures inland. If the 

onshore trend is strong (approximately a 24 hour increase of 2 mb to the local deserts and 4 mb to the 

plateau or more, respectively), the dense fog phase is by-passed completely in the coastal areas (possibly 
, 

18

associated with an eddy or some sort of southerly flow to help lift the inversion) along with good coastal 

visibilities. However, near the inversion in the foothills and mountains, it is likely to be a dense, 

precipitating fog. A rapidly deepening marine layer during the warm season will typically deepen 

around 1000 feet per day or more during these patterns. These conditions should be watched as dense 

fog and drizzle rapidly moving into previously clear areas results in an instantaneous drop in visibility 

and slick wet roads. 

Multi-car chain-reaction accidents occur every few years or so in the Cajon Pass along Interstate 15 east 

of San Bernardino. The usual pattern is an approaching trough with a marine layer rapidly deepening 

against a strong inversion. These types of scenarios produce drizzle or light rain that wets the roadway, 

along with near-zero visibilities in fog. The dense fog generally occurs at or just under the inversion. In 

these cases, it will at times go from clear with unlimited visibilities to zero visibility almost instantly. 

Even thin cloud layers result in hazardous dense fog episodes. 

The 850 mb temperature is useful during the warm season for estimating the depth of the coastal marine 

inversion and approximating the day to day changes in the marine layer depth. Also it helps to 

determine where the fog will be, since if there is an inversion, there will be at least patchy dense fog 

somewhere in the forecast area under the inversion. 

2.12 FORECASTER HINTS – MARINE LAYER: 

a. There is a significant difference in the cloud patterns with the exact same inversion height and 

strength between the transition season and the mid summer season. This is largely because of lower sea 

surface temperatures, increased mid level moisture, and weaker onshore surface pressure gradients 

during the mid summer period. See the “Marine Layer Matrices” in Appendix A for the “expected” 

marine layer characteristics for the transition seasons and for the summer season. In order to help 

illustrate some of the characteristics of the marine layer, Appendix B contains several examples of 

marine layer variability. One case highlights how a stratiform pattern (with afternoon clearing inland 

areas only) transitions to a more convective pattern with multiple cloud layers, then finally to a pattern 

with clearing favored coastal waters and immediate coastal areas rather than inland. Another case shows 

a marine layer cycle (from a shallow marine layer to a deep marine layer, then back to a relatively 

shallow marine layer again). To round out the discussion a shallow convective event, and two dense fog 

cases are also included. 

b. Marine layers with weak inversions (less than 7 C, and generally around 2500 feet deep MSL or 

deeper) allow convection to deepen up into the inversion, sometime reaching the inversion top. Cloud 

tops are closer to the inversion base height for lower, stronger inversions. 

c. The deeper the marine layer, the weaker the inversion is in many cases, and the more convectively 

unstable it is, even “convecting” into 2 or more decks without sunlight due to boundary layer instability, 

especially for layers about 2500 feet deep or deeper. 

d. The change in marine layer height with 850 mb temperature generally becomes more dramatic with 

lower 850 mb temperatures. Marine layer depth is inversely proportional to 850 mb temperature on 

most warm season days. Although on most days a 1 degree C increase (decrease) corresponds to a 200 

foot decrease (increase) in height, eddy circulations might deepen the marine layer while 850 mb 

temperatures remain steady or increase. 
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19

e. During the transition seasons (late spring and early fall), inversion strengths of 10 C or more, for low 

inversions [1500 feet deep or less, and a cloud thickness approaching 1000 feet thick or more)] will 

allow all beaches to clear for an hour or so around mid morning, then clouds move in at the “cloudy 

beaches of the day” [either the Orange County beaches (near KSLI- KSNA) in the north, or the San 

Diego County beaches (KNFG-KNZY) in the south. This is also dependent on eddy location or whether 

or not there is an eddy at all (simple west-northwest flow)]. The thickness of the morning cloudiness (or 

cloud brightness near one beach compared to that of another on the visible satellite imagery or pilot 

reports) can help determine the “cloudy beaches of the day” where cloudiness will not completely clear 

all day. (With little 24 hour change in marine layer depth, surface pressure gradient, surface pressure 

gradient trend, and inversion strength, persistence is often the best forecast). In many cases, the beaches 

in the south are the “cloudy beaches of the day” and the beaches in the north are the “sunny beaches of 

the day”. 

f. For the Inland Valleys (for example the Inland Empire area around KONT) low clouds reach Ontario 

(KONT) for KNKX sounding marine layer depths of 1800 feet or so during the transitions seasons, and 

as much as 2500 feet deep during the summer. This is possibly because of higher sea surface 

temperatures, better afternoon beach clearing, and later development time at the coast during the 

summer. Typically there is a rather large south-to-north sloping of the marine layer depth, with marine 

layers deeper in the south near KNKX than in the north in many cases. Other areas might also need 

deeper marine layers than the 1300, 1800, and around 3000 foot transition season depths to reach the 

coastal valleys, inland valleys, and lower coastal slopes respectively. 

g. The number of hours after sunrise that a low cloud layer typically dissipates is approximately the 

cloud thickness at sunrise divided by 200 feet, since on average the “burn off “ rate is around 200 feet of 

cloud thickness per hour, (assuming no clouds exist above the stratus). Although the “average” for the 

morning is 200 feet per hour, the instantaneous rate climbs to about 350 feet per hour after about 9 AM 

local time during the mid summer. Rates are slightly slower outside of the summer season. (See the 

“Marine Layer Matrix” in Appendix A for more information on clearing times since certain conditions 

can dramatically affect the results of such simple formulas). 

h. The following conditions result in broken skies (mostly cloudy) till at least mid afternoon and 

possibly all day for the following cloud top heights and locations. (Conditions are mainly for days 

beginning with broken cloud decks and a cloud thickness of 800-1000 feet thick or thicker), and 

especially during the transition seasons; 

 

 (1) Coastal Slopes of the Mountains (locations 3000 feet MSL and higher) 

 (a) Stratus deep into the passes in the morning 

 (b) 3500 feet deep or deeper and inversion strength < 7 C (convects into weak inversion) 

 (c) 3500 feet deep with +4.0 mb onshore trend to Plateau 

 (d) Few (if any) clear spots over the coastal waters at sunrise 

 (2) Coastal Valleys 

 (a) All of the above 

 (b) 3500 feet deep with any inversion strength 

 (c) 2500 feet deep with +4.0 mb onshore trend to Plateau 

 (3) Major Coastal Airports 

 (a) All of the above 

 

 (4) Beaches 
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20

 (a) All of the above (with at least broken skies out to the islands, otherwise it may reverse 

 clear). 

 (b) Inversion strength around 10 C or more with any depth during the transition season. 

 

 

i. During the warmest 3 month period in the coastal areas (July through September), the clouds typically 

clear, but linger at the beaches if 

1) There is an upper low over the area 

2) An unusually strong onshore pressure gradient trend develops (2 mb onshore trends to the 

local deserts (for example KLAX-KTRM), and 4 mb onshore trends to the plateau (for example 

KLAX-KTPH). 

3) There is around 6-8 mb of onshore pressure gradient to the local deserts during the morning 

and a Catalina eddy is present 

j. Upper level lows can deepen the marine layer up to more of an “enhanced marine layer” of over 6000 

feet deep, (or simply a surface based moist layer), without strong onshore gradients, since a weak front 

can push into the area, or the airmass is lifted. Approaching upper level lows may deepen the marine 

layer even with residual offshore pressure gradient trends still in force. 

k. Stratus burns from the base upward during the day, and tends to advect in during the early evening at 

around 1/2 to 2/3 the height of the afternoon ceiling. This is partly because stratus that advects in from 

the ocean comes from an area of rather weak daytime heating, so the stratus bases have not been burned 

upward as much as the cloud bases over land. 

l. Non-frontal stratus with a low inversion height moving in from the north along the coast, (if not 

associated with a cold front and in the absence of an eddy) tends to stay rather low when it moves 

inland, and result in dense fog in the coastal areas. Typically southerly flow is a “deepening flow” with 

the marine layer becoming deeper and the cloud bases typically become higher with time. 

m. A weakening or slow moving cold front will occasionally transition into an eddy over the coastal 

waters in the Southern California Bight [the coastal areas and coastal waters between Point Conception 

(KSBA area) and the California/Mexico Border]. It is not unusual to have a low aloft that is reflected in 

the low cloud pattern and basically “acts” like an eddy, with low clouds held in at the coast all day in 

some areas (with a preference for southern areas). 

n. An early stratus arrival is a good bet when the Carlsbad (KCRQ) visibility is restricted to 5 miles or 

less in afternoon haze (means the marine layer low clouds are at the shoreline or only a few miles 

offshore, and the inversion is strong). If a very strong onshore trend develops (such as that associated 

with a trough or low aloft), the low clouds easily move into the coastal airports mid afternoon rather 

than waiting until closer to sundown to move in. 

o. For standard inversion base height (1000 to 3000 feet deep) the following occur around 1⁄2 the height 

of the marine layer inversion base (the “1⁄2 depth” point). This is assuming a strong inversion, so cloud 

tops are close to the inversion base. 

1) Bases are 1⁄2 as high as the inversion 

2) The low clouds will move inland to terrain that is close to 1⁄2 the depth of the inversion, 

for example, 

i. A 1300 foot deep marine layer will generally allow stratus inland to near the 650 

foot contour [lower portion (or western portion) of the coastal valleys] 
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21

ii. An 1800 foot deep marine layer will move inland to approximately the 900 foot 

contour (lower portions of the inland valleys). 

3) There is almost always at least some patchy dense fog at the 1⁄2 depth point. 

4) Although the clouds are typically around 800-1000 feet thick for the more typical 

summer depth range, dense fog will extend lower if the clouds become very thick. 

5) For surface based inversions, the night and morning low clouds typically extend no 

further inland than the mesas (near the 500 foot terrain contour), with tops to around 800 

feet common, resulting in dense fog at several major airports. 

6) With a shallow marine layer and no inversion the coastal regions below 500 feet still have 

 a strong “marine air influence” unless there is strong offshore flow (about 10 knots or 

 more) at the beaches. In these cases the top of the marine layer is assumed to be around 

 500 feet MSL. 

7) With 10 degree C temperature/dewpoint spreads in the lowest part of the evening 

 sounding (for example, surface to 850 mb) the overnight period will most likely remain 

 clear, and a rapid temperature rise is possible the following day. This is especially true if 

 the pressure gradient trend is offshore or near neutral. 

p. At 3000 feet deep and deeper, instead of the 1⁄2 depth point, the elevation level of the inland extent of 

the low clouds along the coastal mountain slopes typically exceeds the coastal inversion base height 

since the inversion near the mountains seems to slope upward into the mountains. 

q. “Inversion base passages” (or “transition zone passages”) associated with the return or retreat of the 

marine layer is similar to cold frontal passages. The warm, dry air above the inversion is replaced by the 

cool, moist marine air (and possibly drizzle or a wetting mist) beneath the inversion. It is sometimes 

accompanied by 10-25 degree 24 hour temperature changes. Rather dramatic changes occur in 

mountainous areas as the inversion rises and falls. With changes from day 1 with a clear afternoon to all 

day cloud cover on day 2, even larger 24 hour temperature changes are possible. Little if any drizzle is 

expected during the marine layer inversion lowering phase, (since the “drizzle” phase is generally 

associated with the deepening phase of the marine layer cycle, and usually there is a rather abrupt end to 

the precipitation when the deepening phase ends). 

r. Most chain reaction accidents occur in the mountains (in particular, the Cajon Pass) with drizzle 

and/or light rain accompanying dense fog. The mountain portion of the pass is between 3000 and 4500 

feet MSL, so inversions approaching around 3000 feet deep or deeper likely produce fog, possibly dense 

in the passes. 

s. A common chain reaction accident pattern involves cyclonic flow, resulting in a marine layer rapidly 

deepening against a strong inversion that is producing drizzle or light rain (possibly enhanced when 

coupled with upslope flow in the mountains). Without ridging to break the pattern, cyclonic flow 

continues to push moisture into the area, resulting in an extended period (possibly many days) of dense 

fog and drizzle for poor visibilities and slick roads. 

 

t. Another common chain reaction accident pattern is a warm rain event (for example warm frontal 

patterns) which also produces very light rain and again, persistent onshore flow. These patterns 

typically persist and warming is sometimes delayed until anti-cyclonic flow (ridging) re-develops. 

u. Checking the marine layer depth on the Vandenberg sounding (KVBG) upstream as well as the 

observations around it (including the offshore islands) for rainfall gives a good estimate of the 

characteristics of an approaching front, such as moist layer depth and/or rainfall rates and amounts. 
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22

v. In many cases the most widespread precipitation during a deep marine layer or shallow front is during 

the rapid deepening stage. 

w. Much of the populated region near the coast experiences dense fog with marine layer depths 

(inversion bases) of around 1000 feet or lower (a common feature of post-offshore flow periods). 

x. Dense fog (less of a problem mid-summer since it is frequently quite patchy then) is typically 

associated with cloud bases down to 800 - 1000 feet below the inversion base, and is quite hazardous 

with marine layers deepening 1000 feet per day to above 2500 feet or so for drizzle and wet roads. 

Conditions are even more critical at depths above 3000 feet deep in the coastal slopes and mountain 

passes. 

y. Fog, locally dense, has a tendency to form in river valleys (or inland valleys) overnight, and is locally 

advected down the river valleys to near the coast. This commonly occurs during mostly clear nights 

after a rainstorm, with patchy marine layers, (for example, 1000-1500 feet deep), or simply with high 

surface relative humidity but slow development of overnight cloudiness. 

z. Valley areas can have high winds below the mountain passes and at the same time have dense fog on 

the coastal side of the valley. Sometimes the dense fog will hang on at the coastline. 

3. HEAT WAVES 

Daily mean temperatures in the southern California coastal areas do indeed deviate from “room 

temperature” on occasion. Hot weather tends to develop when the 500 mb north-south ridge axis moves 

over southern California with the ridge axis overhead. Another potentially more volatile set-up is when 

in addition, the east-west ridge axis pushes north to a position over (or north) of southern California with 

strong offshore winds around the bottom of the high and around the bottom of the east-west ridge axis. 

Figure 10 is a schematic showing the relationship between the subtropical moisture, flow direction, and 

the position in relation to the ridge axis. Shown is the 700 mb relative humidity, wind barbs and 

streamlines. The subtropical moisture tends to remain south of the axis in mainly easterly flow, with the 

deep easterlies occasionally impinging on the ridge axis. At the wind shift line (where the winds shift 

from easterly to southerly, then westerly), some of the moisture advects north of the ridge axis, but the 

airmass tends to dry out as the flow moves north of the axis and joins with the westerlies. The easterly 

flow beneath the 500 mb ridge axis is a key player for heating southern California. The easterly flow 

lowers the inversion, and may eliminate it altogether. The moisture aloft helps disrupt the low clouds, 

causes morning low temperatures to be higher, and raises the temperature-humidity index as well for 

muggy conditions. 

Figure 11 shows heat wave map types 1 through 4. These are schematics of 500 mb patterns commonly 

found with heat waves in extreme southwestern California. All typically occur with easterly low level 

flow at least as far west as the inland valleys, and all the way to the coast during the more extreme, 

widespread events. Type 1 has the 500 mb high center to the north, so weaker synoptic scale subsidence 

occurs. At the same time stronger northeast winds (stronger than those that would occur if the high were 

overhead) can develop around the high as the flow aloft couples with the flow closer to the surface. The 

high to the north allows easterly flow south of the high into the area to lower or dissipate the marine 

layer and associated inversion. Mesoscale mountain wave subsidence is also rather strong with these 

events. Type 2 events have the high center close to if not directly over southern California. With the 

high nearly overhead the synoptic scale subsidence can be very strong. On the other hand the northeast 

flow is weaker since the winds around the high cannot help to supplement the low level flow since 
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23

winds aloft are weak. These are mainly events driven by synoptic scale subsidence. Type 3 events occur 

when the north-south ridge axis is directly overhead. This easily results in strong subsidence. If the 

ridge is displaced slightly to the west, then very strong northeast winds can still develop over the area to 

couple with the subsidence aloft. Type 4 events are when a rather strong east-west ridge axis develops to 

the north. The pattern has little of any west coast “troughing”, so the pattern is dominated by subsidence 

from the surface high pressure areas. 

The occasions when the 500 mb high reaches around 6000 meters can be expected to bring above 

normal temperatures with a few record highs. The mountains, foothills and Inland Empire begin to 

become very vulnerable during weak to offshore pressure gradient patterns (“weak flow patterns”) 

which accompany these scenarios. This generally brings hot northeasterly canyon/pass winds and 

lowers the inversion top to near the higher valleys and mountain/foothill elevations, and on extreme 

occasions, down to sea level. These warm inversion tops tend to raise minimum temperatures in their 

vicinity, possibly into the 70s, with correspondingly higher afternoon temperatures. (The mountains are 

particularly vulnerable). 

Weak flow patterns (table 3) are generally gradients of approximately -3.0 mb from the coast to 

Tonopah (KTPH), near zero (neutral gradient) from the coast to Daggett (KDAG), and +2.0 mb from the 

coast to Thermal (KTRM) or Imperial (KIPL). (These are the gradients to the “plateau”, northern 

deserts, and southern deserts) respectively. These weak flow conditions help to keep the cool marine 

layer to the west, and result in strong warming in the valleys. Also the coast to KLAS gradient of less 

than +3.0 mb is in the “weak” category (another critical and very useful parameter). The pressure 

gradient trend (24 hour change in the pressure gradient) is also very important when coupled with the 

actual gradient. With inversion tops below 960 mb a rapid morning rise in temperatures to above 

seasonal normals is possible as the heat mixes down before the sea breeze becomes established. This is 

especially true if there are no morning low clouds, the inversion is surfaced based, windy offshore flow 

conditions occur, or the inversion becomes very weak or dissipates. Low level easterly flow just above 

the surface is almost always a big help, (as long as it is strong enough to lower the inversion base to the 

surface, begin drying the boundary layer, and weaken the inversion, rather than just strong enough to 

lower and strengthen the inversion). Huge morning temperature jumps occur, with a focus on areas just 

below mountain passes such as Ramona (KRNM), Devore (DEV), and Beaumont (BUO), and even 

further from the passes in the mid-valley regions at locations such as Riverside Arlington (KRAL). 

 

The 850 mb temperature (as well as lower level temperature and wind structure), are valuable for marine 

layer depth and heating determination (table 1 and table 3). The marine layer changes some 200 feet per 

degree C below 4000 feet. With morning offshore flow in the favored areas, 26-30 degrees F can be 

added to the 850 mb temperature (for example, using the KNKX sounding) to approximate the highest 

temperatures (just west of the passes during mid summer, and closer to the coast when some Great Basin 

cold advection occurs below the passes). Temperatures are likely to be rather uniform across the coastal 

plain and valleys at times during these events, as opposed to the usual pattern when coastal temperatures 

are much cooler than valley temperatures. High temperatures in the mountain and desert areas respond 

well to changes in thickness (for example, the temperature changes 1 degree F per every decameter of 

1000-500 mb thickness change). 

Most of the time the offshore flow is seen following the inversion down as the pressure gradients trend 

offshore (an offshore trend from the coast to the local deserts of 2 mb is significant, especially for actual 

surface pressure gradients that are “less onshore” than +3 mb from the coast to the local deserts). This is 

reflected in the marine layer and associated heating. About a 12-15 degree F or more increase in the 

boundary layer temperature (950 mb is a good estimate for boundary layer temperature) allows the flow 

to warm adiabatically at 5.5 degrees F per 1000 feet and “surface the flow”. The “850 mb temperature 
, 

24

plus a constant” (with the constant somewhere between 26 and 30) develops at the surface. A T850 + 26 

to 30 F temperature is prone to extending westward to at least the inland coastal plain (5-25 miles from 

the ocean), and on occasion to the beaches. Low level winds should also be factored in to help make a 

determination (table 3). 

Although the mountains (fig 12) are the coldest zones for lower 850 mb temperatures (lapse rate 

related), mountain highs generally exceed those of the coastal zone for higher 850 mb temperatures. 

The marine influence strongly modulates heating below the inversion (Fig 13 and 14), but only at the 

immediate coast on such very warm days. 

Weak-moderate offshore flow is a very consistent pattern that produces hot days during the summer. 

KLAX-KTPH exceeds 5 mb offshore and surface pressure gradients to the northern deserts and southern 

Nevada become offshore at a mb or two. The gradients to the southern deserts are expected to be near 

neutral. (These “weak offshore flow” gradients are 2 mb more “offshore” than the “weak flow” gradient 

values discussed earlier). With monsoonal thunderstorm episodes that reach the coastal areas, morning 

low temperatures in the upper 60s and lower 70s occasionally occur, possibly with high and mid level 

clouds west of the mountains and a surge of high dewpoints in gulf surge air that moves into the coastal 

areas. On rare occasions, heat indices rise over 105 west of the mountains, and 120 in the deserts during 

these surge patterns. Although southerly flow off the Pacific is typically dry during June and July, 

tropical features moving by to the south (August and September are the key months) makes even 

southerly flow a producer of moisture (including various degrees of cloudiness). The marine layer is 

prone to becoming very sporadic during these patterns. 

The rather strong onshore flow during the spring weakens gradually into September as the strong Pacific 

high and its strong onshore flow continues to be replaced by the monsoon-generated upper level high 

and the associated surface reflection which develops near the 4 corners. This makes it easier for offshore 

flow and possible heat waves to develop late August and into fall. Although heat waves occur during 

any summer month (table 4 – 7), the combination of very strong upper level highs and weak low level 

flow makes late August and September prime months for the hottest heat waves of the year. A graphic 

of heat waves strung end to end along with 950 mb , 850 mb, and inversion top temperatures are 

compared to high temperatures at UCR (in the inland valley area near KONT) and KSAN in Figure 15. 

Minimum temperatures near the inversion base (lower mountain areas) climb, with an associated rise in 

afternoon high temperatures (fig. 16) during these heat waves. Combined with surges of moisture from 

the southeast, or high sea surface temperatures, the resulting high dewpoints cause heat index values to 

skyrocket (however, this is more of a problem in the deserts as dewpoints reach 80 deg F at times). 

Warm season severe weather potential also peaks with all of this heating and moisture in late August 

through early September. Strong offshore flow is sometimes successful in clearing the low clouds 

completely (with no overnight low clouds), and generates a heat wave. One problem that has to be 

watched is if the offshore flow does not become very strong, it has the opposite effect near the coast. As 

the warm, easterly flow moves west over the inversion, this very weak offshore flow causes warming 

aloft, but strengthen the inversion, and makes the low clouds more persistent and the low cloud deck 

more solid. Trying to determine whether the offshore flow will cause warming or cooling near the coast 

is often very tricky. In the spring through early fall period, lowering and strengthening of the inversion 

associated with a quick burst of offshore flow is frequently the case, with cooling at the immediate 

coast. There is a better chance that a stronger offshore flow will clear out the low clouds from the coastal 

areas before May and after September. 

Subsidence around the edge of an approaching tropical storm can bring strong heating all the way to the 

coast. Also the pressure gradient associated with approaching tropical storms might weaken the typical 

onshore surface pressure gradient, or even generate strong heating through offshore flow, and should be 
, 

25

watched. Small features in the subtropical jet have been known to produce sprinkles, followed by strong 

heating and a strong burst of onshore wind. During these unusual events temperatures correspond with a 

spike up to or above the mid 80s at KSAN, however these summer rapid-heating type onshore flow 

events are rather rare. 

Many times after a heat wave the coastal marine layer returns in a rather solid fashion. If the transition 

is slow, the marine layer low cloud deck results in dense fog in the heavily populated coastal areas. If 

the transition is rapid, it lifts up into a stratus deck there. A typical pattern at the end of a heat wave is 

the interior keeps warming up, possibly with a higher afternoon temperature than the previous day, but 

at the same time the cooling marine air results in lower afternoon highs affecting only the areas near the 

coast. An enhanced sea breeze locally develops with heat waves, as the descending easterly flow gets 

entrained into the sea breeze. This increases the onshore flow already induced by the strong ocean to 

desert thermal gradient. During the phase of the marine layer cycle when the surface pressure gradients 

are in the “weak” category (or offshore) strong heating frequently develops (best with a strong offshore 

trend in the pressure gradient). 

There is a transition period spanning the season with the strongest high pressure aloft and the warmest 

sea surface temperatures (late summer/early fall) and the season with the most offshore flow events 

(fall). This helps to explain why the highest temperature ever recorded in San Diego occurred in what is 

technically “fall” (26 September 1963). This makes the late August and though early October period 

prone to heat waves. Some heat wave events are “offshore flow dominated” with strong winds while 

others have weak winds and are “subsidence high dominated”. Many events are a combination of the 

two (combined subsidence high/offshore flow events). The strong “offshore flow dominated heat wave 

events” typically contain wind gusts at or above advisory levels (gusts to around 35 mph) in populated 

areas. Offshore winds are much weaker during the summer than they are during the late fall through 

early spring period, so “subsidence high dominated heat wave events” are by far more common during 

the summer, while “combined subsidence high/offshore flow events” and “offshore flow dominated 

events” are very common during the cool season. Figure 17 is a schematic that combines many aspects 

of the marine layer and the flow into effects on temperature and clouds at both a coastal and inland 

valley site. The schematic is geared toward the transition seasons (spring and fall) and the cloud layer is 

assumed to be approximately 800-1000 feet thick. Other factors like pressure gradients, pressure 

gradient trend, lows aloft, Catalina Eddies, moisture (with or without clouds) aloft, and monsoonal 

moisture in the deserts can all have an effect on the conditions in the schematic and cause conditions to 

deviate significantly from the schematic. Still, the schematic should give a good overview of the 

different scenarios. 

3.1 FORECASTER HINTS – HEAT WAVES; 

3.1.1 High afternoon temperatures reaching west into the mid-coastal plain (around KFUL) 

The parameters below help allow warming adiabatic flow to reach the surface. Adiabatic high 

temperatures are 26 to 30 deg F higher than the 850 mb temperatures. It is probably best to use the 

0000 UTC 850 mb temperature forecast (which are a few degrees F, or a couple of degrees C higher 

than at 1200 UTC). Any 3 of the 11 parameters below can be enough to generate well above normal 

afternoon high temperatures that can reach the mid-coastal plain near the Fullerton/Anaheim area (5-25 

miles inland near KFUL). This is especially true July-September when the beaches generally clear by 

mid afternoon (since persistent beach cloudiness can cool the mid coastal plain more than expected). 

There are 4 distinct map types associated with heat waves in southern California. Brief case studies 

involving examples of each of the 4 map types commonly associated with heat waves are shown in 

Appendix C. 
, 

26

a) Weak (near neutral) to offshore gradients, at or more “offshore” than the following values; 

1) KLAX-KTPH value of -3.0 

2) KLAX-KLAS value of +3.0 (a key parameter) 

3) KLAX-KDAG value of 0.0 (near neutral values are sufficient) 

4) KLAX-KTRM (OR KSAN-KIPL) value of +2.0 

b) Strong highs 

1) East-west upper level ridge axis to the north (preferably with easterly afternoon 700 or 

850 flow at KNKX). 

2) Strong high directly over the region (preferably 590 decameters or more). 

3) Very strong (approaching 6000 meters) 4 - corners high, preferably with easterly 

afternoon 700 or 850 mb flow at KNKX.

c) Large 24 hour changes in the boundary layer temperature (easily approximated using 950 mb 

temperatures); 

1) Large 24 hour increases in the boundary layer temperature (approximately the 950 mb 

temperature) of 12-15 degrees F (or half as much increase in the 850 mb temperature). 

This is a classic example of a “transition day” because there is usually strong warming. 

 

d) Large offshore trends in the pressure gradients 

1) A large (2 mb or more in 24 hours) offshore trend to the local deserts when gradients are 

already in the weak category is significant. This is also an example of a “transition day”. 

e) Rather high morning boundary layer (950 mb) and 850 mb temperature 

1) Morning temperatures at 950 mb and 850 mb in the mid 20s in degrees C (mid 70s in 

degrees F) often result in rather hot days, and upper 20s and 30s in degrees C (80s in 

degrees F) are likely to accompany very hot days. 

f) Morning offshore winds at Devore (DEV), Fremont Canyon (FMC), or Campo (CZZ) 

1) Wind gusts in and below canyons and passes of 15-25 mph adiabatically warms areas in 

and below the passes, possibly reaching the inland coastal sites of KFUL/Anaheim and 

KSNA. If the subsidence and easterly flow is not strong enough, (for example during a 

Catalina Eddy), the easterly wind may be delayed at the coast, or remain above the 

inversion at the coast with little or no coastal warming. Coastal conditions may even be 

cloudier and cooler than expected rather than warmer if an eddy continues or develops. 

g) Low Inversion Tops With High Inversion Top Temperatures 

1) Morning inversion tops near to below 950 mb with inversion top temperatures 

approaching 30 degrees C typically result in rapid morning warming before the sea 

breeze has much affect on slowing down the rapid temperature rise. (Due to the diurnal 

trend, the afternoon inversion top values are typically 2 C higher than morning values). 

h) 1200 UTC coastal area (eg KNKX) surface dewpoint depressions in excess of 10 degrees C 
, 

27

 (which is a common “heat wave” sounding profile, especially with neutral to offshore 

 pressure gradient trends). 

 1) No cool marine layer air to slow the heating 

i). Surface based inversion with a top below 950 mb 

 1) Less marine layer to burn away so heating develops rapidly. On such days, if the 

inversion is surface based, the KFUL/Anaheim high temperature is at least 12 degrees F 

higher than the 950 mb temperature if the pressure gradient trend is no more than slightly 

onshore (somewhat erratic in the fall). 

j) East winds all the way to the coast gusting to 10 mph or more 

 1) Air mass replacement (advection) to warm even the beaches. This could result in reverse 

 heating and/or maximum temperatures based on elevation, especially during the 

 cool season due to cold advection affecting the inland areas. 

k) Generally, the stronger the offshore flow, the later in the afternoon the temperature peaks (delays the 

sea breeze). 

3.1.2 Other areas. 

a). High temperatures just over the 850 mb potential temperature (26 to 30 degrees F above the 850 mb 

temperature) occurs below the favored canyons and passes in the Inland Empire with northerly winds at 

Devore (DEV). Devore is usually the first site that northerly winds surface below mountain passes and 

canyons, since many of the passes and canyons in the forecast area are more vulnerable to easterly flow. 

In the easterly flow cases, strongest Inland Empire winds are often in the Beaumont (KBUO) area. 

b). High temperatures just above the 850 mb potential temperature (26 to 30 degrees F above the 850 mb 

temperature) occur below the favored canyons and passes of the coastal plain with northeasterly winds 

at Fremont Canyon (FMC), especially if gusts at FMC are around 25 mph or more. San Juan Canyon, on 

the coastal side of the Santa Ana Mountains east of KSNA can be as warm or warmer than some inland 

valleys sites (at the mouth of the valleys) such as Chino (KCNO). 

c). High temperatures just over the 850 mb potential temperature (26 to 30 degrees F above the 850 mb 

temperature) occur below the favored canyons and passes in the southern valleys with northeasterly 

winds at Campo (KCZZ), especially if gusts at KCZZ are around 25 mph or more. 

d) A quick approximation for heat index follows; 

1) The heat index is approximately equal to the temperature (in degrees F) when 

 dewpoints reach 59 degrees F. 

2) The heat index is approximately equal to the temperature + dewpoint – 60 

(where the temperature and dewpoint are in degrees F). For example a temperature of 

120 with a dewpoint of 60 minus 60 is approximately a heat index of 120. 

3.1.3 Ingredients for high minimum temperatures 
, 

28

a. On days with low 850 mb temperatures, warmest coastal minimum temperatures are actually higher 

than the warmest desert minimum temperatures, since warm coastal waters warming the boundary layer 

keeps the coast warm (cool season). 

b. As boundary layer and low level temperatures increase, the highest mountain minimums reach 70 

degrees F for 850 mb temperatures in the upper 70s (25 degrees C or more). This reflects the warming, 

and likely the lowering of the inversion and inversion top to (and below) the thermal belt level (the 

1000-4000 foot MSL region). 

c. Minimum temperatures in the higher portions of the valley areas are close to the mountain values, and 

basically match them for the stronger summer offshore flow events (possibly in the 70s or higher). 

d. For the hottest nights mountains westward, warmest minimum temperatures are highest in the 

mountains, and lowest at the coast (inversion height and possibly east wind and mixing related). 

e. For 850 mb temperatures in the upper 20s (degrees C) or higher, highest mountain minimum 

temperatures easily exceed 70 (degrees F) with high temperatures in the Santa Ana Mountains generally 

15-20 degrees above the minimums in the Santa Ana Mountains. 

f. High temperatures in the mountain areas are prone to occur before noon, especially the higher 

mountains around resort level (6000 feet MSL or so). 

3.1.4 Characteristics for ending a warm spell (Large 24 hour changes in high temperatures) 

a. Upper level ridge axis moves inland (reducing coastal subsidence, allowing coastal low clouds to 

develop, the marine layer to deepen, and transition zone to move inland). 

b. Eddy develops (cool marine layer air deepening against the subsidence aloft results in a strong 

inversion, and persistent coastal stratus) This might be signaled by increasing northwest flow over the 

coastal waters, with offshore pressure gradient trends to the north, and onshore pressure gradient trends 

to the east (to the local deserts), which is the classic pressure gradient pattern for coastal eddies. 

 

c. Onshore surface pressure gradient trend around +2 mb to local deserts and +4 mb onshore trend to 

plateau (even if there is a classic heat wave sounding with a 10 degree C dewpoint depression at and 

below 850 mb). This is because such strong onshore trends will usually overpower the initial lack of a 

marine layer). 

d. Strengthening northerly flow has a tendency to cool sea surface temperatures via upwelling, which 

combined with warming flow aloft, tend to strengthen the inversion, and possibly generate more stratus. 

e. In easterly flow aloft, as onshore flow develops at the surface, there is cooling at the base of the rising 

inversion. This strengthens the inversion, and makes the low clouds more persistent and the low cloud 

deck more solid. 

 

f. Surface pressure gradient contours that become parallel to the coastline often accompany an increase 

in stratus. 

 4. SUMMARY AND CONCLUSION 
, 

29

Many events that occur in southern California are “high impact” because of the number of people 

affected. The coastal marine layer wind, temperature, and moisture profile is responsible for many of the 

high impact events, such as travel on the freeways through passes and air travel as well. For much of the 

year the coastal marine layer cloud deck plays a strong role in southern California weather. The cloud 

deck is typically 800-1000 feet thick. The top of the cloud deck is near the base of the temperature 

inversion when the inversion is strong (especially when the height of the inversion base is near 

“average” for the season). The average inversion base (approximately the marine layer depth) falls in 

height from around 2400 feet MSL in early spring to approximately 1300 feet MSL mid summer. The 

marine layer inversion base is typically higher in the south and toward the mountains to the east. When 

the inversion is weak, the cloud tops can actually be significantly higher than the inversion bases as the 

clouds convect into the area between the inversion base and the inversion top. This is especially true 

near the mountains and with “weaker” inversions. During mid summer the coastal airports clear at 

approximately 1700 UTC (around 10 am local standard time), and the low clouds move back in again 

around 0400 - 0800 UTC (around 8 pm to midnight local standard time). It tends to arrive earlier and 

depart later during the transition season. When the marine layer is deep, its cooling effects reach all the 

way into the deserts. A chance of showers develops for depths up to 6000 feet deep, and showers are 

likely if the layer deepens up to around 6250 feet MSL (800 mb) or more and the marine layer remains 

relatively saturated. 

Many factors affect stratus. Ridging to the north, rising heights, and offshore flow aloft bring warming 

via subsidence aloft, which helps lower the inversion. At the same time the associated low level 

northwest flow over the outer waters (30-60 miles offshore) often generates a coastal eddy, which tends 

to deepen the marine layer, counteract coastal warming, and make the clouds more persistent near the 

coast. If upper level heights and thicknesses are increasing, inland areas and areas at higher elevations 

are likely to see higher temperatures than those 24 hours prior at the same time that coastal areas 

experience lower afternoon high temperatures than those 24 hours prior (because of more persistent 

marine layer cloud cover at the coast). As for the beaches, the “preferred beaches for clearing” are the 

only beaches that see any sun at all. The beaches in the northern areas are typically the preferred 

beaches for clearing (or “beaches of the day for clearing”), while the southern beaches are the “preferred 

beaches for cloudiness”. Even weak upper level low pressure systems can move over and deepen the 

marine layer for more low clouds, in spite of the pressure gradient trends. (This assumes, of course, the 

inversion does not weaken so much that the clouds actually decrease). On the other side of the coin, if 

the mean mid level relative humidity in the 700-500 mb layer reaches the mid 40s (percent) as the upper 

level low passes over, thunderstorm development is possible, and the marine layer low clouds become 

very disorganized, patchy, or dissipate. What seems to be a pattern at times is after an episode of coastal 

thunderstorms, the marine layer low cloud pattern becomes more organized and comes roaring onshore 

by the following morning (especially during the late spring through early fall period). 

The height of the marine inversion oscillates quite a bit. Perhaps the strongest feature that causes the 

inversion to oscillate is the fight between the Pacific high and the Monsoonal high. The Pacific high and 

its subsidence will tend to dominate the equation during the spring and early summer, with its large scale 

subsidence over the entire area, and a low level northwesterly jet over the coastal waters. As a result, 

desert heating during the spring and early summer enhances the onshore flow at the coast by deepening 

the thermal low in the deserts and increasing the coast-to-desert pressure gradient. This results in 

relatively cool afternoon temperatures in the coastal areas and a more persistent low cloud layer as the 

strong onshore flow pulls the marine air in beneath the inversion, which also strengthens the inversion 

(mainly May through early July). 

By mid July and continuing into September, the Monsoonal high (along with its moist mid level jet, low 

level surges of moisture into the deserts from the southeast, weaker surface pressure gradient, and 

occasional upward motion from disturbances in the flow) tends to weaken the inversion. This helps the 
, 

30

marine layer pattern to become very erratic. This monsoonal high develops over the interior and 

overshadows the Pacific high at times. Clouds aloft reduce the overnight thermal radiation along the 

coast and reduce the heating in the desert. This results in a weaker east-west (onshore) pressure gradient, 

a weaker inversion, and better afternoon clearing of the marine layer. This pattern is easily seen in 700 

mb dewpoints reaching 0 C and 850-800 mb dewpoints of around 10 C or more for the “lopsided 

inversion” type of pattern. The rapid fall in dewpoint (commonly found with subsidence from the Pacific 

high at the inversion level) is nearly absent during a strong monsoonal event. Although this profile 

favors monsoonal convection, it results in a very erratic low cloud pattern. Also it is common for the 

marine layer inversion to weaken during surges of monsoonal moisture aloft, making it difficult for the 

marine layer to form (disrupting the formation of the marine layer cloudiness). Closer to the surface, 

another strong effect on the marine layer is the surge of low level gulf moisture into the deserts. This 

weakens the thermal low in the deserts via a surge of cooler, moist air from decaying MCS outflow 

boundaries that develop over Mexico or Arizona, or simply a surge of cool, moist low level air 

generated by a huge north-south pressure gradient from the deserts to the southern tip of Baja. The 

combination of the two, especially with easterly flow might keep the low clouds out completely or only 

allow them to develop during the late night hours. These surges typically show up sometime during the 

late night through mid morning hours. The leading edge can have a blast of southerly winds 15-20 mph, 

and typically drop off a bit thereafter. The surface pressure in the deserts can rise 5 mb and the relative 

humidity and dewpoint jump up 30 percent and 30 degrees F respectively in a matter of hours. 

Dewpoints might even surpass 80 degrees F. These surges bring higher temperatures than expected to 

the coastal and valley areas due to a decrease in onshore gradients, and lower temperatures than 

expected to the deserts. The pressure gradient change will sometimes occur during the day, making the 

morning airmass different than that of the afternoon period. Temperature forecasting becomes very 

tricky when the onshore pressure gradient across the extreme southern part of the state from the coast to 

the local deserts falls to only 4 mb during such an episode. 

Outflow boundaries and easterly waves (as well as waves in the subtropical jet from the southwest) are 

frequently seen rippling along the top of the marine layer. Most of the time the waves at the top of the 

marine layer propagate to the west, and systematically dissipate parts of the low cloud deck (especially 

in areas where the clouds are thin) and may disrupt the night and morning low cloud pattern. In addition, 

if high pressure inland becomes strong enough to lower the marine layer down to 500 feet MSL or so, 

the marine layer will most likely clear each afternoon all land areas, and have trouble moving back in 

overnight (if it comes back in at all). The change in marine layer height generally increases with lower 

850 mb temperatures. Although most changes are 200 feet per degree C, eddy circulations, on occasion, 

actually deepen the marine layer while 850 mb temperatures remain steady or increase. 

 

Flow direction also has serious effects on the marine layer. An eddy-like flow (southerly flow) along 

the coast has a tendency to keep the low clouds in all day, capped by a rather strong inversion. Eddies 

typically spin up during the mid morning with very little notice, become strong (10 knots sustained) 

within a few hours, and help keep the low clouds from clearing near the coast, especially from the 

KCRQ area south. Predictions of whether or not there is an eddy results in substantial differences in the 

afternoon temperature and wind direction forecasts (for example, southerly eddy winds verses the 

typical westerly winds). 

Re-development of the low clouds can be a challenge as well. Moisture aloft, offshore surface pressure 

gradients, a weak inversion, offshore surface pressure gradient trends, and moderate to strong offshore 

flow delay the low cloud development till the late night hours. Sometimes the inland edge of the marine 

air stays offshore and little if any low clouds develop. On the other hand, a low, strong inversion, a 

strong surface onshore pressure gradient, a strong onshore surface pressure gradient trend, or an 

approaching upper level trough or low often results in early re-development or re-introduction of the low 
, 

31

clouds into the coastal areas, especially if there is still a rather solid bank of low clouds just offshore 

during the afternoon. A tricky scenario occurs during occasions when the patchy low clouds come in 

during the afternoon and evening, only to drain out during the late night after the sea breeze weakens 

(especially true during the transition seasons when longer, cooler nights produce better drainage winds). 

This affects air traffic in and out of the area during the evening aviation rush hour. Isolated bands of 

low clouds frequently form via confluence due to flow around the islands or by opposing flows over the 

coastal waters (coastal convergence zone). 

At times the marine layer cloud deck is thin and exists only in a thin, elevated layer at to just below the 

inversion. This is a characteristic of a “low level inverted V” sounding, and skies may clear rapidly 

during the morning with such thin layers, even though the marine layer might be rather deep (for 

example, 4000 feet deep). This type of thin layer typically happens near the end of a deepening event, 

when the marine layer becomes quite deep, and the inversion weakens. If there are numerous holes in 

the deck during the morning, then a reverse clearing event is a good bet. (Reverse clearing is when the 

low clouds clear at the coast, but convection in the inland areas delays the inland area clearing until 

later). These events often have very patchy cloudiness before sunrise, followed by an explosion of nearsunrise through mid-morning development.

Because of the way it hugs the bottom of the almost daily inversion, dense fog is evident almost every 

day during the spring-fall period, with coastal dense fog and very shallow inversions rather prevalent in 

the fall. For the typical 1000-3000 foot deep marine layer, at least patchy dense fog is often found at an 

elevation halfway between sea level and the top of the marine layer, or 800-1000 feet below the top of 

the marine layer (whichever is lowest). Toward the end of a slowly weakening Santa Ana wind event, 

dense fog typically occurs in the highly populated coastal areas for about 2 days as the marine layer 

depth re-adjusts to the normal height range of somewhere between 1000 and 3000 feet deep. If the 

marine layer continues to deepen, the low clouds and dense fog eventually reach the mountains. On the 

other hand, the marine layer can become very shallow and be pushed offshore when the surface pressure 

gradients become rather weak (near neutral), then offshore (negative), with strong heating aloft and 

inland. These are the types of scenarios that evolve into heat waves. When the marine layer returns, it is 

typically rather shallow and results in dense fog in the coastal areas, advects inland as the coastal marine 

layer deepens, and finally reaches the mountains at 3000 feet deep or so. The fog is likely to have a big 

impact on air, marine, and highway traffic. Add a little drizzle to make the roads slick, and the effect is 

even stronger. Power consumption is strongly driven by these events. Knowing what causes the 

variations in these phenomena helps to give a “heads up” on what to expect when such things occur. 

When the surface pressure gradients are in the “weak” category (or offshore) strong heating usually 

develops (especially with a strong offshore trend in the pressure gradient). There is a transition period 

between the period with the strongest high pressure aloft and warmest sea surface temperatures (late 

summer/early fall) and the period with the most strong offshore flow events (fall). This helps to explain 

why the highest temperature ever recorded in San Diego occurred in what is technically “fall” (26 

September 1963). This makes the late August and though early October period prone to heat waves. 

Some heat wave events are “offshore flow dominated” with strong winds while others have weak winds 

and are “subsidence high dominated”. Many events are a combination of the two (combined subsidence 

high/offshore flow events). The strong offshore flow dominated heat wave events typically contain 

wind gusts at or above advisory levels (gusts to around 35 mph) in populated areas. Offshore winds are 

much weaker during the summer than they are during the late fall through early spring period, so 

“subsidence high dominated heat wave events” are by far more common during the summer, while 

“combined subsidence high/offshore flow events” and “offshore flow dominated events” are very 

common during the cool season. Near the end of summer the first few fronts result in surface high 

pressure building over the plateau and locally into the deserts, which serve to weaken the onshore 

pressure gradient (and sea breeze), and even result in heat waves at the coast that are commonly seen in 
, 

32

September and October. Other factors like pressure gradients, pressure gradient trend, lows aloft, 

Catalina Eddies, moisture (with or without clouds) aloft, and monsoonal moisture in the deserts can all 

have an effect on the conditions in the schematic (Fig. 17) and cause conditions to deviate significantly 

from the schematic. Still, this schematic should give a good overview on the interactions. 

It can easily be seen that there are many different features that affect boundary layer conditions in 

southern California, with some of the features having opposite effects from one pattern to another. For 

example, during the summer, moderate subsidence can actually cool the coastal areas by strengthening 

the inversion and making the low clouds more persistent (Fig. 17). During the winter the same amount 

of subsidence serves to clear the skies and warm the coastal areas. Other factors like pressure gradients, 

pressure gradient trend, lows aloft, Catalina Eddies, moisture aloft, and monsoonal moisture in the 

deserts can all have an effect on the conditions in the schematic. One final point to be made is 

sometimes the marine layer almost seems to be de-coupled from the low level flow. Stratus has been 

known to tenaciously hold on at the coast, even with 5 mb of offshore surface pressure gradient from the 

coast to KDAG, and 2 mb of offshore pressure gradient from the coast to KTRM, during a period of 

offshore pressure gradient trends! This shows that even though offshore gradients of these magnitudes 

frequently result in the clearing of the low clouds, it is not guaranteed to do so. Although more 

interrogation into these issues in needed, it is hoped that at least some light has been shed on the more 

important details associated with low clouds, dense fog, and heat waves in southern California. 

5. BIBLIOGRAPHY 

Atkin, Daniel and Joseph Dandrea, 1998: Composite Maps of Selected Warm Events in San Diego. 

Western Region Technical Attachment No. 98-09. (Available from the National Weather Service, 125 

S. State St., Salt Lake City, UT 84103). 

http://newweb.wrh.noaa.gov/wrh/98TAs/9809/index.html

Bruno, D., and G. Ryan, 2000: Climate of Los Angeles, California. NOAA Technical Memorandum, 

NWS WR-261. 

http://www.ntis.gov/

Fox, A. D., 1978: A study of a Southern California coastal storm using satellite imagery. Preprints, 

Conference on Weather Forecasting and Analysis and Aviation Meteorology, Amer. Meteor. Soc., 

Boston, 125-131. 

Isla, E. M., S. Vanderburg, C. Medjber, and D. Paschall, 2004: Climate of San Diego, California (2nd

Edition). NOAA Technical Memorandum, NWS WR-270. 

http://newweb.wrh.noaa.gov/wrh/techMemos/270.pdf

Martin, Greg. 2000: Examples of the Advantages of ACARS Data. Western Region Technical 

Attachment No. 00-07. (Available from the National Weather Service, 125 S. State St., Salt Lake City, 

UT 84103). 

http://newweb.wrh.noaa.gov/wrh/00TAs/0007/index.html

Miller, Miguel, 2005: The Weather Guide, (3rd Edition). National Weather Service, San Diego CA. 

http://newweb.wrh.noaa.gov/sgx/research/Guide/The_Weather_Guide.pdf

NCEP Reanalysis Data. NOAA-CIRES ESRL/PSD Climate Diagnostics Branch, Boulder, Colorado, 

USA 

http://www.cdc.noaa.gov/
, 

33

Rosenthal, J., 1972: Point Mugu Forecasters Handbook. Pacific Missile Range Tech. Publ. 

PMR-TP-72-1. 278 pp. [Available from Pacific Missile Range, Pt. Mugu, CA 93042-5000.] 

Small, Ivory, 1995: Santa Ana Winds and the Fire Outbreak of Fall 1993. NOAA Technical 

Memorandum NWS WR-230. 

http://www.ntis.gov/

Small, Ivory J., 1999a: An observational study of Island Effect Bands: Precipitation Producers in 

Southern California. Western Region Technical Attachment No. 99-18, 11 pp. (Available from the 

National Weather Service, 125 S. State St., Salt Lake City, UT 84103). 

http://newweb.wrh.noaa.gov/wrh/99TAs/9918/index.html

Small, Ivory J., 1999b: An observational study of a California Bight Coastal Convergence Zone: 

Precipitation producers in Southern California. Western Region Technical Attachment No. 99-19, 11 

pp. (Available from the National Weather Service, 125 S. State St., Salt Lake City, UT 84103). 

http://newweb.wrh.noaa.gov/wrh/99TAs/9919/index.html

Small, Ivory J. and Daniel V. Atkin, 2003: A WES Simulation of the Extreme Santa Ana Wind Event of 

6-7 January 2003. Western Region Ta-Lite No. 03-08. (Available from the National Weather Service, 

125 S. State St., Salt Lake City, UT 84103). 

http://newweb.wrh.noaa.gov/wrh/talite0308.htm

Small, Ivory J and Daniel V.Atkin, 2004: A Pacific Storm As Seen Via The WES: Focus on a Gap 

Wind Enhanced Convergence Zone and Associated Flooding. Western Region Ta-Lite No. 04-03. 

(Available from the National Weather Service, 125 S. State St., Salt Lake City, UT 84103). 

http://newweb.wrh.noaa.gov/wrh/talite0403.htm

Small, Ivory J, and Joseph Dandrea, 2003: A Low Level Blocking Ridge Along With Downward Motion 

Aloft in the Right Front Quadrant of the Upper Level Jet – A Recipe for Minor Flooding and High 

Winds. Western Region Technical Attachment No. 03-05. (Available from the National Weather 

Service, 125 S. State St., Salt Lake City, UT 84103). 

http://newweb.wrh.noaa.gov/wrhq/03TAs/0305/index.html

Small, Ivory J., and Joseph Dandrea, 2004: Some Interesting Characteristics of Southern California 

Wildfires and the Resulting Waterman Canyon Flash Flood/Mudslide. Western Region Ta-Lite No. 04-

29. (Available from the National Weather Service, 125 S. State St., Salt Lake City, UT 84103). 

http://newweb.wrh.noaa.gov/wrh/talite0429.htm

Small, Ivory J. and Philip Gonsalves, 2005: A Dramatic Shift From a Warm Monsoonal Pattern to a 

Cold Northwest Express Pattern. Western Region Ta-Lite No. 05-05. (Available from the National 

Weather Service, 125 S. State St., Salt Lake City, UT 84103). 

http://newweb.wrh.noaa.gov/wrh/talite0505.pdf

Small, Ivory J., Mackechnie, and B. Bower, 2000: Mesoscale Interactions Triggering Severe 

Thunderstorms and Flash Flooding in Southern California - July 1999. Western Region Technical 

Attachment No. 00-01, 18 pp. (Available from the National Weather Service, 125 S. State St., Salt Lake 

City, UT 84103). 

http://newweb.wrh.noaa.gov/wrh/00TAs/0001/index.html
, 

34

Small, I., Martin, G., Ladochy, S., and Brown, J., 2002: Topographical and Synoptic Influences on Cold 

Season Severe Weather Events in California. Preprints of the 16th Conference on Probability and 

Statistics, Orlando, FL, Amer. Meteor Soc., 146-153. 

http://ams.confex.com/ams/pdfpapers/28708.pdf

Small, I., Martin, G., Ladochy, S., and Brown, J., 2002: Topographical and Synoptic Influences on Cold 

Season Severe Weather Events in California. Final Report. 

http://www.comet.ucar.edu/outreach/0124241.htm

Fig. 1. Terrain map of the WFO SGX CWFA. Color coding in the 

legend is in thousands of feet MSL. 
, 

35

Fig. 2. 1200 UTC KNKX mean sounding values for 1998-2000. For the upper figure, the 

thick black sounding with triangles is the mean summer sounding, the dashed blue sounding 

with ovals is the June mean, and the thin red sounding with rectangles is the August mean. 

For the lower figure, the blue sounding with rectangles is the June mean and the red sounding 

with triangles is the August mean. The green triangle (850 mb temperature around 27 C) and 

the black triangle (inversion top temperature around 30 C) are usually associated with at least 

105 F in the warmest inland valley areas (and 110 F or more with 10 - 20 knot gusts below 

canyons and passes during the mid summer period, or afternoon east winds at 850 and 700 mb 

during the mid summer period). 
, 

36

Fig. 3. The chart shows the summer trends for the selected parameters for each half-month 

period based on the 1200 UTC KNKX sounding values for 1998-2000. The 850 mb height 

gradient from KNKX to KDRA is also shown. Since heights are higher at KNKX during the 

entire period, all 850 mb height gradient values are onshore (positive). The 500 mb heights 

(blue, diamonds) and 1000-500 mb thickness (green, squares) have peaks during the second 

half of July. The 650 mb relative humidity (brown, circles, and is basically mid level 

monsoonal jet moisture) continues to increase, and peaks at the end of August, spilling into 

early September. The 850 mb height gradient (red, triangles) decreases into at least 

September. (This makes for the most volatile time period for strong to severe convection 

from mid August to mid September with a peak around September 1st, corresponding 

closely with the moisture maximum consisting of values over 40 %). This moisture 

maximum results in a peak in the probability of thunderstorm outflows from the south and 

southeast that affect the marine layer. Summer peaks in 500 mb heights 1000-500 mb 

thickness, and 650 mb moisture (close to the 700 mb value) are 591 decameters, 579 

decameters, and 47 percent respectively. 
, 

37

MEAN 2000-2003 SURFACE PRESSURE GRADIENT AT 

1200 UTC FROM THE COAST TO THE LOWER 

COLORADO RIVER VALLEY (LSF-RIC)

2

2.7

5.3

3.8

2.4

6.4 6.1 6.1

1.9

4.4 4.1 4.1

0

1

2

3

4

5

6

7

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

MONTH

PRESSURE GRADIENT (MB)

Fig. 4. The above chart shows the character of the annual surface pressure gradient at 1200 UTC for each month 

between Las Flores (LSF) [near Camp Pendleton (KNFG) at the coast] to Rice Valley (RIC) [near Blythe (KBLH) in 

the lower Colorado River Valley area]. During the spring and early summer strong heating inland areas and the 

building Pacific high result in increasing onshore flow and onshore gradients. This strengthens the inversion below 

the subsiding air at the same time that the marine layer deepens. The pressure gradient rises above 6.0 mb between 

April and June, with an annual maximum in June, which helps to explain why the marine layer is most persistent 

during the period April - June. The stronger onshore gradients may also help the strong northwest winds of spring to 

“surface” in the mountains and deserts. There is a rapid decrease in the gradient thereafter ( 33 % from June to 

August), partly due to cooler conditions inland under falling heights/thicknesses with the change in sun angle. There 

are also moisture surges into the deserts from the southeast to help cool the deserts and weaken the pressure gradient 

at times. Another factor is the sea surface temperature increase to an annual maximum in August and September. 

This combination results in the “shallowest” inversion and marine layer of the summer season for better clearing 

along the coast in August and September. Some locations reach their annual temperature maxima in September due 

to the combination of these conditions. There is a continued decrease in the pressure gradient to an annual minimum 

in December as high pressure areas develop to the north and drop into the plateau, which is reflected in the December 

peak in offshore flow. The mean gradient for every month is onshore. 
, 

38

Fig. 5. Basically, the above graph shows how changes in the 850 mb temperature affects the inversion 

base height, inversion strength, and their relative values for June (deep marine layer), August (summer 

marine layer depth minimum) and the summer mean based on the 1200 UTC Miramar (KNKX) 

sounding. Lower 850 mb temperatures correlate well with deeper marine layers and weaker inversions. 

Higher 850 mb temperatures correlate well with shallower marine layers and stronger inversions. (The 

inversion usually weakens as the marine layer deepens, and strengthens as the marine layer becomes 

“more shallow”).
, 

39

Fig. 6. The figure is a schematic of an “Enhanced Marine Layer Event” (or a “Hybrid Weak 

Front/Marine Layer” Event). The airmass under the inversion is assumed to be relatively 

saturated (approaching or exceeding 90 % relative humidity from just above the surface to the 

base of the inversion for thick cloud development) until the event begins to wind down, on 

average, toward the end of day 3. (If the airmass is not relatively saturated, less precipitation 

occurs). The event progresses from the coastal dense fog stage, through the rapid deepening 

stages with drizzle and/or light rain, and ends with the lowering/re-establishment of the marine 

layer low inversion. The upper, light green contour is the height of the inversion base (or roughly 

the cloud top). The lower, dark green line is the base of the clouds. The vertical bars are the 

thickness of the clouds. The thin blue shaded region along the bottom is the drizzle period. The 

thick blue shaded region along the bottom is the light rain period. The upper x- axis is the time in 

hours from the beginning of the event. The lower x- axis is the estimated surface pressure 

gradient from the coast to the local deserts (for example, KLAX-KDAG). The schematic 

assumes a “Nevada Low” type of synoptic pattern (with a strong trough development or passage 

to the north and east, and a surface low in the interior). 
, 

40

Fig. 7. These are examples of soundings that may be associated with a typical marine layer cycle. (a) The low, strong 

inversion marks the beginning of a transition stage from offshore to strengthening onshore flow. (b) The middle of the 

rapid deepening stage (which typically coincides with the onset of widespread drizzle or light rain). (c) The end of the 

rapid deepening stage and development of the low level inverted V, (with precipitation, but a general decrease in 

precipitation ensues due to the following “thinning” of the cloud deck). (d) The transition stage from onshore to 

offshore trend. [Schematic shows the weakening of the upper (old) inversion and re-establishment of the lower (new) 

inversion and re-developing marine layer. A jet aloft may also result in an elevated inversion]. (e) The marine layer has 

become re-established and the old inversion has been wiped out. Sounding d can be a period marked by a disorganized 

marine layer clouds. There can be a combination of mostly residual stratus, stratocumulus, and a few cumulus, 

followed by daytime boundary layer convection (with the low deck of cumulus beginning to “fill in” any clear spots as 

the regime transforms to a more convective marine layer pattern). Some events skip past sounding d and go directly 

from sounding c to sounding e, resulting in a simple lowering of the high inversion with a sharp increase in wind speed 

and/or a wind shift just above the inversion. 
, 

41

R2 = 0.6984

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

0 5 10 15 20 25 30 35

850 MB TEMPERATURE (DEG C)

INVERSION HEIGHT (FT)

Fig. 8. The above graph depicts marine layer depth (curve and bubbles) for onshore 850 mb height gradients (blue bubbles) 

and offshore 850 mb height gradients (clear bubbles) for June-September 1998-2000. 

WARM SEASON MEAN INVERSION BASE HEIGHT BASED ON 850 MB TEMPERATURE

850 MB 

TEMPERATURE 

(DEGREES C) 

30 25 20 15 10 5 2 

APPROXIMATE 

INVERSION 

BASE HEIGHT 

(FEET MSL)

Most 

often 

around 

1000 ft 

Most 

often 

around 

1000 ft 

2,000 3,000 4,000 6,500 10,000 

Starting with 20 C approximately = 2000 ft, the 

inversion base changes at nearly 200 ft per 

degree either side of 2000 ft up to around 4,000 

feet MSL, and down to 1000 ft MSL. 

(The inversion tends to fall no lower than 800-

1000 feet MSL during the midsummer period 

unless it is very warm aloft or there is east wind 

below coastal canyons and passes, which is 

typically a transition season or cool season 

phenomena). 

 

Inversion 

base changes 

500 feet per 

degree 

4,000 - 6,500 

feet MSL 

Inversion base 

changes at 

1,000 feet per 

degree C 

6,500 - 10,000 

feet MSL 

Table. 1. Basically, warm season inversions heights average 1300 feet and 850 mb temperatures around 23 C mid-summer, with a 

200 ft per degree C change to either side. One exception is the inversion height tends to fall to no lower than around 800-1000 feet 

MSL unless it is very warm aloft or east wind develops below the passes and canyons. Gusts approaching 30 mph often wipe out 

the marine layer, as can very strong subsidence aloft. For higher inversions the changes are larger per degree C (500 feet per degree 

C 4000-6500 feet MSL, and 1,000 feet per degree C 6500-10000 feet MSL). Events such as eddy circulations, onshore trends, and 

stronger onshore gradients than what is expected for a particular 850 mb temperature may tend to raise the inversion above these 

values. A “non-eddy” pattern of northwest flow, offshore trends, or weaker gradients than what is typical may tend to result in 

inversion heights lower than the above values for a given 850 mb temperature. Locally significant boundary layer rains are possible 

with 850 mb temperatures in the lower single digits since depths are generally around 7,000-10,000 feet. However such events are 

usually absent in the middle of the warm season, when very deep marine layers are very rare. 
, 

42

Fig. 9. The above figure shows the Low Level Storm Probability of Precipitation (LLSPOP) in the coastal and valley 

areas based only on marine layer depths 4000-7900 feet deep for the cool seasons (defined as October – May) of 

1998-2000. The LLSPOP is the percentage of stations with measurable rainfall for a 48 hour period ending a 4 pm 

local standard time of day 2. Basically, the LLSPOP assumes that much of the rainfall from a storm occurs within a 

48 hour period, so it is basically a 48 hour areal coverage. For example, over the 48 hour period, if 90 of 100 stations 

report measurable precipitation, the LLSPOP is 90 %. The marine layer depth was defined as the top of the surface 

based 60 % relative humidity layer during a storm at the KNKX sounding site. The correlation is better if there is a 

nearly saturated (at least 90 % relative humidity) air mass or an 80 % relative humidity layer somewhere between 

850 and 700 mb. Although there is a rather large amount of variation (since the amount of saturation of the marine 

layer is a big component, not included in the curve), depths around 6000 feet are considered to be in the “chance “ 

category, with a changeover to “likely” at around 6250 feet (about 800 mb or so). The two-day time period for 

LLSPOP is designed to catch most marine layer or weak frontal events, so LLSPOP can be larger than the 6 hour 

POP or 12 hour POP. Basically, just over 3000 ft deep is a slight chance of rain (10 percent), just over 5000 ft is a 

slight chance (20 percent), a chance is around 6000 feet (30-50 percent), just over 6000 ft (near 800 mb) is likely (60 

percent), and just over 7000 feet is categorical (80 percent), The likely and low end categorical regions mark the 

changeover to “high POP, Low QPF events (with a high probability of precipitation, but a low quantitative 

precipitation forecast). 

Low Level Storm Probability of Precipitation 

(LLSPOP) based on Marine Layer Depth

00

0 5 0

0000 7

7 0 12

14 61 100 0 81

48 4 23 28 100

100 94

29 96100 89 97 45

21

22

17

77

73

54 68

57

53

45

24

33

35

33 R2

 = 0.5483

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 10 20 30 40 50 60 70 80 90 100 110

Low Level Storm Probability of Precipitation (LLSPOP) 

Marine Layer Depth (ft)
, 

43

QUICK STORM PROBABILITY OF PRECIPITATION ESTIMATES (SPOP) 

BASED ON 

MARINE LAYER AND “LOWER MID-LEVEL” MOISTURE COMBINED

LOW LEVEL SPOP (LLSPOP)

Marine layer depth just over 3000 ft = Slight Chance (10 percent) 

Marine layer depth just over 5000 ft = Slight Chance (20 percent) 

Marine layer depth just over 6000 ft = Likely (60 percent) 

Marine layer depth just over 7000 ft = Categorical (80 percent) 

LOWER MID-LEVEL SPOP (LMLSPOP) 

Lower Mid-Level SPOP is based on the 850-700 mb relative humidity. Approximate lower bounds for the 850-700 mb 

relative humidity are; 

65 % = slight chance 

70 % = chance 

75 % = likely 

80 % = categorical 

This is based on 52 available storms from 1998-2002 cool season (October – May) data, and should still be applicable for 

the warm season. The average areal coverage of the precipitation (the average SPOPs) for 850-700 mb relative humidity 

values of 80%-84%, 85%-89%, 90%-95%, and 95%-100% were 73%, 86%, 95%, and 97% respectively. Basically, for 

850-700 mb relative humidity of 75% or more, the SPOP at least matches the relative humidity, and an 80 percent spike 

in the relative humidity in the 850-700 mb layer is essentially a “rain layer” since this elevated wet layer will result in 

measurable rain in many cases. 

Values may be appropriate for use during the summer monsoon over the mountains and adjacent deserts, (and possibly 

representative for all areas).

For example, for the Lower Mid-Level SPOP: 

With an 850-700 mb relative humidity of 80 % the best estimate is “a categorical rain probability of 80 % or more”. It 

must be remembered that although there is a high probability of measurable rain, if the dynamics are well to the north, 

actual rainfall totals can be minimal. 

COMBINED (OR FINAL) SPOP

DEFINED AS THE LOWER MID LEVEL SPOP IF THE LOWER MID-LEVEL SPOP IS 80% OR HIGHER, 

OTHERWISE IT IS THE LOW LEVEL SPOP 

For example, 

1. The SPOP for a 7 or 8 thousand foot deep moist layer is likely to be a high SPOP (greater than 80 %) low QPF (less 

than 1/4 inch) event. 

2. The SPOP for a 10000 ft deep moist layer with a 90 % 850 -700 mb relative humidity is categorical, with urban 

flooding possible. 

3. The SPOP for a 20000 ft deep moist layer with a 90 % 850-700 mb relative humidity is also categorical, with 

widespread urban flooding likely, and flash flood possible. This is especially true for strong low level winds (for example 

at 850 mb), reaching 25 knots. 

For elevated precipitation events, for example high based convection or overrunning (anywhere and any season) 50 % 

relative humidity down to 650 mb with a 70 % or higher maximum usually provides at least sprinkles at the surface (with 

a better chance of reaching the ground at the higher elevations such as the mountains). An upper level low may be needed 

to squeeze rainfall out of lower mid level relative humidities of around 75 percent or less during the summer west of the 

mountains.

 
, 

44

 

EXAMPLES OF QUICK SPOP ESTIMATES

 SPOP FORMULA = 

A MARINE LAYER JUST ABOVE 3000 FEET DEEP IS APPROXIMATELY A 10 PERCENT SPOP 

A MARINE LAYER JUST ABOVE 5000 FEET DEEP IS APPROXIMATELY A 20 PERCENT SPOP 

A MARINE LAYER JUST ABOVE 6000 FEET DEEP IS APPROXIMATELY A 60 PERCENT SPOP 

A MARINE LAYER JUST ABOVE 7000 FEET DEEP IS APPROXIMATELY AN 80 PERCENT SPOP 

OR

THE MEAN RELATIVE HUMIDITY OF THE 850-700 MB LAYER IFTHE 850-700 MB LAYER RELATIVE HUMIDITY 

IS 80 % OR MORE 

Marine 

Layer Depth

[Depth of the 60 % layer 

(in ft MSL)] 

Relative humidity of the 

850-700 mb layer Rain Chance Calculations 

Just over 4000 feet deep Below 60 % 

(fairly dry mid levels) 

10 %

(Slight Chance) 

Just over 5000 feet deep Below 60 % 

(fairly dry mid levels) 

20 %

(Slight chance) 

Just over 6000 feet deep Below 60 % 

(fairly dry mid levels) 

60 %

(Likely) 

Just over 6000 feet deep 80 %

(wet mid levels) 

80 % mid level layer 

is the final SPOP since it is above 80 % 

(Categorical SPOPs) 

 Just over 7000 feet deep 

 (High SPOP/Low QPF) 

Below 60 % 

(fairly dry mid levels) 

80 %

(Categorical) 

Just over 8000 feet deep 

(High SPOP/Low QPF) 

Below 60 % 

(fairly dry mid levels) 

Near 100 %

(Categorical SPOPs) 

TABLE 2. Estimated Storm Probability of Precipitation (SPOP) based on the boundary layer 

[marine layer (or moist layer) depth] and the 850-700 mb relative humidity (Lower mid-level 

moisture). There is a big gradient between 20 % (just over 5000 feet) and 60 % (just over 6000 

feet) SPOP, with much variability involved. These values work best when 500 mb southerly 

winds exceed about 60 knots, and such excessive wind speed may result in increased SPOPs. 

Southwesterly flow along with a 564 decameter heights or lower also favors precipitation. 

Depths around 7-8 thousand feet are more “High SPOP, Low QPF” type events (high storm 

probability of precipitation, low quantitative precipitation forecast) since durations are generally 

short which corresponds to a “high POP, low QPF” situation as well. Values are especially true 

during the rapid deepening period and during night and morning hours when measurable 

precipitation seems more probable. If the trajectory is west to northwest and the moisture depth is 

8000 feet or less, the low level moisture is swept out by the Santa Ynez mountains, virtually 

“shutting off the precip” northern areas, but moisture just west of Point Conception is generally 

unaffected, and can still sweep in for heavier precipitation southern areas. 
, 

45

Fig. 10 is a schematic showing the relationship between the subtropical moisture, flow direction, 

and the position in relation to the ridge axis. Shown is the 700 mb relative humidity (in percent) 

shaded above 50 percent, wind barbs (in knots), and streamlines (solid white contours). The 

subtropical moisture tends to remain south of the axis in mainly easterly flow, with the deep 

easterlies occasionally impinging on the ridge axis. At the wind shift line (where the winds shift 

from easterly to southerly, then westerly), some of the moisture advects north of the ridge axis, but 

the airmass tends to dry out as the flow moves north of the axis and joins with the westerlies. The 

easterly flow beneath the ridge axis is a key player for heating southern California. The easterly 

flow lowers the inversion height. This moisture aloft helps disrupt the low clouds, causing morning 

low temperatures to be higher, and raising the temperature humidity index for muggy conditions. 
, 

46

Figure 11. Heat Wave Map Types 1 through 4 shown above are schematics of 500 mb patterns 

commonly found with heat waves in extreme southwestern California. All typically occur with easterly 

low level flow at least as far west as the Inland Valleys, and all the way to the coast for the more 

extreme widespread events. Type 1 (upper left) has the high center to the north, so weaker synoptic 

scale subsidence occurs, but stronger northeast winds can occur around the high as the flow aloft 

couples with the flow closer to the surface. The high to the north allows easterly flow south of the high 

into the area to lower or dissipate the marine layer and associated inversion. Mesoscale mountain wave 

subsidence is also rather strong with these events. Type 2 events (upper right) have the high center 

close to, if not directly, over southern California. With the high nearly overhead the synoptic scale 

subsidence can be very strong. On the other hand the northeast flow is weaker since the winds around 

the high cannot help to supplement any low level flow since winds aloft are weak. These are mainly 

events driven by synoptic scale subsidence. Type 3 events (lower left) occur when the north-south ridge 

axis is directly overhead. This easily results in strong subsidence. If the ridge is displaced slightly to 

the west, then very strong northeast winds can develop over the area to couple with the subsidence 

aloft. Type 4 events (lower right) are when a rather strong east-west ridge axis develops to the north. 

The pattern has little of any west coast “troughing”, so the pattern is dominated by subsidence from the 

surface high pressure areas. 
, 

47

LOCATION “WEAK FLOW” 

[APPROXIMATE WARM SEASON 

PRESSURE GRADIENT UPPER 

BOUNDS FOR STRONG VALLEY 

WARMING] 

 (Strong coastal warming is possible 

with easterly canyon winds)] 

“WEAK-MODERATE OFFSHORE FLOW” 

[TYPICAL WARM SEASON PRESSURE 

GRADIENT VALUES FOR STRONG COASTAL 

WARMING] 

 (Values are about 2 mb more offshore than the 

weak flow values)]

COAST - KLAS +3.0 (Weak Onshore) 0.0 (Neutral)

COAST - TRM +2.0 (Weak Onshore) 0.0 (Neutral) 

COAST - KIPL +2.0 (Weak Onshore) 0.0 (Neutral) 

COAST - KDAG 0.0 (Neutral) -2.0 (Weak Offshore)

KLAS - KCDC -3.5 (Weak Offshore) -5.0 (Moderate Offshore) 

COAST - KTPH -3.5 (Weak Offshore) -5.0 (Moderate Offshore)

COAST - KWMC -7.0 (Weak Offshore) -9.0 (Moderate Offshore) 

EFFECTS Strong valley warming to 850 mb potential 

temperature expected. Strong warming 

possible coastal areas under an upper ridge 

or easterly flow below 700 mb. Generally 

at least 100 warmest inland valley 

locations. Basically near record warmth. 

High likelihood of strong valley warming and inland 

coastal area warming to the 850 mb potential 

temperature. Generally at least low 80s at KSAN and 

at least 90 mid coastal plain sites such as KFUL. 

Especially true under an upper ridge aloft, easterly 

flow below 700 mb, or with an inversion top 

temperature maximum below 960 mb. Basically at or 

above record levels is a good bet. 

Table. 3. Optimal upper bounds of selected surface pressure gradient categories for above normal 

temperatures during the warm season (June – September). “COAST” is either KLAX or KSAN. 

Key gradient values are shown in bold. 
, 

48

Fig. 12. Chart shows the highest expected maximum temperature in each region for the 

coastal (green), mountain (blue), valley (orange), and desert (red) regions respectively based 

on 850 mb temperature for June-September 1998-2000. Note that for the warmest days, the 

warmest values are in the deserts, valleys, mountains, and coasts respectively. For the coolest 

days, the coolest values are not at the coast, but in the mountains, with the valleys slightly 

cooler than coastal areas. During the cooler days of the warm season, mountain highs (blue 

curve) are the coldest areas on the cool end of the chart with lower 850 mb temperatures 

invading the mountains (possibly since this scenario can have a deep marine layer with an 

unstable lapse rate that cools rapidly with height. Also, reverse clearing results in reverse 

heating, so coastal areas can get more sunshine and become a bit warmer than the valleys and 

lower mountain slopes). Higher 850 mb temperatures result in mountain highs that are 

actually warmer than coastal areas (at the warm end of the chart the warm inversion air 

invades the mountains). On most summer days, maximum temperatures are coolest at the 

coast (due to the proximity to the cool ocean), and are higher in the mountains, valleys and 

deserts respectively. This graph is meant to give a simple comparison. During certain 

conditions, such as the occasional day with cold advection into the mountain and valley areas, 

the temperature comparison between the coast, valleys, and mountains can reverse, with the 

highest temperatures at the coast, the lowest temperatures in the mountains, and the valleys 

somewhere in-between due to the adiabatic temperature profile (steady cooling with height) 

during such events. Discrepencies would more likely be seen at the coast on the warm end 

since the marine layer is perhaps the largest variable to affect the temperature during the 

warm season. For example, with 850 and 700 mb winds from the east during the afternoon 

and 850 mb temperatures around 30 C the highest coastal temperature is more likely to 

exceed 100. With the same conditions, except adding a strong onshore pressure gradient 

trend, the regional maximum temperature for the coastal areas is probably around 90. Much 

less effect is seen further away from the marine layer influence in the valleys, mountains, and 

deserts respectively. 

 
, 

49

 

 

Fig. 13. The graphic shows examples of diurnal temperature curves near the coast. The green curve with 

squares shows a rather typical profile. This typical curve begins with a cloudy morning with temperatures 

near the sea surface temperature, then temperatures steadily climb when the clouds clear mid morning or 

so. The sea breeze can be seen to temporarily "knock down" or stall the temperature rise somewhat at 

around noon, but temperatures can climb a bit more til about 2 or 3 pm, when the downward trend 

generally begins. The blue curve with triangles is an example of an exceptionally cloudy day when the 

marine layer clears rather late (around noon or later). Overnight low temperatures are a bit warmer due to 

the earlier cloud cover during the evening prior to these "persistent marine layer" days. When the low 

clouds finally clear at around noon, the temperature jumps up rapidly, but still peaks lower than what is 

expected for a clear day due to the delayed heating. The orange curve with circles is an example of a day 

when offshore flow dominates. The temperature might start out lower, mainly due to the drier air and lack 

of insulating cloud cover, but can rise rather steadily after sunrise. The temperature continues to rise, and 

peaks later in the afternoon, showing that either the sea breeze is delayed, or the sea breeze simply 

brings relatively warm, dry continental air previously deposited offshore (by northeast flow aloft) back to 

the coastal areas well into the afternoon. The red curve with triangles (the curve with the warmest values) 

is an example of a mountain wave that surfaces at night briefly, the cold pooling and drainage wind returns 

after the wave temporarily lifts off the surface, then the wave surfaces again and remains on the surface. 

There is a rapid rise, possibly well before sunrise when the wave finally remains at the surface. There is a 

much higher afternoon temperature, and a later temperature peak than the norm. If the wave cannot 

surface during the overnight hours, the rapid rise can occur mid morning when mixing can "surface" the 

hot air from above a very low inversion, since the hot air in a shallow inversion is very close to the surface. 

During very strong events, temperatures can stay rather warm (upper 60s and even 70s) overnight. 

Sometimes the surfacing of the winds can make temperatures oscillate wildly as the cold pooling at night 

battles the mixing effect of the winds. Oscillations occasionally exceed 20 degrees F in one hour, 

especially in and below canyons (which includes some valley areas such as KRNM and KRAL). 

DIURNAL TEMPERATURE CURVES NEAR THE COAST

77 77 77

79

81

83

86

89

92 93 94 95 94 93

91

89

84

79

75

72

69

72

74

76

78

81

84

86 86

84

82

80

78

76

73

70

67

73

76 75

78 77 77

75 74 73 72 71 70

68 67

68 68 68 69

73

75 74 73 73 72 71 71 70 70 70 69

63 64

75

66

66 66 66

66 66 67

87

68

65

71

66

66 65 65 64 66 65

69 69 69

68 68 68

68 68

60

70

80

90

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

HOUR (LOCAL STANDARD TIME)

TEMPERATURE (DEG F)
, 

50

 

Fig 14. Example of the movement of the transition zone associated with the marine layer air and inversion layer. 

Temperatures are shown in degrees F. The transition zone acts like a “mini frontal passage”, with 20 degree 24 hour 

temperature falls possible after the transition zone sweeps by (and on occasion, in only an hour or two). Drizzle often 

occurs with rapidly deepening marine layers, especially during the transition seasons. Afternoon temperatures on the 

coastal side of the Santa Ana Mountains above the inversion can be higher than the western portion of the inland 

valleys beneath the inversion as the marine layer air flows through canyons and passes into the inland valleys. After a 

“lowering trend”, (resulting from a marine layer inversion that lowered and pushed toward the coast as a transition 

zone), a deepening trend can push the transition zone inland from the coast, then into the valleys. It is basically the 

“rapid change zone” not only for the current temperature gradient across the transition zone, but also for day to day 

changes. Moisture also changes rapidly across the zone. This is important at the end of a hot, dry episode (since 

before the transition zone arrives, very low dewpoints and high temperatures can continue above the inversion as the 

winds increase aloft). 
, 

51

Fig. 15. Summer days with weak gradients only (less than 3 mb onshore from the coast to 

the lower Colorado River Valley) end to end, for the period June-September 2000-2003. 

The pressure gradient is from LSF (near KNFG in the coastal areas) to RIC (near KBLH 

near the Arizona border). The graph shows how the U. C. Riverside high temperature 

(highest, red curve) and the San Diego Lindbergh Field high temperature (KSAN, which is 

the second highest curve, in blue) relate to selected KNKX sounding temperatures. The 

lowest, second lowest, and third lowest curves in the graphic are the 850 mb temperature 

(blue curve, with triangles), the 950 mb temperature (green curve with squares) and the 

inversion top temperature (red curve with squares) respectively. The graphic basically 

strings together all of the weak gradient days for 3 consecutive summers for comparison. 

The 950 mb temperature (the second lowest from the bottom, in green) is a good 

approximation of the boundary layer temperature. The Lindbergh Field high temperature is 

approximately the 850 mb temperature or slightly higher if the 850 mb temperature reaches 

the upper 20s in degrees C (about 80 F), especially with east winds in the passes (for 

example, KCZZ). With east to northeast wind gusts to about 30 mph in the mountains 

during the warm season the adiabatic temperature (about 26-30 degrees F higher than the 

850 mb temperature) can surface all the way to the coastal areas to around 5 miles from the 

coastline. With clear skies and east winds at coastal sites such warming can reach the 

coastline (with a better chance if there is a high overhead and strong subsidence aloft. 
, 

52

Fig. 16. The preliminary data in the above graphic shows how KSOX (Santa Ana Radar, near 3092 feet MSL) 

minimum and maximum temperatures relate during a 12 month period. Maximum and minimum temperatures 

are somewhat correlated. The difference between minimum and maximum temperatures averaged approximately 

15 degrees F during the warm season (afternoon maximum temperatures in the upper 80s are often associated 

with minimum temperatures in the lower 70s in areas such as the foothills). The difference is smaller for lower 

temperatures, and larger for higher temperatures (closer to a 10 degree difference at high temperatures around 50 

deg F, and about a 20 degree difference for high temperatures around 100 F). This relationship is relatively 

common for lower mountain locations. Basically, there is somewhere in the neighborhood of a 25 percent 

difference between maximum and minimum temperatures, and one is easily estimated from the other by adding 

(or subtracting) 25 percent. 

MAXIMUM/MINIMUM TEMPERATURE RELATIONSHIP AT KSOX 

(OCTOBER 1 2002 - SEPTEMBER 30 2003)

R2 = 0.87

0

10

20

30

40

50

60

70

80

90

30 40 50 60 70 80 90 100

MAXIMUM TEMPERATURE (DEG F)

MINIMUM TEMPERATURE

 (DEG F)
, 

53

 

SCHEMATIC OF THE HIGH TEMPERATURE AT A VALLEY LOCATION JUST BELOW A MOUNTAIN PASS 

VERSUS THE HIGH TEMPERATURE AT THE COAST DURING THE TRANSITION SEASON

68

100

75

68

95

95

68

72

90

70

85 90

70

100

75

90

DAY1 DAY2 DAY3 DAY4 DAY5 DAY6 DAY7 DAY8

Valley Site

Coastal Site

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

The marine layer

with a

moderately

strong 8 degree

C inversion in

place at 2000

feet develops

with a standard

clearing pattern

that clears inland

first 

Weak offshore

flow with

mesoscale

subsidence

slowly lowers the

inversion to 1500

feet, but also

strengthens the

inversion at the

coast to 10

degrees C with

low clouds

lingering at the

beaches all day

A strong

"offshore flow

driven" warming

event and

associated

mesoscale

subsidence

develops and

wipes out the

inversion, but

cold advection

counteracts

some of the

adiabatic

warming and

keeps inland

areas cooler

than the coast

The cold air

advection

weakens inland,

but the marine

layer redevelops at the

coast, deepens

under a strong

inversion to 500

feet deep and

begins to effect

the immediate

coast

The

strengthening

onshore flow

rapidly lifts the

strong inversion

at 1000 feet per

day to 1500 feet,

which keeps the

beaches and

coastal airports

cloudy, but

coastal plain

areas further

inland eventually

clear

The strong

inversion rapidly

lifts to 2500 feet,

with coastal

areas and

coastal valleys

remaining

cloudy, but

inland valleys

eventually clear

by afternoon

The marine layer

rapidly deepens

to 3500 feet

deep, which can

result in a

reverse clearing

pattern that

clears at the

coast first if

there are large

breaks in

cloudiness over

the coastal

waters, or

remains cloudy

everywhere if

overcast

offshore

The marine layer

is wiped out as a

strong

"subsidence

driven" warming

event develops,

along with mainly

large scale

subsidence,

which results in

a standard

clearing pattern

that clears inland

first

HIG H TEM PERATURE ( DEG F )

 

 

Fig. 17. The above schematic shows how the temperature for a coastal site (blue curve with squares) and a valley 

site near the foothills (red curve with triangles) respond to changes in the marine layer depth and the character of the 

flow. The schematic is geared toward the transition seasons (spring and fall). The cloud layer is assumed to be 

approximately 800-1000 feet thick. Other factors like pressure gradients, pressure gradient trend, lows aloft, Catalina 

Eddies, moisture (with or without clouds) aloft, and monsoonal moisture in the deserts can all have an effect on the 

conditions in the schematic and cause conditions to deviate significantly from the schematic. 
, 

54

 Table 4. AFTERNOON HIGH TEMPERATURES

Morning 

(1200 UTC)

 850 MB temperature

At or above

27 degrees C

Highest Inland Empire temperature of at least 105 for any

inversion depth. (14 of 15 days) 

AND 

Highest Inland Empire temperature of at least 107 for low 

inversions (near or below the August average of 1300 feet MSL).

(9 of 9 days) 

 Table 5. AFTERNOON HIGH TEMPERATURES

Morning (1200 UTC)

inversion top 

temperature

30 degrees C or higher

Inland coastal plain hot spot of at least 90 

(15 of 17 days)

AND

Inland Empire hot spot of at least 105 degrees F 

(17 of 17 days).

 Table 6. AFTERNOON HIGH TEMPERATURES

Afternoon (0000 UTC) 

wind

east at 850 or 700 mb 

at any speed 

AND

Afternoon (0000 UTC) 

 850 mb/inversion top 

temperatures at least 

27/30 C

Highest Inland Empire temperature 

 at least 110 degrees F (7 of 8 days)

Table 7. OVERNIGHT LOW TEMPERATURES

Morning (1200 UTC) 

850 mb wind

easterly at any speed 

AND

Morning (1200 UTC) 

850 mb dewpoint 

temperature greater 

than 13 degrees C

 

Highest overnight minimum temperatures coastal and valley areas at or 

above 70 degrees F (4 of 4 days). 

WARM SEASON TEMPERATURE CORRELATION TABLES 

(from June-September 1998-2000 data)
, 

55

APPENDIX A. MARINE LAYER MATRICES 
, 

56

 

 

TRANSITION SEASON MARINE LAYER MATRIX (LATE SPRING/EARLY FALL) 

INVERSION 

STRENGTH

INVERSION BASE CHARACTER

around 1300 ft or less Patchy dense fog with beaches affected shallow end, and mesas and western edge of coastal valleys deep end of height range. Clear 

everywhere by late morning 

 1300 - 1999 ft Patchy dense fog affecting mesas and coastal valleys shallow end, and coastal valleys and western inland valleys deep end of height 

range. Clear all areas by noon 

 2000 - 2499 ft Patchy dense fog into all valleys. Clear all areas by noon 

 2500 - 3000 ft Patchy dense fog into all valleys. 2nd convective deck forms, likely to reverse clear, and is slow to clear, with all areas clear mid 

afternoon. Cloud tops near mountains reach height of approximately the top of the KNKX inversion with foothills clearing last. 

3000 - 4000 ft Patchy dense fog to coastal slopes. 2nd convective deck forms, likely to reverse clear, but slow to clear, and takes til late afternoon for 

all areas to clear. Cloud tops near mountains reach height of approximately the top of the KNKX inversion with mountain slopes 

clearing last. 

WEAK 

(< 7.0 C) 

 

4000 ft deep and up Patchy dense fog to coastal slopes. At least a couple of convective decks typically form, likely to reverse clear, but inland valleys and 

lower mountain slopes may not clear. Cloud tops near mountains reach height of approximately the top of the KNKX inversion with 

best cloud coverage on slopes. 

around 1300 ft or less Areas of dense fog with beaches affected shallow end and mesas and western edge of coastal valleys affected deep end of height 

range. Clears everywhere by late morning 

1300 - 1999 ft Areas of dense fog affecting mesas and coastal valleys shallow end, and coastal valleys and western inland valleys deep end of height 

range. Clear by noon (except near 9 C, after temporary morning clearing, areas of clouds reform or advect in at the beaches of the 

preferred “beaches of the day for clouds” around noon). 

2000 - 2499 ft Areas of dense fog affecting coastal and inland valleys. Clears by noon all areas (except near 9 C, after temporary morning clearing, 

areas of clouds reform or advect back in at most beaches around noon). 

2500 - 3000 ft Areas of dense fog, mainly in the inland valleys. A 2nd convective deck forms, but still clears by mid afternoon all areas (except 

cloudy at most beaches near 9 C).

3000 - 4000 ft Areas of dense fog higher inland valleys and lower coastal slopes of the mountains. A 2nd convective deck forms. Clouds become 

scattered inland valleys in the afternoon but stays overcast remaining coastal areas and coastal valley areas. 

MODERATELY 

STRONG 

“Typical 

Inversion” 

(7.0 - 9.9 C)

4000 ft deep and up Areas of dense fog lower slopes. Multiple convective decks are expected. Skies become scattered inland valleys in the afternoon but 

stays overcast remaining coastal areas and coastal valley areas. (Assumes overcast offshore, or it may reverse clear). 

around 1300 ft or less Areas of dense fog with beaches affected shallow end, and mesas and western edge of coastal valleys deep end of height range. May 

clear approximately an hour or so at the beaches late morning, but returns to the beaches in the “county of the day for clouds” just 

before noon (If no eddy, then the “favored beaches of the day for cloudiness” will likely be in San Diego County, and via coastal 

blocking and island effect stratus bands. The Orange County Beaches from KSNA to KSLI are the favored “beaches of the day for 

clearing”). Erratic in Fall. 

1300 - 1999 ft Areas of dense fog affecting mesas and coastal valleys shallow end, and coastal valleys and western inland valleys deep end of height 

range. Clears for approximately an hour or so all beaches late morning, but returns to the beaches in the “county of the day” just 

before noon. (If no eddy, then stratus affects mainly the favored San Diego County beaches, via coastal blocking and island effect 

stratus bands). Erratic in Fall. 

2000 - 2499 ft Areas of dense fog affecting coastal valleys and inland valleys, but only inland valleys clear by early afternoon. Coastal airports 

remain cloudy. 

2500 - 3000 ft Areas of dense fog higher portions of inland valleys, but only inland valleys clear by early afternoon while coastal areas and coastal 

valleys remain cloudy. 2nd convective deck forms. 

3000 - 4000 ft Areas of dense fog inland valleys and lower coastal slopes. Only inland valleys clear by early afternoon while coastal areas and 

coastal valleys remain cloudy. 2nd convective deck forms. 

STRONG 

(10.0 C OR 

MORE)

 

4000 ft deep and up Areas of dense fog coastal slopes, becoming scattered inland valleys in the afternoon, and remains overcast coastal areas and coastal 

valleys. Multiple convective decks are expected. Above 5000 feet expect cloudy conditions to the coastal slopes all day, although 

reverse clearing may occur along the immediate coastal strip if it is clear offshore during the morning. 

Table A1. Approximate marine layer characteristics (for a cloud thickness of around 800-1000 feet thick or thicker). Lower sea surface temperatures (generally lower 60s or 

lower at the local buoys and mid 60s or lower at the beaches) may result in stronger inversions than mid summer for the 1000-2000 ft deep region, and along with cooler conditions 

and overall deeper inversions. This may serve to make low clouds more persistent than mid summer. For the more typical marine layer depths (up to 3000 feet deep, the clouds can 

normally be expected to extend inland to the terrain height corresponding to 1⁄2 the depth of the marine layer (“the half-depth point”). For example, a marine layer inversion base at 1300 feet 

MSL supports low clouds moving inland to near the 650 foot MSL terrain contour. 

Clearing time assumes approximately 80 % or more coverage west of the inland edge of the marine layer around sunrise and sufficiently thick (800-1000 feet thick or thicker) or the 

clouds are likely to clear sooner than usual. Works best for neutral to slightly onshore trends to the local deserts. If cloud bases are high, and the airmass below 3000 feet is so dry 

that the LCL approaches 3000 feet, the cloud deck is probably thin, and will generally clear everywhere). For moderately strong onshore trends (around 2 mb or more to the local 

deserts and around 4 mb or more to the plateau, add 1000 feet to the depth to obtain slower clearing). For moderately strong offshore trends (2 mb or more to the local deserts, and 4 

mb or more to the plateau, subtract 1000 feet from the depth to show better improvement). Offshore trends during “weak gradients” (+3.0 mb or less KLAX-KLAS) are often very 

tricky to deal with and significantly lower tops/inversion heights. Pilot reports and ACARS data give additional clues on trend. Results might be offset by strong Catalina Eddy 

formation, which can supply clouds to advect in, and change the “favored beaches for cloudiness”, but based on the position and depth of the Eddy, the low clouds can be pulled out 

of some coastal areas. In general, if there is weak easterly flow aloft and/or weak offshore surface pressure gradient trends, low cloud development may be delayed till the following morning. For 

fairly weak inversions with deep marine layers, (typically around 2500 feet deep MSL or deeper with patchy/ or just areas of low clouds overnight) there can be a dramatic increase in cloud 

coverage to form a ceiling during the morning (low clouds fill in the gaps in coverage mainly through boundary layer convection). For the lower, stronger inversions cloud tops are generally 

around a few hundred feet above the base of the inversion, but with deep marine layers (typically with weaker inversions that slope more gently in the vertical), cloud tops will 

attempt to convect upward to near the top of the inversion, especially inland valleys and near the mountains. Patchy dense fog extends down to 800-1000 feet or so below base of 

inversion for the more “average” marine layer depths of 1000-3000 feet deep (MSL). Marine layer clouds are prone to “reverse clearing” with weakening inversions, often seen as a 

“patch of clearing” developing over the northern portion of the California Bight Region downwind of the Santa Ynez Mountains that spreads south and east. Persistent stratus about 

10 miles or so offshore may re-develop at the coastline mid afternoon, or may advect in just before sundown. On a few occasions, afternoon and evening low clouds may “drain out” 

if significant draining winds develop. Transition season marine layers seem to deliver a thicker cloud deck than mid summer marine layers, (even when the depth is the same), and 

could produce more drizzle at lower marine layer depths (such as 2500 feet or so). Conditions will at times deviate significantly from those indicated in this table. 
, 

57

 

 

 

SUMMER MARINE LAYER MATRIX (JULY – SEPTEMBER) 

INVERSION 

STRENGTH

INVERSION BASE CHARACTER

around 1300 ft or less Patchy dense fog with the beaches affected shallow end, and mesas and western edge of coastal valleys deep end of the height range. Clear 

everywhere by late morning 

 1300 - 1999 ft Patchy dense fog affecting mesas and coastal valleys shallow end, and coastal valleys and western edge of the inland valleys deep end of 

height range. Clear all areas by noon 

 2000 - 2499 ft Patchy dense fog into all valleys. Clear all areas by noon 

 2500 - 3000 ft Patchy dense fog into all valleys. 2nd convective deck forms, convects (or re-convects) in some areas, and is slower to clear, with all areas 

clear by early afternoon. 

3000 - 4000 ft Patchy dense fog to coastal slopes. 2nd convective deck forms, likely to reverse clear, but slow to clear, and takes til late afternoon for all 

areas to clear. Cloud tops near mountains reach height of approximately the top of the KNKX inversion with mountain slopes clearing last. 

WEAK 

(< 7.0 C) 

 

 

4000 ft deep and up Patchy dense fog to coastal slopes. A couple of convective decks can form, likely to reverse clear. Cloud tops near mountains reach height of 

approximately the top of the KNKX inversion with best cloud coverage on slopes. 

around 1300 ft or less Patchy dense fog with beaches affected shallow end and mesas and western edge of coastal valleys affected deep end of height range. Clears 

everywhere by late morning 

1300 - 1999 ft Patchy dense fog affecting mesas and coastal valleys shallow end, and coastal valleys and western inland valleys deep end of height range. 

Clear by noon (except near 9 C, some patchy low clouds may persist into early afternoon in the preferred “county of the day for clouds” 

where the thickest clouds are). 

2000 - 2499 ft Patchy dense fog affecting coastal and inland valleys. Clears by noon all areas (except near 9 C, some patchy low clouds may persist into 

early afternoon in the preferred “county of the day for clouds” under thickest clouds). 

2500 - 3000 ft Patchy dense fog, mainly in the inland valleys. A 2nd convective deck may form, but still clears by mid afternoon all areas (except patchy 

clouds at some beaches well into the afternoon near 9 C).

3000 - 4000 ft Patchy dense fog to coastal slopes. A 2nd convective deck may form, but still clears by mid afternoon all areas (except patchy clouds at some 

beaches near 9 C where the thickest clouds are). 

MODERATELY 

STRONG 

“Typical Inversion” 

(7.0 - 9.9 C)

4000 ft deep and up Patchy dense fog lower slopes. Multiple convective decks typically form, but still clears by mid afternoon all areas (except patchy clouds at 

preferred “beaches of the day” near 9 C under the thickest clouds). 

around 1300 ft or less Patchy dense fog with beaches affected shallow end, and mesas and western edge of coastal valleys deep end of height range, with beach 

clearing before noon. (If no eddy, then the thicker clouds and “favored beaches of the day for clouds” are in San Diego County from 

approximately KCRQ to KSDM, and possibly via coastal blocking and island effect stratus bands). Those areas clear last. 

1300 - 1999 ft Patchy dense fog affecting mesas and coastal valleys. Clears at the beaches just before noon. [If no eddy, then the thicker clouds and 

“favored beaches of the day for clouds” are in San Diego County (approximately KCRQ to KSDM), and possibly via coastal blocking and 

island effect stratus bands, which clear last around noon]. 

2000 - 2499 ft Patchy dense fog affecting coastal valleys, but inland valleys generally remain clear. Clears at the “beaches of the day for clearing” mid 

afternoon. 

2500 - 3000 ft Patchy dense fog inland valleys. Generally clears all areas by mid afternoon. 

3000 - 4000 ft (Rare mid summer) Patchy dense fog inland valleys and lower coastal slopes. Generally clears all areas by mid afternoon. 

STRONG 

(10.0 C OR MORE)

4000 ft deep and up (Very Rare mid summer). Patchy dense fog coastal slopes. Generally clears all areas by mid afternoon 

Table. A2. Approximate marine layer characteristics (for cloud thickness of around 800-1000 feet or thicker). Summer days are warmer with weaker onshore gradients, along with more mid 

level moisture, hence clear better than transition season days (valleys almost always clear). Also warmer water temperatures (generally upper 60s or higher local buoys and around 70 or higher coastal 

waters observing sites) results in weaker inversions and poorer/slower nighttime re-development, especially in the 1000-2000 foot deep range. In general, temperature increases in the inversion are 

more dramatic for a “flatter” inversion, which improves mixing and clearing for the deeper marine layers. Deeper marine layers (typically around 2500 feet or more AGL) allows boundary layer 

convection into areas that are clear at around 1500-1600 UTC or so west of the inland edge of the marine layer air. For the more typical marine layer depths (up to 3000 feet deep), the clouds can 

normally be expected to extend inland to the terrain height corresponding to 1⁄2 the depth of the marine layer (“the half-depth point”). For example, a marine layer inversion base at 1300 feet MSL 

supports low clouds moving inland to near the 650 foot MSL terrain contour. A deeper marine layer might be needed to reach Ontario during the mid summer period in comparison to the transition 

season (possibly 2500 ft deep compared to 1800 ft deep). This is likely due to more strong eddy development in transition season, possibly a thicker cloud layer during the transition season in 

comparison to mid summer, and the later re-development of the low clouds during the mid summer overnight periods. With very rapid deepening (4 mb onshore trend) to the local deserts, and 

especially with 6-8 mb onshore gradients, conditions resemble transition season conditions, except the valleys will almost never remain cloudy all day. 

Clearing time assumes approximately 80 % or more coverage west of the inland edge of the marine layer around sunrise and sufficiently thick (at least 800-1000 feet thick) or the clouds are likely to 

clear sooner than usual. Matrix works best for neutral to slightly onshore trends to the local deserts. If cloud bases are high, and the airmass below 3000 feet is so dry that the LCL approaches 3000 

feet MSL, the cloud deck is probably thin, and will generally clear everywhere. For moderately strong onshore trends [2 mb or more to the local deserts (for example KSAN to KTRM) and 4 mb or 

more to the plateau (for example, KLAX to KTPH), add 1000 feet to the depth)] For moderately strong offshore trends (2 mb or more to the local deserts and 4 mb or more to the plateau, subtract 

1000 feet from the depth). Offshore trends during “weak gradients” (for example, +3.0 mb or less KLAX-KLAS) tend to be very tricky to deal with and can significantly lower the inversion top and 

inversion height. Pilot reports and ACARS data help to give clues on trend. Results might be offset by strong Catalina Eddy formation, which rather frequently supplies clouds to advect in, but based 

on the position and depth, eddies can pull low clouds out of some coastal areas. In general, if there is weak easterly flow aloft and/or a weak offshore surface pressure gradient trend, low cloud 

development may be delayed till the following morning. For fairly weak inversions with deep marine layers, (typically around 2500 feet deep MSL or deeper with patchy/ or just areas of low clouds 

overnight) there can be a dramatic increase in cloud coverage to form a ceiling during the morning (low clouds fill in the gaps in coverage mainly through boundary layer convection). For the lower, 

strong inversions cloud tops are generally around a few hundred feet above the base of the inversion, but with deep marine layers (with weak inversions) cloud tops tend to convect upward to near the 

top of the inversion, especially inland valleys and near the mountains. Patchy dense fog extends down to around 800-1000 feet below base of the inversion for the standard 1000-3000 foot inversion. 

Marine layer clouds are prone to “reverse clearing” with weakening inversions, often seen as a “patch of clearing” developing over the northern bight (downwind of the Santa Ynez Mountains north of 

KSBA) that spreads south and east. Persistent stratus up to 5-10 miles off the coast may re-develop at the coastline mid or late afternoon, or advect in just before sundown as the heating slows. 

Conditions will at times deviate significantly from those indicated in this table. 
, 

58

APPENDIX B. MARINE LAYER CASE STUDIES
, 

59

MARINE LAYER DEEPENING 

EVENT

(Transition season, deepening around 1000 feet per day) 

During the period 15-22 June 2003 the marine cloud tops deepen from 

2000 to 5000 feet deep with pilot reports of 6000 feet near the 

mountains by the end of the period. 

On the 18th at the beginning of the event the morning cloud bases were 

1200 feet with afternoon tops of 2200 feet. Normally, 2200 foot deep 

marine layers clear at the coastal airports, but with the 10 degree C 

inversion and 1000 ft/day deepening, ceilings continue past noon 

coastal airports, with partly cloudy skies at best coastal plain and 

southern valleys in the afternoon. 

On the 19th, a convective cloud deck (1800 feet) was visible beneath a 

higher stratiform deck (about 2500 feet) during the morning, with a 

top around 3000 feet. Since the 1000 ft/day deepening makes the 

clearing pattern resemble that of a 4000 foot deep marine layer, there 

was no clearing coastal valleys, and little clearing inland valleys. 

Fig. B1. Graphic shows the marine layer cloud tops deepening 

from 2000 feet deep to about 6500 feet deep, than back down 

to 4000 feet deep. The upper line (light green) is the inversion 

base at KNKX. The lower line (thick green) is the cloud base 

at KSAN. The blue circles are pilot reports near the coast. The 

red triangles are pilot reported cloud tops near the mountains, 

which appear to be higher than those near the coast, showing 

an upward slope in the marine layer cloud top height and 

marine layer inversion base height. The diurnal rising/falling 

of the LCL is easily seen early in the event, and the multiple 

cloud decks develop during the middle and latter portions of 

the event. Essentially, the bases burn away from the “bottom 

up” during the day, then advect in again at around 1⁄2 the 

afternoon base height in many cases (since the essentially 

“unburned” bases advecting in from the ocean are still low). 

The clouds also re-develop and thicken while the bases lower 

at night as the typical nighttime strengthening of the inversion 

occurs (and LCL lowers). Clouds also re-develop during the 

daytime via convection during deep marine layer episodes. The 

inversion typically weakens during the day up until late 

afternoon. 

0000 UTC 19 June 2003 

0000 UTC 20 June 2003

0000 UTC 21 June 2003

Fig. B2. Visible Satellite Imagery 
, 

60

On the 20th, the main morning cloud base was around 2500 feet and top around 

5000 feet. The airmass became a very unstable layer. The saturated airmass with a 

strong lapse rate was so unstable it began to convect into 2 lower decks overnight, 

without solar heating. It produced 3 clouds decks (at 1800, 2500, and 4200 

hundred feet as seen on 1200 UTC sounding and graphic). Enough cold advection 

to lower the 850 mb temperature to the mid teens (eventually around 10, then into 

the single digits), along with subsidence downwind of the Santa Ynez mountains 

cleared the coastal waters, and resulted in reverse clearing. The difference 

between the bases and inversion (tops) results in clouds thickening from 

nearly1000 to approximately 3500 feet thick. 

Rainfall amounts were heaviest in the southern coastal and valley areas. 

(Convection in the south develops on spiral bands around the eddy, which are 

intensified in regions of blocked flow parallel to the coast). The heaviest amounts 

were Spring Valley (0.13), Escondido (0.10), La Mesa (0.09), Montgomery Field 

(0.06), Rancho Bernardo (0.05), and National City (0.05). Without a thick 

isothermal layer to aide convection higher mountain slopes, 

KSAN Visibility (nmi)

0

2

4

6

8

10

12

Sun Jun 15 01:51:00Sun Jun 15 10:37:00Sun Jun 15 17:26:00Sun Jun 15 23:51:00

Mon Jun 16 05:51:00Mon Jun 16 12:51:00Mon Jun 16 17:51:00Mon Jun 16 22:51:00

Tue Jun 17 06:51:00Tue Jun 17 13:51:00Tue Jun 17 19:25:00

Wed Jun 18 00:36:00Wed Jun 18 06:51:00Wed Jun 18 14:51:00Wed Jun 18 18:51:00Wed Jun 18 23:51:00

Thu Jun 19 05:51:00Thu Jun 19 13:51:00Thu Jun 19 19:51:00

Fri Jun 20 00:51:00 2003Fri Jun 20 07:23:00 2003Fri Jun 20 13:51:00 2003Fri Jun 20 17:51:00 2003Fri Jun 20 22:51:00 2003

Sat Jun 21 05:51:00 2003Sat Jun 21 13:51:00 2003Sat Jun 21 18:51:00 2003Sat Jun 21 23:51:00 2003

Sun Jun 22 05:51:00Sun Jun 22 12:51:00Sun Jun 22 20:37:00

Mon Jun 23 02:51:00Mon Jun 23 07:51:00Mon Jun 23 14:51:00Mon Jun 23 22:51:00

Tue Jun 24 06:51:00

 Fig. B3. San Diego (KSAN) visibility in miles. 

 mountain amounts were meager at best. Only KSOX in the Santa Ana 

Mountains reported rain (0.04). With bases just below 1000 feet for the 

first few days, visibilites dipped to or below 3 miles in the morning, 

otherwise visibilities were good. 

GRADIENT/

TRENDS 

(June 2003)

6/15 

12Z

6/16 

12Z

6/17 

12Z

6/18 

12Z

6/19 

12Z

6/20 

12Z

6/21 

12Z

6/22 

12Z

6/23 

12Z

LAX-SFO -5.2 -2.1 

(3.1)

-1.5 

(0.6)

-1.9 

(0.4)

0.0 

(1.9)

0.3 

(0.3)

-0.4 

(-0.7)

-0.7 

(-0.3)

0.7 

(1.4)

LAX-WMC -1.6 -2.7 

(-1.1)

-2.0 

(0.7)

3.1 

(5.1)

4.2 

(1.1)

5.5 

(1.3)

3.1 

(-2.4)

-0.1 

(-3.2)

3.7 

(3.8)

LAX-TPH -0.1 0.6 

(0.7)

0.0 

(-0.6)

1.8 

(1.8)

6.8 

(5.0)

8.1 

(1.3)

6.4 

(-1.7)

4.8 

(-1.6)

6.8 

(2.0)

LAX-LAS 3.4 3.4 

(0.0)

4.0 

(0.6)

3.9 

(-0.1)

7.4 

(3.5)

9.6 

(2.2)

7.6 

(-2.0)

6.7 

(-0.9)

10.0 

(3.3)

SAN-IPL 5.7 6.3 

(0.6)

6.3 

(0.0)

6.7 

(0.4)

8.0 

(1.3)

8.0 

(0.0)

6.7 

(-1.3)

6.7 

(-0.0)

7.7 

(1.0)

SAN-YUM 5.6 6.1 

(0.5)

mm 6.1 7.6 

(1.5)

7.3 

(-0.3)

6.7 

(-0.6)

5.2 

(-0.5)

7.9 

(2.7)

Table B1. Pressure gradients (black) onshore trends (blue), and offshore 

trends (red negative values) beneath. 

 Fig. B4. KNKX Sounding data 
, 

61

 

:SOUTHWESTERN CALIFORNIA TEMPERATURE AND PRECIPITATION SUMMARY 

:NATIONAL WEATHER SERVICE SAN DIEGO CA 

:810 PM PDT FRI JUN 20 2003 

: 

:***** AMENDED FOR IMPERIAL BEACH AND CUYAMACA PARK ***** 

: 

:TODAYS HIGH AND OVERNIGHT LOW TEMPERATURES AS OF 5 PM TODAY. 

: * DENOTES 24 HOUR HIGH TEMPERATURES BETWEEN 4 PM YESTERDAY 

: AND 4 PM TODAY. OCCASIONALLY THE HIGH TEMPERATURE MAY HAVE 

: OCCURRED AFTER 4 PM YESTERDAY. 

: 

:PRECIPITATION FOR THE PAST 24 HOURS. M DENOTES MISSING. 

: T DENOTES TRACE OF PRECIPITATION 

: 

.B SAN 0620 P DH16/TX/TN/PPD/SD 

: 

: ID : STATION ELEV : HIGH / LOW / PCPN / SNODEP 

: FEET INCHES 

: 

: ...COASTAL AREAS... 

: 

YBLC1: * YORBA LINDA 350 : 65 / M / 0.00 / 

FUL : FULLERTON AIRPORT 96 : 68 / 60 / 0.00 / 

ANAC1: * ANAHEIM 335 : 67 / 61 / 0.00 / 

STAC1: * SANTA ANA 135 : 70 / 60 / 0.00 / 

SNA : JOHN WAYNE AIRPORT 55 : 70 / 61 / T / 

3L3 : * NEWPORT BEACH 10 : 66 / 62 / 0.00 / 

LAGC1: * LAGUNA BEACH 35 : 74 / 59 / 0.00 / 

SJYC1: * SAN JUAN CANYON 375 : 65 / 58 / T / 

L34 : * OCEANSIDE HARBOR 10 : 65 / 59 / 0.00 / 

OKB : OCEANSIDE AIRPORT 28 : 68 / 59 / 0.03 / 

VSTC1: * VISTA 330 : 65 / 57 / 0.00 / 

CRQ : CARLSBAD AIRPORT 328 : 66 / 58 / T / 

SOL : SOLANA BEACH 75 : M / M / 0.00 / 

DMRC1: * DEL MAR 100 : 67 / 58 / 0.00 / 

NKX : MIRAMAR 477 : 68 / 58 / T / 

MYF : MONTGOMERY FIELD 420 : 64 / 57 / 0.06 / 

SWDC1: SEA WORLD SAN DIEGO 10 : 67 / 58 / 0.03 / 

SAN : SAN DIEGO 15 : 65 / 60 / T / 

CDOC1: * CORONADO 25 : 65 / 58 / 0.04 / 

L13 : * CABRILLO NATL MNMT 364 : 67 / 57 / 0.06 / 

NATC1: * NATIONAL CITY 30 : 71 / 56 / 0.05 / 

CVAC1: * CHULA VISTA 56 : M / M / M / 

IPLC1: * IMPERIAL BEACH 22 : 72 / 60 / 0.00 / 

SDM : BROWN FIELD 525 : 66 / 56 / 0.06 / 

: 

: ...INLAND AREAS... 

: 

ONT : ONTARIO 943 : 63 / 58 / T / 

L67 : RIALTO 1420 : 66 / 57 / 0.00 / 

SBOC1: * SAN BERNARDINO 1125 : M / M / M / 

UCR : RIVERSIDE 986 : 70 / 57 / 0.02 / 

EORC1: * ELSINORE 1285 : 74 / 46 / 0.00 / 

HEMC1: * HEMET 1655 : 66 / 51 / 0.00 / 

TEMC1: * TEMECULA 1020 : 67 / 56 / 0.00 / 

FALC1: * FALLBROOK 698 : 63 / 57 / 0.02 / 

ESCC1: * ESCONDIDO 600 : 67 / 46 / 0.10 / 

ASCC1: * WILD ANIMAL PARK 420 : 70 / 58 / 0.00 / 

RAMC1: RAMONA 1393 : 60 / 54 / 0.01 / 

SGX : RANCHO BERNARDO 690 : 66 / 56 / 0.05 / 

POWC1: * POWAY 648 : 68 / 57 / 0.00 / 

ALPC1: * ALPINE 1735 : M / M / M / 

SNTC1: * SANTEE 340 : M / M / M / 

ELJC1: * EL CAJON 405 : 65 / 57 / 0.00 / 

LMAC1: * LA MESA 530 : 77 / 56 / 0.09 / 

SPVC1: * SPRING VALLEY 270 : 72 / 58 / 0.13 / 

LMGC1: * LEMON GROVE 427 : 64 / 57 / 0.00 / 

: 

: ...MOUNTAIN AREAS... 

: 

SOX : SANTA ANA RADAR 3092 : 52 / 49 / 0.04 / 

WRIC1: * WRIGHTWOOD 5980 : 72 / 39 / 0.00 / 

ARWC1: * LAKE ARROWHEAD 5205 : 68 / 54 / 0.00 / 

BBLC1: * BIG BEAR LAKE 6760 : 75 / 42 / 0.00 / 

IDYC1: * IDYLLWILD 5380 : 69 / 41 / 0.00 / 

PLRC1: * PALOMAR MOUNTAIN 5550 : 71 / 41 / 0.00 / 

JUL : JULIAN 4240 : 52 / 45 / 0.01 / 

CUYC1: * CUYAMACA PARK 4870 : 75 / 51 / 0.00 / 

MTLC1: * MT. LAGUNA 5760 : M / M / M / 

CZZ : CAMPO 2615 : 60 / 51 / 0.00 / 

: 

: ...DESERT AREAS... 

: 

HESC1: * HESPERIA 3055 : M / M / M / 

APLC1: APPLE VALLEY 2780 : 75 / 54 / 0.00 / 

PSP : PALM SPRINGS 425 : 85 / 67 / 0.00 / 

IDOC1: * INDIO -21 : 103 / 69 / 0.00 / 

TRM : THERMAL -117 : 90 / 70 / 0.00 / 

BROC1: BORREGO 805 : 85 / 61 / 0.00 / 

OCTC1: * OCOTILLO WELLS 390 : 102 / 65 / 0.00 / 

TYEC1: * TWENTYNINE PALMS 1975 : 98 / 62 / 0.00 /

Near the end of the event the boundary layer 

near surface begins to dry out, and the low level 

inverted V sounding accompanies decreasing 

rainfall. 

With the shallower marine layers the drizzle is 

more of a night and morning phenomena, but 

when it begins to convect around the mountains 

in an isothermal inversion and around 6000 feet 

deep or deeper, the precipitation can become as 

prevalent (if not more) during the afternoon, 

with sprinkles/light rain becoming the dominant 

type. 

HIGH TEMPERATURES VERSUS ELEVATION 

DURING A TYPICAL REVERSE CLEARING 

PATTERN 

(JUNE 20 2003)

0

1000

2000

3000

4000

5000

6000

7000

40 50 60 70 80

High Temperature (degrees F)

Height (feet)

Fig. B5. The above figure shows a temperature 

profile typical of a reverse clearing pattern. The 

region at approximately 3-4 thousand feet 

around the mountains stays coolest (lowest 

temperatures on the sounding, and may remain 

cloudy all day). The lowest high temperatures 

are in the lower mountains just below the 

inversion base. Above and below the inversion 

base there are clear skies and better heating. It is 

over 20 degrees warmer near 7000 feet MSL 

than it is near the inversion base near 3500 feet 

MSL. 
, 

62

 

 

MARINE LAYER CYCLE

(Lowering event, then deepening event) 

During this May 2000 case the marine layer clears all land areas day 1 (19 May 2000), but does not clear at most 

beaches day 2 (20 May 2000). Also a huge increase in inland temperatures occurred. Anticipating the possibility of 

this scenario during the evening shift on 19 May 2000, the author mentioned in the Area Forecast Discussion (AFD) 

that the latest runs have “the valleys down right hot...especially by Sunday”. A lowering/strengthening inversion was 

expected to result in this warming. Basically, the increasing heights, thicknesses, 850 mb and 950 mb temperatures, 

along with easterly flow aloft would bringing the warmer air in/above the inversion closer to sea level (and further 

west), forcing the inversion down. The figures indicate this fact over the 24 hour period. Also stated in the AFD was 

“the tricky part of this whole scenario is the coastal low clouds and fog. It may hang tough in some location through a 

good portion of the day. For now....I will think along the lines of at least a few hours of sunshine for the beaches 

during the afternoon”. As expected, the lowering and strengthening of the inversion made the low clouds so 

persistent that clearing was limited to only the far northwest corner of the forecast area (not an unusual occurrence for 

the more “concave” coastline in that area). Since strong heating is generally limited to the land areas, the warming 

over the ocean was insufficient to clear the low clouds or even get close to breaking the inversion. A dewpoint 

inversion has also developed. Afternoon high temperatures are higher everywhere, except over the coastal plain, 

where increased cloud cover and a strengthened sea breeze (due to strong inland heating) actually results in lower 

high temperatures than on day 1. Temperatures to over 105 in the valleys result in some weak convection over the 

mountains, even with little moisture in the sounding. Significant temperature increases in the 950 mb region (around 

1500-1800 feet) to around 73 degrees F (approximately 23 C) or higher are usually needed for significant warm-ups 

in the coastal plain areas (in which the high temperature at KSAN is approximately a few degrees lower than the 950 

mb temperature as a first guess). The 950 mb temperature was around 68 F in this case. As the deepening trend 

progresses on the 22nd, the cooling is largest over the inland coastal plain area (KFUL), and in the valleys eastward on 

the 23rd, with the cooling maximum associated with the “thermal transition zone” progressing inland, essentially 

following the inversion base. In the far inland areas in places such as the higher mountains and deserts, lowering 850 

mb temperatures and thickness falls play a strong part in the cooling process. As usual some dense fog was found 

just under the inversion base. 

Fig. B6. 1200 UTC 19 May 2000 KNKX sounding (dashed) 

overlayed with 1200 UTC 20 May 2000 sounding (solid) showing 

lowering and strengthening of the inversion.
, 

63

GRADIENTS 

AND TRENDS 

(May 2000)

5/18 

12Z

5/19 

12Z

5/20 

12Z

5/21 

12Z

5/22 

12Z

5/23 

12Z

5/24 

12Z

KLAX-KSFO -4.1 -3.4 

(0.6)

-3.2 

(0.2)

-1.4 

(1.8)

-3.9 

(-2.5)

0.3 

(4.2)

1.1 

(0.9)

KLAX-KWMC -8.2 -5.7 

(2.5)

-8.0 

(-2.3)

-8.2 

(-0.2)

-4.8 

(3.4)

3.2 

(8.0)

0.4 

(2.8)

KLAX-KTPH -3.7 -4.0 

(-0.3)

-5.0 

(-1.0)

-4.3 

(0.7)

-2.9 

(1.4)

-0.2 

(2.7)

1.6 

(1.8)

KLAX-KLAS 0.9 0.4 

(-0.5)

0.1 

(-0.3)

-0.5 

(-0.6)

1.0 

(1.5)

4.7 

(3.7)

7.5 

(2.8)

KSAN-KIPL M 3.7 3.7 

(0.0)

4.7 

(1.0)

4.7 

(0.0)

5.0 

(0.3)

7.0 

(2.0)

KSAN-KYUM 4.8 3.1 

(-1.7)

5.0 

(1.9)

5.1 

(0.1)

5.3 

(0.2)

6.0 

(0.7)

7.6 

(1.6)

Fig. B2. Pressure gradients (top) and 24 hour pressure gradient trends (bottom) in 

parentheses (offshore in red, underlined, and onshore in blue). 

Fig. B7. 2345 UTC 20 May 2000 visible satellite 

imagery. 
, 

64

Parameter 5/18 

12Z

5/19 

00Z

5/19 

12Z

5/20 

00Z

5/20 

12Z

5/21 

00Z

5/21 

12Z

5/22 

00Z

5/22 

12Z

5/23 

00Z

5/23 

12Z

5/24 

00Z

inversion base height in 

mb 942 953 968 950 981 963 981 956 966 953 934 933

inversion base height in 

meters 614 537 407 560 270 416 261 483 381 517 672 699

inversion base height in 

feet 2015 1761 1335 1837 886 1365 856 1585 1251 1696 2204 2293

inversion base 

temperature in degrees 

C

10.2 16.6 12.8 21.4 13.8 20.2 13.0 17.0 13.2 15.8 11.8 12.4

inversion top height in 

mb 850 925 925 925 925 902 814 911 896 879 837 850

inversion top height in 

meters 1486 793 794 791 776 990 872 906 1039 1221 1621 1504

inversion top height in 

feet 4876 2602 2605 2595 2546 3248 2861 2973 3409 4006 5319 4630

inversion top 

temperature in degrees 

C

16.4 21.4 21.0 24.8 24.4 26.0 26.8 29.0 29.6 27.6 24.8 23.6

inversion strength in 

degrees C 6.2 4.8 7.2 3.4 10.6 5.8 13.8 12.0 16.4 11.8 13.0 11.2

Temperature at 

850 mb in degrees C

16.4 17.6 18.0 19.4 20.2 22.0 23.2 24.6 26.0 25.8 22.0 23.6

1000-500 mb thickness 

in decameters 563 569 569 572 570 574 573 575 576 576 573 573

Site (elev) THU 18th FRI 19th SAT 20th SUN 21st MON 22nd TUE 23rd

KSAN (15) 70 72 70 68 65 65 (trace)

KFUL (96 ) 81 85 83 81 74 70

BUO (2600) 90 94 101 106 105 88

BBL (6760) 66 70 74 missing 82 missing

KTRM (-117) 101 104 111 114 110 105

Table. B3. Example of a cycle in the marine layer depth (inversion lowering from near 2000 feet to below 1000 feet then back up 

above 2000 feet) and the effect on selected locations. All heights are in feet MSL. Temperatures in upper portion of table in degrees 

C. Corresponding maximum temperatures for 18 May 2000 through 23 May 2000 are shown in the lower portion of the table in 

degrees F. Sites are coastal, inland coastal, valley, mountain, and desert respectively. Station elevations are in feet MSL. Deepening 

of the marine layer to near 2500 feet deep resulted in widespread cooling, with even a trace of drizzle. As the inversion lowered and 

strengthened on the 21st, afternoon highs were lower near the coast in response to the persistent cloud cover, but actually went up 

valleys, mountains, and deserts. The rise was in response to rising temperatures aloft, increasing thicknesses, and very high inversion 

top temperatures lowering enough to effect the inland areas, but not warm the coastal areas. On 22 June 2000 as the cooling spread 

inland, SAN fell only 3 deg F, since it was already rather cool. The thickness remained high, so Beaumont (BUO) only fell 1 deg F. At 

the same time, KFUL dropped 7 deg F on the inland coastal plain area, reflecting the maximum cooling band spreading inland. As

this thermal transition zone passes Beaumont on the 23rd the 24 hour change was whopping 17 degrees F. 
, 

65

 

SHALLOW CONVECTION 

EVENT 

(deep moist layer with isothermal layer aloft). 

 

At 1200 UTC 30 April 2003 the sounding shows a 

nearly saturated layer below 3200 feet, but the inversion 

is much weaker than the approximately 7 degrees C 

usually needed to cap a solid overnight deck (in this 

case, it is nearly isothermal above the moist layer). 

At 1430 UTC there is a very weak eddy shown by the 

southerly wind at the surface on the raob and the outer 

coastal convergence zone just off the Orange County 

coast, stretching south to northern Baja. Skies are clear 

over the coastal plain. 

Since the marine layer moisture extends inland under the 

3000 foot inversion, and it was clear at 1600 UTC west 

of the moisture boundary, some boundary layer 

convection was possible. 

At 1900 UTC the clear skies in the coastal plain begins 

to convect, with the more stable sea breeze air moving 

inland behind the sea breeze front. 

By 2300 UTC the coastal plain has stabilized, but now 

the afternoon convection along the mountain slopes has 

developed, since the cloudiness can distribute into an 

isothermal layer or weak inversion layer above the moist 

low levels. (Moist layers capped by an isothermal layer 

can produce locally heavy afternoon showers, especially 

around the mountains, for moist layers around 6000 feet 

deep or deeper). This differs from strong, low inversion 

patterns (which cap cloud tops to only a few hundred 

feet into the inversion layer). 

Fig. B9. 1430 UTC 30 April 2003 visible satellite 

imagery 

Fig. B10. 1900 UTC 30 April 2003 visible satellite 

imagery 

Fig. B11. 2300 UTC 30 April 2003 visible satellite 

imagery

Fig. B8. KNKX Sounding data. 
, 

66

DENSE FOG (Chain reaction accident in the Cajon Pass on 26 January 2001) 

Although they do not always occur under these conditions, this scenario is a common one. Surface pressures were 

falling to the north and to the interior as a surface low approached the Pacific Northwest. Aloft, an upper level low 

pressure system was bearing down on the state with increasing cyclonic flow, hence a rapidly deepening surface based 

moist layer which moves into the coastal slopes. The moist layer deepened well into the Cajon Pass, possibly also 

wetting the road. This dense fog layer could have been rather deep. 

Dense fog in the Cajon Pass along Interstate 15 near Devore (DEV) led to 78 vehicle accidents, one of which included 

a 26 car pileup that sent 9 people to hospitals with minor to moderate injuries. (KNKX soundings on next page). 

Fig. B12. 1200 UTC 26 January 2001 surface analysis (mb) and 500 mb heights (decameters), 

temperatures (deg C), and winds (knots). 
, 

67

Fig. B13. KNKX sounding data. 
, 

68

DENSE FOG (Chain reaction accident in the Cajon Pass on 1 April 2004) 

The scenario was a fairly common one. Surface pressures were falling to the north and to the 

interior as a surface low approached the Pacific Northwest. Aloft, an upper level low pressure 

system was bearing down on the state with increasingly cyclonic flow, hence a rapidly deepening 

marine layer. There was light rain reported. The marine layer deepened well into the Cajon Pass 

with dense fog (with precipitation wetting the road to make the problem even worse). Notice the 

low level inverted “V” sounding, with the actual fog layer rather thin (in this case) and just beneath 

the inversion. (KNKX soundings on next page). 

Dense fog in the Cajon Pass along Interstate 15 led to 66 vehicles in a chain reaction accident. 

Fig. B14. 1200 UTC 1 April 2004 surface analysis (mb) and 500 mb heights (decameters), temperatures 

(degrees), and winds (knots). 
, 

69

Fig. B15. KNKX Sounding data 
, 

70

APPENDIX C. HEAT WAVE CASES 
, 

71

 Type 1. High to The North Event (mesoscale subsidence via strong offshore/downslope winds) 

(All time record 111 F at San Diego) 

On 26 September 1963 relatively strong high pressure developed aloft, and combined with very 

strong surface high pressure at the lower levels, they produced very high temperatures. The 500 

mb high initially developed over San Diego due to a building ridge in response to a developing low 

further offshore. At the peak of the heat wave on the 26th, a “high pressure to the north” pattern 

(rather than a high directly overhead) with offshore flow combined with 850 mb temperatures to 

near 26 C in the basin. The conditions resulted in the highest temperature in history at the official 

San Diego observing site (111 degrees F). Gusty winds that reach all the way to the coast 

basically guarantees record breaking heat during the warm season. Winds gusted to 18 miles per 

hour all the way to the coast in this case. The downward motion was mainly due to mesoscale 

subsidence associated with mountain wave activity. Note that the record was set in what is 

technically considered to be the “fall” season rather than summer. 

 

 Fig. C1. 26 September 1963 reanalysis 500 mb heights (decameters) and 700 mb heights (decameters). 
, 

72

Fig. C2. 26 September 1963 reanalysis 850 mb heights (decameters) and 850 mb temperatures 

(degrees K). The 850 mb temperature at KNKX is 26 deg C (79 deg F). 
, 

73

Type 2. High Overhead Event (Subsidence Driven Heating) on 12 September 2000 

On 12 September 2000 very hot conditions developed as far west as the coastal plain. Lindbergh Field hit 88, and some coastal temperatures 

were over 100. There was a large high centered over southern California. The KLAX-KLAS surface pressure gradient was offshore (-2.0), well 

under the “weak category optimal” upper bound of +3.0 mb onshore, and KLAX-KTPH was nearly -6.0 mb offshore, allowing the warmup to 

occur all the way to the coast. The 850 mb/Top of the inversion temperature couplet was well above normal at 25 C/29 C. The winds aloft 

during the morning in the 850 to 700 mb layer were east at about 10 knots. Combined with the low inversion, this scenario usually (and did) 

result in a rapid warmup coastal and valley areas. The strong offshore gradients brought the 850 mb potential temperature of near 103 into the 

valleys and nearly so into the inland coastal plain. West of the mountains, minimum temperatures were generally in the 60s, even thought there 

was a rather high temperature at the top of the inversion. This may be because the early morning east winds were generally confined to the 

foothills (which saw some 70s) in the morning, and the since the 850 mb dew points were only about 5 C. Surface dew points were only in the 

lower 50s. Some high temperatures were Newport Beach 74, Oceanside Harbor 75, Lindbergh Field 88, Anaheim 98, Ontario 105, Riverside 

110, Big Bear Lake 82, Hesperia 107, and Thermal 111. (KNKX sounding data on the next page). 

SURFACE PRESSURE GRADIENT 

(1200 UTC) 

DATE: 

11 SEP 2000 

DATE: 

12 SEP 2000 

LAX-SFO -1.6 -2.6 

LAX-WMC -3.3 -5.3 

LAX-TPH -2.6 -5.9 

LAX-LAS 1.0 -2.0 

SAN-YUM M M 

Table C1. Surface pressure gradient data 

Figure C3. 1200 UTC 12 September 2000 500 mb heights in decameters, temperatures in degrees C, and 

winds in knots (left), along with station plots and mean sea level pressure in mb (right). 
, 

74

Fig. C2. KNKX sounding data. 
, 

75

SURFACE PRESSURE GRADIENT 

(1200 UTC) 

DATE: 

11 SEP 2000 

DATE: 

12 SEP 2000 

LAX-SFO -1.6 -2.6 

LAX-WMC -3.3 -5.3 

LAX-TPH -2.6 -5.9 

LAX-LAS 1.0 -2.0 

SAN-YUM M M 

Table C1. Surface pressure gradient data 

ASUS66 KSGX 130028 AMD 

RTPSGX 

:SOUTHWESTERN CALIFORNIA TEMPERATURE AND PRECIPITATION 

SUMMARY 

:NATIONAL WEATHER SERVICE SAN DIEGO CA 

:530 PM PDT TUE SEP 12 2000 

: 

:***** UPDATED ***** 

: 

:TODAYS HIGH AND OVERNIGHT LOW TEMPERATURES AS OF 5 PM 

TODAY. 

: * DENOTES 24 HOUR HIGH TEMPERATURES AS OF 4 PM 

YESTERDAY. 

: OCCASIONALLY THE HIGH TEMPERATURE MAY HAVE OCCURRED 

AFTER 

: 4 PM THE PREVIOUS DAY. 

: 

:PRECIPITATION FOR THE PAST 24 HOURS. M DENOTES MISSING.

: T DENOTES TRACE OF PRECIPITATION 

: 

.B SAN 0912 P DH16/TX/TN/PPD/SD 

: 

: ID : STATION ELEV : HIGH / LOW / PCPN / 

SNODEP 

: FEET 

INCHES 

: 

: ...COASTAL AREAS... 

: 

FUL : FULLERTON AIRPORT 96 : 96 / 63 / 0.00 / 

ANAC1: * ANAHEIM 335 : 98 / 64 / 0.00 / 

STAC1: * SANTA ANA 135 : 94 / 67 / 0.00 / 

SNA : JOHN WAYNE AIRPORT 55 : 89 / 65 / 0.00 / 

3L3 : * NEWPORT BEACH 10 : 74 / 62 / 0.00 / 

:XX1 : * S JUAN CAPISTRANO 730 : 89 / 58 / 0.00 / 

L34 : * OCEANSIDE HARBOR 10 : 75 / 60 / 0.00 / 

OKB : OCEANSIDE AIRPORT 28 : 87 / 55 / 0.00 / 

VSTC1: * VISTA 330 : 101 / 66 / 0.00 / 

CRQ : CARLSBAD AIRPORT 328 : 87 / 64 / 0.00 / 

DMRC1: * DEL MAR 100 : 92 / 74 / 0.00 / 

NKX : MIRAMAR 477 : 94 / 64 / 0.00 / 

MYF : MONTGOMERY FIELD 420 : 93 / 67 / 0.00 / 

SWDC1: SEA WORLD SAN DIEGO 10 : 79 / 64 / 0.00 / 

SAN : SAN DIEGO 15 : 88 / 69 / 0.00 / 

CDOC1: * CORONADO 25 : 85 / 67 / 0.00 / 

L13 : * CABRILLO NATL MNMT 364 : 83 / 64 / 0.00 / 

NATC1: * NATIONAL CITY 30 : M / M / M / 

CVAC1: * CHULA VISTA 56 : 89 / 65 / 0.00 / 

IPLC1: * IMPERIAL BEACH 22 : 81 / 62 / 0.00 / 

SDM : BROWN FIELD 525 : 96 / 67 / 0.00 / 

: 

: ...INLAND AREAS... 

: 

ONT : ONTARIO 943 : 105 / 66 / 0.00 / 

SBOC1: * SAN BERNARDINO 1125 : 105 / 64 / 0.00 / 

UCR : RIVERSIDE 986 : 110 / 62 / 0.00 / 

BUO : BEAUMONT 2600 : 105 / 59 / 0.00 / 

EORC1: * ELSINORE 1285 : 105 / 61 / 0.00 / 

HEMC1: * HEMET 1655 : M / M / M / 

TEMC1: * TEMECULA 1020 : M / M / M / 

FALC1: * FALLBROOK 698 : 99 / 68 / 0.00 / 

ESCC1: * ESCONDIDO 600 : M / M / M / 

ASCC1: * WILD ANIMAL PARK 420 : 104 / M / 0.00 / 

RNM : RAMONA 1393 : 102 / 61 / 0.00 / 

SGX : RANCHO BERNARDO 690 : 95 / 64 / 0.00 / 

POWC1: * POWAY 648 : M / M / M / 

ALPC1: * ALPINE 1735 : 96 / 73 / 0.00 / 

SNTC1: * SANTEE 340 : M / M / M / 

ELJC1: * EL CAJON 405 : 102 / 56 / 0.00 / 

LMAC1: * LA MESA 530 : 94 / 70 / 0.00 / 

SPVC1: * SPRING VALLEY 270 : M / M / M / 

LMGC1: * LEMON GROVE 427 : 94 / 64 / 0.00 / 

: 

: ...MOUNTAIN AREAS... 

: 

WRIC1: * WRIGHTWOOD 5980 : 83 / 47 / 0.00 / 

:RUN : RUNNING SPRINGS 6080 : 92 / 63 / 0.00 / 

BBLC1: * BIG BEAR LAKE 6790 : 82 / 42 / 0.00 / 

IDYC1: * IDYLLWILD 5380 : 91 / 49 / 0.00 / 

PLRC1: * PALOMAR MOUNTAIN 5550 : M / M / M / 

JUL : JULIAN 4240 : 93 / 64 / 0.00 / 

CUYC1: * CUYAMACA PARK 4870 : 89 / 58 / 0.00 / 

MTLC1: * MT. LAGUNA 5760 : 85 / 57 / 0.00 / 

CZZ : CAMPO 2615 : 98 / 56 / 0.00 / 

: 

: ...DESERT AREAS... 

: 

HESC1: * HESPERIA 3055 : 107 / 72 / 0.00 / 

PSP : PALM SPRINGS 425 : 108 / 79 / 0.00 / 

IDOC1: * INDIO -21 : 108 / 76 / 0.00 / 

TRM : THERMAL -117 : 111 / 65 / 0.00 / 

BROC1: BORREGO 805 : 106 / 76 / 0.00 / 

TYEC1: * TWENTYNINE PALMS 1975 : 101 / 62 / 0.00 / 

: 

.END 
, 

76

Type 3. North-South Oriented Ridge Event (Subsidence and Wind Driven) 

The 29 September 1998 event is an example of an offshore flow event during a warm season month. It is 

characterized by the 500 mb ridge axis and upper level high center analyzed very close to the California coast. 

There is a gradient to the Great Basin plateau (COAST to KTPH) in excess of 10 mb offshore. The 1200 UTC 29 

September 1999 KNKX sounding shows very little upper level support (only a weak east wind at 850 mb). However, 

there is significant low level flow to help clear the extremely shallow surface based inversion via mixing and airmass 

replacement associated with the easterly flow by midday. The cold air advection from the Great Basin into the 

deserts results in a “reverse heating” pattern, where the high temperatures are higher west of the mountains (up to 

108) than they are in the deserts (up to 102). This anomalous situation produces higher minimum temperatures west 

of the mountains than east of the mountains (78 verses 77). The cold advection in the mountains results in highest 

maximum and highest minimum temperatures much lower than their valley counterparts (more typical of a cool 

season pattern). A placement of the upper level high further westward may have resulted in stronger winds since 

more upper level northeast flow can develop. (KNKX sounding data on the next page). 

SURFACE PRESSURE GRADIENT 

(1200 UTC) 

DATE: 

29 Sep 1999 

LAX-SFO -1.3 

LAX-WMC Missing 

LAX-TPH -10.8 

LAX-LAS -8.1 

SAN-YUM 0.8 

Table C2. Surface pressure gradient data 

Figure C5. 1200 UTC 29 September 1999 500 mb heights in decameters, temperatures in degrees C, and winds in 

knots (left), along with station plots and mean sea level pressure in mb (right). 
, 

77

0000 UTC 30 September 1999 KNKX sounding not available 

Fig. C6. KNKX sounding data. 
, 

78

SURFACE PRESSURE GRADIENT 

(1200 UTC) 

DATE: 

29 Sep 1999 

LAX-SFO -1.3 

LAX-WMC Missing 

LAX-TPH -10.8 

LAX-LAS -8.1 

SAN-YUM 0.8 

Table C2. Surface pressure gradient data 

National Weather Service - San Diego, California 

ZCZC LAXRTPSGX 

TTAA00 KSGX 300022 AMD 

:SOUTHWESTERN CALIFORNIA TEMPERATURE AND PRECIPITATION 

SUMMARY 

:NATIONAL WEATHER SERVICE SAN DIEGO CA 

:530 PM PDT WED SEP 29 1999 

: 

:***** UPDATED ***** 

: 

: TODAYS HIGH AND OVERNIGHT LOW TEMPERATURES AS OF 0.00 

TODAY. 

: * DENOTES 24 HOUR HIGH TEMPERATURES AS OF 4 PM 

YESTERDAY. 

: OCCASIONALLY THE HIGH TEMPERATURE MAY HAVE OCCURRED 

AFTER 

: 4 PM THE PREVIOUS DAY. 

: 

:PRECIPITATION FOR THE PAST 24 HOURS. M DENOTES MISSING. 

: T DENOTES TRACE OF PRECIPITATION 

: 

.B SAN 0929 P DH16/TX/TN/PPD/SD 

: 

: ID : STATION ELEV : HIGH / LOW / PCPN / 

SNODEP 

: FEET 

INCHES 

: 

: ...COASTAL AREAS... 

: 

FUL : FULLERTON AIRPORT 96 : 104 / 59 / 0.00 / 

ANAC1: * ANAHEIM 335 : 105 / 59 / 0.00 / 

STAC1: * SANTA ANA 135 : 99 / 56 / 0.00 / 

SNA : JOHN WAYNE AIRPORT 55 : 97 / 57 / 0.00 / 

3L3 : * NEWPORT BEACH 10 : 83 / 58 / 0.00 / 

: : * S JUAN CAPISTRANO 730 89 / 55 / 0.00 / 

L34 : * OCEANSIDE HARBOR 10 : 73 / 54 / 0.00 / 

OKB : OCEANSIDE AIRPORT 28 : 92 / 50 / 0.00 / 

VSTC1: * VISTA 330 : 105 / 56 / 0.00 / 

CRQ : CARLSBAD AIRPORT 328 : 92 / 60 / 0.00 / 

DMRC1: * DEL MAR 100 : 85 / 52 / 0.00 / 

NKX : MIRAMAR 477 : 97 / 54 / 0.00 / 

MYF : MONTGOMERY FIELD 420 : 99 / 56 / 0.00 / 

SWDC1: SEA WORLD SAN DIEGO 10 : 81 / 50 / 0.00 / 

SAN : SAN DIEGO 15 : 90 / 59 / 0.00 / 

CDOC1: * CORONADO 25 : 85 / 56 / 0.00 / 

L13 : * CABRILLO NATL MNMT 364 : 94 / 57 / 0.00 / 

NATC1: * NATIONAL CITY 30 : 95 / 53 / 0.00 / 

CVAC1: * CHULA VISTA 56 : 96 / 55 / 0.00 / 

IPLC1: * IMPERIAL BEACH 22 : M / M / M / 

SDM : BROWN FIELD 525 : 99 / 55 / 0.00 / 

: ...INLAND AREAS... 

: 

ONT : ONTARIO 943 : 102 / 62 / 0.00 / 

SBOC1: * SAN BERNARDINO 1125 : M / M / M / 

UCR : RIVERSIDE 986 : 103 / 61 / 0.00 / 

BUO : BEAUMONT 2600 : 96 / 71 / 0.00 / 

EORC1: * ELSINORE 1285 : 102 / 63 / 0.00 / 

HEMC1: * HEMET 1655 : 105 / 57 / 0.00 / 

TEMC1: * TEMECULA 1020 : M / M / M / 

FALC1: * FALLBROOK 698 : 103 / 78 / 0.00 / 

ESCC1: * ESCONDIDO 600 : 104 / 57 / 0.00 / 

ASCC1: * WILD ANIMAL PARK 420 : 108 / 56 / 0.00 / 

RNM : RAMONA 1393 : 102 / 59 / 0.00 / 

SGX : RANCHO BERNARDO 690 : 97 / 58 / 0.00 / 

POWC1: * POWAY 648 : 102 / 55 / 0.00 / 

ALPC1: * ALPINE 1735 : 101 / M / 0.00 / 

SNTC1: * SANTEE 340 : 104 / 53 / 0.00 / 

ELJC1: * EL CAJON 405 : 105 / 55 / 0.00 / 

LMAC1: * LA MESA 530 : 101 / 62 / 0.00 / 

SPVC1: * SPRING VALLEY 270 : 104 / 54 / 0.00 / 

LMGC1: * LEMON GROVE 427 : 77 / 55 / 0.00 /

: ...MOUNTAIN AREAS... 

: 

WRIC1: * WRIGHTWOOD 5980 : 73 / 37 / 0.00 / 

CTL : CRESTLINE 4650 : M / M / M / 

BBLC1: * BIG BEAR LAKE 6790 : 74 / 37 / 0.00 / 

IDYC1: * IDYLLWILD 5380 : 86 / 48 / 0.00 / 

PLRC1: * PALOMAR MOUNTAIN 5550 : 83 / 63 / 0.00 / 

JUL : JULIAN 4240 : 84 / 65 / 0.00 / 

CUYC1: * CUYAMACA PARK 4870 : 84 / 61 / 0.00 / 

MTLC1: * MT. LAGUNA 5760 : 76 / 60 / 0.00 / 

CZZ : CAMPO 2615 : 88 / 61 / 0.00 / 

: 

: ...DESERT AREAS... 

: 

: : * HESPERIA 3290 88 / 51 / 0.00 / 

PSP : PALM SPRINGS 425 : 100 / 77 / 0.00 / 

IDOC1: * INDIO -21 : 100 / M / 0.00 / 

TRM : THERMAL -117 : 98 / 55 / 0.00 / 

BROC1: BORREGO 805 : 102 / 67 / 0.00 / 

TYEC1: * TWENTYNINE PALMS 1975 : 93 / 54 / 0.00 / 

: 
, 

79

Type 4. “East-West Ridge Axis to The North” Event (Subsidence Driven) 

On 3 September 1998 very hot conditions developed as far west as the coastal plain. Lindbergh Field 

(KSAN) hit 92, and some coastal temperatures were over 100. There was an east-west ridge axis to the 

north, common for very warm days. The KLAX-KLAS surface pressure gradient was well under the 

“weak category” upper bound of 3.0 mb onshore, reporting in at -2.2, and KLAX-KTPH was around -5.0 

mb offshore, allowing the warmup to occur all the way to the coast. The 850 mb/Top of the inversion 

temperature couplet was about 24 C/30 C. The winds aloft during the morning at 850 and 700 mb were 

east at about 10 knots, good for warming the coastal and valley areas. Combined with the strong offshore 

gradients it brought the 850 mb potential temperature of near 101 into the valleys and inland coastal 

plain. Minimum temperature most areas were in the 70s, due to the very high temperatures at the top of 

the inversion, 850 mb dew points of around 10 C, and easterly winds below 700 mb. Some high 

temperatures were Newport Beach 92, Oceanside Harbor 92, Lindbergh Field 92, Santa Ana 101, 

Ontario 100, Wild Animal Park 102, Big Bear Lake 71, Hesperia 96, and Thermal 110. Large scale 

subsidence was the main factor driving the strong heating since winds were light. (KNKX soundings on 

next page).

SURFACE PRESSURE GRADIENT 

(1200 UTC) 

DATE: 

2 SEP 1998 

DATE: 

3 SEP 1998 

LAX-SFO -1.9 -0.6 

LAX-WMC -7.3 -5.6 

LAX-TPH -6.7 -5.2 

LAX-LAS -3.8 -2.2 

SAN-YUM 3.4 3.5 

Table C3. Surface pressure gradient data 

Figure C7. 1200 UTC 3 September 1998 500 mb heights in decameters, temperatures in degrees C, and winds in knots 

(left), along with station plots and mean sea level pressure in mb (right).
, 

80

Fig. C8. KNKX sounding data. (Note that strong downslope warming can occur with 

clouds and limited sunshine). 
, 

81

SURFACE PRESSURE GRADIENT 

(1200 UTC) 

DATE: 

2 SEP 1998 

DATE: 

3 SEP 1998 

LAX-SFO -1.9 -0.6 

LAX-WMC -7.3 -5.6 

LAX-TPH -6.7 -5.2 

LAX-LAS -3.8 -2.2 

SAN-YUM 3.4 3.5 

Table C3. Surface pressure gradient data 

ZCZC LAXSTPSAN 

TTAA00 KSGX 040023 AMD 

:SOUTHWESTERN CALIFORNIA TEMPERATURE AND PRECIPITATION SUMMARY 

:NATIONAL WEATHER SERVICE SAN DIEGO CA 

:530 PM PDT THU SEP 03 1998 

: 

:***** UPDATED ***** 

: 

: TODAYS HIGH AND OVERNIGHT LOW TEMPERATURES AS OF 5 PM TODAY. 

: * DENOTES 24 HOUR HIGH AND LOW TEMPERATURES AS OF 4 PM TODAY. 

: OCCASIONALLY THE HIGH TEMPERATURE MAY HAVE OCCURRED AFTER 

: 4 PM THE PREVIOUS DAY. 

: 

:PRECIPITATION FOR THE PAST 24 HOURS. M DENOTES MISSING. 

: T DENOTES TRACE OF PRECIPITATION + DENOTES NOT AVAILABLE 

: 

.B SAN 0903 P DH16/TX/TN/PPD/SD 

: 

: ID STATION ELEV HIGH LOW PCPN / 

SNODEP 

: FEET 

INCHES 

: 

: ...COASTAL AREAS... 

: 

ANAC1: * ANAHEIM 335 : 99 / 75 / 0 

STAC1: * SANTA ANA 135 : 101 / 72 / 0 

NZJ : EL TORO 383 : 99 / 71 / 0.03 

3L3 : * NEWPORT BEACH 10 : 92 / 78 / 0 

: * SAN JUAN CAPISTRANO 730 95 / 71 / 0.03 

L34 : * OCEANSIDE HARBOR 10 : 92 / 70 / 0 

VSTC1: * VISTA 330 : 100 / 71 / 0.18 

CRQ : CARLSBAD AIRPORT 328 : 95 / 73 / 0.01 

DMRC1: * DEL MAR 100 : 90 / 72 / 0.21 

NKX : MIRAMAR 477 : 95 / 73 / 0.09 

MYF : MONTGOMERY FIELD 420 : 95 / 75 / 0.07 

SAN : SAN DIEGO 15 : 92 / 72 / 0.03 

CDOC1: * CORONADO 25 : 92 / 71 / 0.04 

L13 : * CABRILLO NATL MNMT 364 : 92 / 70 / 0.03 

NATC1: * NATIONAL CITY 50 : 96 / 71 / 0.05 

CVAC1: * CHULA VISTA 75 : 93 / 72 / 0 

IPLC1: * IMPERIAL BEACH 15 : 89 / 66 / 0.07 

SDM : BROWN FIELD 525 : 96 / 69 / 0.07 

: 

: ...INLAND AREAS... 

: 

ONT : ONTARIO 943 : 100 / 73 / T 

SBD : SAN BERNARDINO 1125 : 102 / 74 / + 

UCR : RIVERSIDE 986 : 101 / 74 / 0 

BUO : BEAUMONT 2600 : 95 / 71 / 0 

EORC1: * ELSINORE 1285 : 100 / 72 / 0 

HEMC1: * HEMET 1655 : 92 / 70 / 0 

TEMC1: * TEMECULA 1020 : 102 / 74 / 0.02 

FALC1: * FALLBROOK 730 : 102 / 70 / 0.01 

ESCC1: * ESCONDIDO 650 : 101 / 74 / 0.10 

ASCC1: * WILD ANIMAL PARK 750 : 102 / 71 / 0 

RNM : RAMONA 1140 : 98 / 73 / 0.01 

SGX : RANCHO BERNARDO 690 : 94 / 71 / 0.05 

POWC1: * POWAY 525 : M / M / M 

ALPC1: * ALPINE 1860 : 95 / 71 / 0 

SNTC1: * SANTEE 370 : M / M / M 

ELJC1: * EL CAJON 450 : 98 / 75 / 0.10 

LMAC1: * LA MESA 540 : 97 / 74 / 0.50 

SPVC1: * SPRING VALLEY 425 : 97 / 74 / 0.11 

LMGC1: * LEMON GROVE 460 : 97 / 71 / 0 

: 

: ...MOUNTAIN AREAS... 

: 

WRIC1: * WRIGHTWOOD 6200 : 76 / 53 / 0 

CTL : CRESTLINE 4650 : 85 / 67 / 0 

BBLC1: * BIG BEAR LAKE 6790 : 71 / 51 / 0.04 

IDYC1: * IDYLLWILD 5380 : 85 / 62 / 0.03 

FMNC1: FREMONT CANYON 1781 : 102 / 70 / 0 

PLRC1: * PALOMAR MOUNTAIN 5650 : 77 / 58 / 0.26 

JUL : JULIAN 4130 : 87 / 66 / 0 

CUYC1: * CUYAMACA PARK 4600 : 85 / 66 / 0.10 

MTLC1: * MT. LAGUNA 6220 : 73 / 60 / 0.05 

CZZ : CAMPO 2615 : 89 / 66 / 0.19 

: 

: ...DESERT AREAS... 

: 

: * HESPERIA 3290 96 / 66 / 0 

: YUCCA VALLEY 3260 93 / 69 / 0 

PSP : PALM SPRINGS 425 : 105 / 87 / T 

IDOC1: * INDIO -21 : 107 / 87 / 0 

TRM : THERMAL -117 : 110 / 85 / T 

BROC1: BORREGO 500 : 104 / 82 / 0.04 

TYEC1: * TWENTYNINE PALMS 1975 : 100 / 77 / 0 

: 

.END