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NOAA Technical Memorandum NWS ER-106

CLIMATOLOGY OF STORM DATA EVENTS

FOR THE NWS MOUNT HOLLY 

FORECAST AREA

Alan Cope

National Weather Service Office

Mount Holly, NJ

Scientific Services Division

Eastern Region Headquarters

Bohemia, NY

February 2016
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Table of Contents

Abstract ................................................................................................. 1

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

2. Data and Methodology .......................................................................... 4

 

 2.1 Data Sources ................................................................................. 4

 2.2 Quality Control .............................................................................. 5

 2.3 Data Analysis ............................................................................... 5

3. Results ............................................................................................. 8

 3.1 General Results .............................................................................. 8

 3.2 Cool Season Events ........................................................................ 8

 3.3 Warm Season Events ........................................................................ 15

4. Summary and Conclusions ..................................................................... 21

Acknowledgements ................................................................................. 22

References .......................................................................................... 22

Figures...............................................................................................
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ABSTRACT

Storm-related and other significant weather events in the National Weather Service Mount Holly 

forecast area, mainly during the period from 1993 to early 2014, have been examined to 

determine temporal and spatial patterns of their occurrence. Cool-season events include winter 

storms, heavy snow and ice storms, as well as less severe events such as “winter weather” and 

dense fog. Related phenomena that occur entirely or mainly during the cool season, such as high 

winds, extreme cold and coastal flooding, are also covered. Warm-season events include severe 

thunderstorms (damaging winds and large hail), tornadoes, flash floods, lightning damage, 

excessive heat and drought. Numerous graphs are presented to show the frequency of events by 

year, month, time of day and areas (counties) affected. These results should provide a better 

understanding of the likelihood and distribution of significant weather events in the region 

served by the Mount Holly National Weather Service Forecast Office. 

_________________________________________________

1. Introduction

Forecasters at the National Weather Service

(NWS) Forecast Office in Mount Holly, NJ

(WFO PHI) are responsible for issuing 

watches, warnings, and advisories for many 

types of potentially hazardous weather 

phenomena that can occur within their

geographic area of responsibility. The 

decision to issue a warning or related 

product is usually based on several factors, 

including recent observations, numerical or 

statistical guidance, and the forecaster’s own 

professional experience. Of course, this 

experience is gained over a number of years

and can vary considerably from one 

forecaster to another. This study is 

motivated in part by the belief that a good 

knowledge of the climatology of one’s 

forecast area can at least partially 

compensate for the lack of experience, e.g., 

for a forecaster new to the area. 

Climatological records in the NWS tend to 

focus on normals and extremes in 

temperatures and precipitation amounts. 

However, standardized records of many 

other weather phenomena have been 

maintained since the mid-1990s by the 

National Centers for Environmental Data 

(NCEI). Although climatological normals 

are typically based on a 30-year period of 

record, this study’s somewhat shorter period

(generally 21 years) should be sufficient to 

derive some useful information about the 

temporal and spatial distribution of 

significant weather events within the WFO 

PHI forecast area. At WFO PHI, Storm Data 

records have been produced primarily by 

one dedicated individual since 1994, and the 

quality of these records is believed to be 

consistently high. 

The WFO PHI forecast area covers 34 

counties in New Jersey, eastern 

Pennsylvania, Delaware, and northeast 

Maryland, as shown in Fig. 1. Elevation 
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ranges from sea level along the New Jersey 

and Delaware coasts and the shores of 

Delaware and Chesapeake Bays to around 

2000 feet MSL in the Pocono Mountains of 

Carbon and Monroe Counties in 

Pennsylvania. Approximately the 

southeastern half of the area is a rather flat 

coastal plain, while the northwestern half 

consists of rolling piedmont hills rising up to 

the mountains in the far northwest. These 

two regions are separated by the Fall Line 

which runs from northeast New Jersey 

southwestward across northwest 

Philadelphia and into the northeast corner of 

Maryland (see Fig. 1). 

It should be noted that the WFO PHI 

forecast area took its present form in 1995 as 

part of the NWS modernization and 

restructuring program during the 1990s. 

Prior to 1995 the forecast area included the 

eastern half of Pennsylvania and the 

southern half of New Jersey, but did not 

include northern New Jersey or any part of 

Delaware or Maryland. However, all records 

used for this study correspond to the current 

forecast area. 

The next section of this document (Data and 

Methodology) will describe the data sources, 

quality control and analysis methods used in 

this study. The following section (Results) 

will describe overall results from the study, 

followed by results for specific Storm Data 

phenomena; first those that occur primarily 

in the cool season and then those mainly 

associated with warm weather. The final 

section will summarize conclusions from the 

study.

2. Data and Methodology

2.1 Data Sources

The National Centers for Environmental 

Information (NCEI; formerly the National 

Climatic Data Center, NCDC) of the 

National Oceanic and Atmospheric 

Administration (NOAA) maintain a Storm 

Events Database of significant weather 

events in the United States. The database 

includes records of 48 types of events as

defined by NWS directive (National 

Weather Service 2007) from 1996 to the 

present. In addition, records of severe 

thunderstorm winds and large hail extend 

back to 1955, and tornado records go back 

to 1950. Event data and descriptions are

submitted monthly by local NWS field 

offices for their area of responsibility, to be

included in the NWS Storm Data

publication as well as the Storm Events 

database. This database is available online at 

http://www.ncdc.noaa.gov/stormevents/.

All available storm events within the WFO 

PHI forecast area from January 1950

through March 2014 were downloaded from 

the above NCDC web site. To supplement 

the data beyond convective severe weather, 

additional reports from local hard-copy 

Storm Data records for 1993-95 were added 

to form a 21-year database of multiple event 

types, containing well over 33,000 records. 

Cool season events generally begin in late 

1993 and continue through March 2014, 

while most warm season events include the 

calendar years 1993 through 2013.

Although records of severe convective 

events extend back to the 1950’s, only 
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records from 1984 onward were used for 

damaging winds and hail. This provided an 

even 30-year period of record for those 

events. Also, a significant increase in those 

reports within the study area was noted in

the mid- to late-1980s, after the NWS began 

a local storm warning verification program 

in 1979 (Pearson and David 1979). This 

increase in the 1980s was also noted in other 

parts of the U.S. by Hales (1987). For 

tornadoes, although the quality of some 

early reports is suspect, the full record back 

to 1950 was used owing to the combined 

rarity and significance of those events. 

2.2 Quality Control

Since the early to mid-1990s there have 

been numerous changes to the way Storm 

Data events are reported. Also, different 

software has been used over time for data 

entry and formatting. One of the main issues 

with the data record is the changes in 

allowable event types over the years; the 

current list of 48 events types was 

established only in 2007 (National Weather 

Service 2005). For example, prior to 2003 

there are numerous entries for “Snow”, 

“Wintry Mix” and “Freezing Rain”, but 

from 2003 onwards these were all listed as 

“Winter Weather”. For this study, different 

types of winter weather prior to 2003 were 

similarly combined. As another example, no 

coastal flood events were reported from 

December 2002 through October 2005; 

instead they were reported as “Astronomical 

High Tide”. Many other previously allowed

event types have been eliminated, such as 

“Unseasonably Warm”, “Below Normal 

Precipitation”, “First Snow”, etc.; none of 

these were included in the current study. 

Of the 48 types of storm events, the 

following 12 types which occur exclusively 

or predominantly during the cool season 

(roughly November through March) were 

chosen for more detailed analysis: winter

storm, heavy snow, ice storm, blizzard, 

winter weather, extreme cold, wind chill, 

high wind, strong wind, coastal flood, 

astronomical low tide (blowout tide) and

dense fog. Certain cool-season events such 

as freezing fog, lake-effect snow and sleet 

were excluded because they are very rare or 

non-existent in the WFO PHI forecast area. 

Eleven predominantly warm-season types 

were also chosen: thunderstorm wind, large 

hail, lightning, tornadoes, funnel cloud, flash 

flood, flood, heavy rain, drought, wildfire 

and excessive heat. Hurricanes, 

waterspouts, rip currents and other marine 

and tropical phenomena were excluded due 

to the very limited number of reports 

available. Table 1 shows the events selected,

their predominant season, period of record 

for each event type, and total number of 

events. 

A few reports from the database were 

identified as duplicates; certain others

lacked a time stamp. After screening for 

incomplete record periods and missing data 

elements, and removing event types deemed 

to be of lesser significance or too infrequent 

for valid results, about 28,600 records were 

retained for further analysis. 

2.3 Data Analysis

Storm Data events are designated as either 

zone based or county based (see Table 1). 

Zone-based events are mainly the more 

widespread cool-season events such as 
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winter storms, coastal flooding, etc., while 

county-based events are localized, mainly 

warm-season events such as damaging 

thunderstorm winds, flash floods, etc. On 

any given day, the maximum number of 

zone-based verifications, i.e., events, is 

limited by the number of zones (actually 

counties in this study; see below), but there 

is no such limit for county-based events

since there can be multiple events in one or 

more counties.

In the WFO PHI forecast area, forecast 

zones boundaries mostly correspond to 

counties, except for certain counties that are 

split into two separate zones. The five NJ 

and DE Atlantic Coastal counties are each 

divided into an inland section and a 

relatively narrow strip along the immediate 

coastal area. These five coastal zones have 

been in place for the entire period of this 

study. Four other inland counties 

(Burlington in NJ and Chester, Montgomery 

and Bucks in PA) are split into two zones, 

but these splits occurred during the study 

period. For the best overall consistency in 

this study, all Storm Data events, whether 

zone-based or county-based, are located by 

county rather than zone. This may result in 

the loss of some information in the 

immediate coastal areas. 

Records for each event type were stratified 

by year, month, hour (mainly for shorterduration local events) and location by 

county. From these results, the number and 

percentage of event-days per month was 

computed, as well as the frequency 

distribution of event days according to the 

number of events per day. Analysis of the 

data was done using several locally-written 

BASIC computer programs and standard 

spreadsheet software. The latter was also 

used to create tables and graphs for each 

weather event type. Maps of event 

distributions by county were created using 

the web-based GPS Visualizer program

[http://www.gpsvisualizer.com].

3. Results

3.1 General Results

The most numerous event types in the WFO 

PHI area (Fig. 2) were excessive heat, 

followed by damaging thunderstorm winds, 

winter weather (below warning criteria), and 

strong non-convective winds. Most events 

types showed considerable year-to-year 

variability (see individual descriptions 

below), while combined events (Fig. 3) 

showed less variability but perhaps a gradual

rising trend. 

The graph of all events by month (Fig. 4a)

shows a bi-modal distribution with peaks in 

mid-summer and mid-winter. July had the 

most events, mainly excessive heat, while 

October had the least. Figures 4b and 4c

show the monthly distributions of combined 

cool and warm season events, respectively. 

The most frequent cool-season events are 

winter weather (below winter storm criteria) 

and strong winds (below high wind criteria). 

The most frequent warm-season events are 

excessive heat and damaging thunderstorm 

winds. 

Convectively driven events such as hail and 

flash floods showed a strong seasonal and 

diurnal signal, while most cool-season 

events showed strong seasonal but much 

weaker diurnal signals. Winter 
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weather/storm events increased from south 

to north as expected, while convectively 

driven events were more evenly distributed 

across the study area. 

Figure 5 shows the distribution of all events 

by county across the WFO PHI area. There 

does not seem to be any naturally based 

pattern to the distribution, although there are 

such patterns for many specific event types, 

as will be shown below. This distribution

may be explained partly by county area and 

population, which may affect county-based 

event types but not zone-based events. Note 

the higher number of events for counties 

around Philadelphia, and the lower number 

for the sparsely populated Maryland Eastern 

Shore. Burlington County, NJ, has the 

greatest number of events (1213) and it is 

also the location of WFO PHI, which 

suggests that proximity to the weather office 

might also be a factor. 

3.2 Cool Season Events

3.2.1 Winter Storms

For purposes of Storm Data, the term 

“winter storm” refers to an event where 

there are hazards from multiple precipitation 

types, including snow, sleet and freezing 

rain (National Weather Service 2007). If 

precipitation is all snow, then a heavy snow 

event may be recorded; if all freezing rain, 

an ice storm. The combined total amount 

must exceed predefined warning thresholds,

which may vary from one part of the 

forecast area to another. There is a degree of 

subjectivity in determining whether mixed 

precipitation exceeds the criteria, especially 

if significant sleet is involved, since there is 

no formal warning criterion for sleet and no 

local criterion is used at WFO PHI.

A total of 1342 winter storm events occurred 

during the study period. Figures 6a through 

6e show the distribution of events by year, 

month, event size and area. The number of 

events per season varies from less than 10 in 

2001-02 and 2011-12 to over 100 in 1993-

94, 2000-01 and 2002-03 (Fig. 6a). They 

can occur any time from late October 

through early April, but are most frequent 

during February (Fig. 6b). However, Fig. 6c

shows the chance of a winter storm event 

somewhere in the forecast area is about the 

same (just under 8 percent) in both January 

and February. Comparing Figs. 6b and 6c

shows that February has 491/47 = 10.5 

events (counties affected) per storm day on 

average, while January has 346/50 = 6.9 

events per day, so February winter storm 

days tend to be somewhat more widespread.

Overall, the distribution of winter storm 

“size”, or number of counties affected, 

ranges from one county to the maximum of 

34 counties in the WFO PHI forecast area 

(Fig. 6d). However, most storm days affect 

four counties or less. The frequency of 

winter storm events decreases sharply from 

north to south across the area (Fig. 6e), as 

would be expected from both latitude and 

elevation.

3.2.2 Heavy Snow

As mentioned above, a heavy snow event is 

a winter storm in which the hazard is due to 

snow only, with snow exceeding specific 

thresholds. For the WFO PHI forecast area, 
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the thresholds vary depending on location: 

either 4 or 6 inches in 12 hours, or either 6 

or 8 inches in 24 hours. The lower values 

apply to areas mainly south of Philadelphia 

with higher values to the north.

A total of 823 heavy snow events occurred 

during the study period, indicating that pure 

snow events occur at about three-fifths the 

rate of mixed precipitation events. Figures 

7a to 7e show the distribution of heavy snow 

events by year, month, etc. The annual 

variation (Fig. 7a) is again quite large. 

Heavy snow events can occur from October 

through April (Fig. 7b), but are most 

common in February. February also retains a 

slight edge in the number of heavy snow 

days per month (Fig. 7c). Most heavy snow 

days have five or less events (Fig. 7d); 

however, “big” days with more than 20 

events account for more than half of all 

heavy snow events. The geographic 

distribution (Fig. 7e) shows that heavy snow 

frequency varies by about a factor of three 

from north to south across the WFO PHI

forecast area. 

3.2.3 Ice Storms

Ice storms involve ice accretion on trees and 

power lines as the primary hazard. To 

qualify as an ice storm the accretion must 

exceed a locally-defined threshold. For the 

WFO PHI forecast area during the study 

period, this threshold was generally 1⁄4 inch 

of ice. However, starting with the winter of 

2008-09 the threshold was raised to 1⁄2 inch 

for the seven northernmost counties. 

There were a total of 147 ice storm events 

during the study period; they are somewhat 

rare compared to heavy snow or mixed 

precipitation type events. Of this total, about 

half occurred during the winter of 1993-94 

(Fig. 8a). Several years had no events. The 

ice storm “season” is confined to November 

through March (Fig. 8b), with January 

having the most events by a considerable 

margin. January also has the most ice storm 

days. (These results are likely somewhat 

skewed by the one dominant winter noted 

above.) Ice storm days tend to cover smaller 

regions within the forecast area (Fig. 8d); 

with one exception, they all affected less 

than 10 counties. Geographic distribution is 

shown in Fig. 8e, indicating the vast 

majority of ice storm events occur north and 

west of Philadelphia. This could be due to 

the increased likelihood of milder oceanic 

air moving inland across the coastal plain.

3.2.4 Blizzards

A blizzard is a type of winter event defined 

in terms of wind speed and visibility. 

Specifically, a blizzard event requires “...(1)

sustained winds or frequent gusts 30 kts (35 

mph) or greater, and (2) falling and/or 

blowing snow reducing visibility frequently 

to less than 1/4 mile...”, which last for at

least three hours (National Weather Service 

2007). Blizzards are quite rare in the WFO 

PHI forecast area: during the study period 

there were only five blizzard days, affecting 

a total of 56 counties. All the blizzards 

occurred during the period December 

through March. Because of the rarity, 

detailed plots of blizzard distributions are 

not likely meaningful and are not shown. 
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However, blizzard events are included in the 

combined winter storm section below. 

3.2.5 Combined Winter-Storm 

Verifying Events

Figures 9a through 9e show the distributions 

of all 2368 “winter storm” type events 

combined, including heavy snow, ice storms 

and blizzards. Results for this combination 

are presented primarily for comparison with 

the combined “winter weather” events in the 

following section. There is a great deal of 

inter-annual variability in overall winter 

“storminess”, but no clear trend towards 

increasing or decreasing. Except for ice 

storms in the 1993-94 season, these 

combined totals are dominated by winter 

storm events (mixed precipitation-type) and 

heavy snow events. February is the overall 

stormiest month (Fig. 9b) although, again, 

January has somewhat more days with at 

least one winter-storm type event. For both 

months, about 1 in 7.5 days has at least one

event (Fig. 9c). 

The distributions by event size (Fig. 9d) and 

by county (Fig. 9e) are consistent with 

results already shown; i.e., most days with 

winter-storm type events only have a few, 

and there is about a four-fold increase from 

south to north in the frequency of events. 

The number of events for Burlington 

County, NJ (81) seems slightly high

compared to surrounding counties, but this 

could be partly explained by the relatively 

large area of the county, or perhaps the 

location of the WFO PHI forecast office in 

that county. 

3.2.6 Combined Winter Weather 

Events

There is a secondary class of winter 

precipitation events which fall in the 

category of “winter weather”. These are 

events not severe enough to verify a winter 

storm warning, but still able to pose a 

significant hazard, and would verify a winter 

weather advisory in the zones/counties 

where they occur. Prior to 2003 these 

events were classified as “snow”, “freezing 

rain” or a “wintry mix”, but subsequently 

they have been merged into the single 

“winter weather” category. 

In the WFO PHI forecast area, the local 

criteria for a winter weather advisory vary 

somewhat from north to south. Generally, 

locations north of Philadelphia require at 

least three inches of snow in 12 hours, while 

locations south require at least two inches. A 

trace or more of freezing rain verifies an 

advisory anywhere. 

During the period of study there were a total 

of 3866 winter weather events, or about twothirds again as many as the combined winter 

storm events. The distributions of winter 

weather events are shown in Fig. 10a 

through Fig. 10e. There seems to be 

somewhat less inter-annual variability with 

winter weather versus winter storms (Fig. 

10a). Aside from the variability, there also 

appears to be an overall increase in the 

number of events per year, whereas no such 

trend was noted for winter storms. The 

reason for this is not clear; one possible 

explanation is increasing societal sensitivity 

to smaller or less severe winter events.
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Figure 10b shows the monthly distribution 

of winter weather events. January has the 

most events followed by February and 

December, in contrast with winter storm 

events which are most favored in February.

The same pattern holds as well for winter 

weather days with at least one event (Fig. 

10c), but such days occur at about the rate of 

1 in 5 for both January and February. Figure 

10d shows a pattern similar to Fig. 9d for 

winter storms, but is slightly less skewed 

toward the low end. In both cases the chance 

of a day with 14 or less events is about the 

same (50 percent) as the chance of a day 

with 15 or more. Finally, the areal 

distribution of winter weather events (Fig. 

10e) is similar to winter storms, with a 

three- to four-fold increase from Delmarva 

northward to the Pennsylvania Pocono 

Mountains.

3.2.7 Extreme Cold

Extreme cold events are characterized by a 

wind chill temperature at or below certain

locally defined threshold values which 

depend on the climatology of a particular 

region. For the WFO PHI forecast area, the 

wind chill threshold is -25°F. An extreme 

cold event verifies a wind chill warning for 

that time and area. 

Distributions of extreme cold events are 

depicted in Figs. 11a through 11e. These 

events are somewhat rare, averaging slightly 

less than 20 per winter season. Many years

had no events at all (Fig. 11a). January is by 

far the dominant month, containing over 

two-thirds (69 percent) of all events (Fig. 

11b) and 14 of 20 total extreme cold days 

(Fig. 11c) during the period of study. When 

extreme cold occurs it often affects all 34 

counties (Fig. 11d). Events are rather 

evenly distributed across the forecast area 

(Fig. 11e) with inland areas slightly favored.

3.2.8 Wind Chill

Less extreme cold events are designated as 

“wind chill” events. These correspond to 

wind chill advisories rather than warnings, 

and the WFO PHI advisory threshold varies 

from -15°F in the north to -10°F central and 

south. Wind chill events have been recorded 

only since 2006 or less than half the period 

of study, with 236 occurrences compared to 

over 400 extreme cold events. Therefore, 

for the most part, detailed wind chill results 

are not included here. However, it can be 

noted that the monthly distribution is similar 

to extreme cold, while the geographic 

distribution shows more of a south to north 

gradient. Also, there was only one wind chill 

day affecting the entire forecast area; most 

days affected only a few counties. 

3.2.9 High Wind

High wind events are defined as “Sustained 

non-convective winds of 40 mph or greater 

lasting for 1 hour or longer or winds 

(sustained or gusts) of 58 mph for any 

duration...” (National Weather Service 

2007). In practice for the WFO PHI area it is 

almost always the wind gusts that meet or 

exceed criteria. Expectation of these 

conditions would warrant issuance of a High 

Wind Warning. Current Storm Data rules 

state that high wind events should not 
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include convective storms or tropical 

cyclones; however some past hurricanes 

have generated high wind entries (National 

Weather Service 2007).

Figures 12a through 12e show the 

distribution of high wind events for the 

WFO PHI area. There is considerable 

variability from year to year (Fig. 12a) with 

the 1995-96 winter season standing out for 

its large number of events. Being nonconvective, these high winds are primarily a 

cool season phenomenon (Fig. 12b), 

occurring mostly from November through 

March, but also as early as September or as 

late as June. However, most of the 

September events are due to hurricanes 

Floyd (1999) and Isabel (2003). The number 

of high wind days (Fig. 12c) shows an 

approximately even distribution from 

November through March. Most high wind 

days have relatively few events (Fig. 12d); 

115 of the 142 days have 6 events or fewer. 

The geographic distribution (Fig. 12e) 

shows high wind events most common over 

coastal section of New Jersey and Delaware, 

and generally decreasing farther inland. 

There is no apparent increase over the 

higher terrain of Carbon and Monroe 

counties in Pennsylvania (18 and 15 events, 

respectively) as one might expect. Certain 

areas of the Maryland Eastern Shore seem to 

suffer from a lack of reports. 

3.2.10 Strong Wind

Strong wind events are similar to high winds 

but less severe. The criteria are sustained

winds of 25 to 39 mph or wind gust of 46 to 

57 mph. When these conditions are 

expected, a wind advisory would normally 

be issued for the affected area. 

A total of 3375 strong wind events occurred 

during the study period, or about 4.5 times 

the number of high wind events. This ratio 

would be somewhat higher except that 

almost no strong wind events were recorded 

for the years 1993-1996. Figures 13a 

through 13e show the distribution of strong 

wind events; overall they are similar to high 

wind events. Figure 13a shows somewhat 

less year to year variation than Fig. 12a. The 

cool season dominance remains (Fig. 13b) 

although these events have occurred in all 

months. The distribution of strong wind 

days is somewhat smoother, perhaps due 

simply to the larger sample size. The 

proportion of days with several strong wind 

events, e.g., more than six, is significantly 

greater than for high winds (compare Fig. 

13d with Fig. 12d). The geographic 

distribution again favors the Atlantic Coast 

as well as the western shore of Delaware 

Bay. 

3.2.11 Coastal Floods

Coastal flooding occurs when waters along 

the ocean shore, bay shore or other tidal 

areas rise above normal levels due to 

meteorological effects (primarily wind) and 

flood adjacent coastal land areas. In Storm 

Data coastal flooding is classified as minor, 

moderate or severe. Within the WFO PHI 

area, coastal or tidal flooding can occur 

along the Atlantic coast, the shores of 

Delaware and Chesapeake Bays, and along 

the tidal portion of the Delaware River as far 

upstream as Trenton, NJ. This means that 
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22 of 34 counties in the study area are 

vulnerable to, and have experienced, coastal 

flooding under suitable conditions.

Figures 14a to 14e show the distributions of 

769 coastal flood events during the period of 

study. The year-to-year variation (Fig. 14a) 

ranges from no events in the 1994-95 season 

to 111 events in 2006-07 (July to June). 

These events are generally more frequent 

during the cooler and transitional months 

(Fig. 14b), as they are often associated with 

East Coast extra-tropical cyclones, or 

“nor’easters”. The high value for October 

and the low value for February are likely 

just random variations due to limited sample 

size. (October does include numerous 

reports from post-tropical storm Sandy.) Fig. 

14c shows a similar pattern for coastal flood 

days, except that March is just slightly 

behind October. This results from March 

having more coastal flood days with events 

affecting just one to three counties. Overall, 

about half of all coastal-flood days have

three events or less (Fig. 14d). The 

geographic distribution in Fig. 14e shows 

coastal flooding most frequent along the 

New Jersey Atlantic shore (except 

northernmost Monmouth County), followed 

by Delaware Bay and the lower (tidal) 

Delaware River. Tidal flooding is much less 

frequent along the upper eastern shore of 

Chesapeake Bay. 

3.2.12 Blowout Tides

Abnormally low or “blowout” tides occur 

when strong, sustained northwest winds 

blow water out of the lower Delaware River, 

Delaware Bay and the back bays along the 

New Jersey and Delaware coast. This often 

occurs on the “back side” of a strong 

nor’easter. The resulting low water levels 

are a hazard to marine navigation, especially 

small boats in normally shallow water. The 

same 22 counties that experience tidal 

flooding have also experienced blowout

tides, except for Mercer County, NJ, with no 

blowout tide reports during the study period. 

Figures 15a to 15e show the distributions for 

236 blowout tide events. These events were 

not recorded in Storm Data before 2007 

(Fig. 15a) but have occurred every year 

since. During that same period (2007-2014) 

there were 220 coastal flood events, 

suggesting the two types occur with about 

equal frequency. The monthly frequency 

rises steadily through the winter months 

(Fig. 15b) but falls sharply in the spring; 

thus blowouts are much more confined to 

the cool season than coastal floods. 

Blowout tide days show a very similar 

pattern (Fig. 15c) with February having the 

most days by a wide margin. Details of these 

distributions should not be taken too literally 

owing to the rather small sample size. 

Blowout days do seem to affect more 

counties at once; compare Fig. 15d with 

14d. The geographic distribution of blowout

tides is also quite different from coastal 

flooding, with blowouts most common along 

the lower Delaware River and Delaware Bay 

rather than the Atlantic Coast (Fig. 15e). 

3.2.13 Dense Fog

Fog is considered to be “dense” when it 

reduces visibility to one-quarter mile or less. 

For Storm Data records there is the 
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additional requirement that it must 

significantly impact transportation or 

commerce (National Weather Service 2007). 

Therefore the results presented here should 

not be interpreted as a general fog 

climatology, since they may be skewed 

towards times and areas of increased traffic 

or other commercial activity.

The distribution of dense fog events is 

shown in Figs. 16a to 16f. The inter-annual 

distribution (Fig. 16a) is quite variable, 

ranging from zero events in some years to a 

maximum of 128 events from July 1999 to 

June 2000. Figure 16b shows the 

distribution of events by month, with 514 

out of the 592 total (87 percent) occurring 

during the months of November through 

February. The monthly distribution of dense 

fog days, with at least one event (Fig. 16c), 

is similar but shifted to slightly earlier in the 

cool season. Half of all dense fog days 

affected eight counties or less (Fig. 16d) but 

there were several days with events in all 34 

counties. Fig. 16e shows the distribution of 

starting hours for all events, which may 

typically continue for several hours. Dense 

fog in the WFO PHI area is mostly a 

nocturnal phenomenon, especially 

considering the longer nighttime period 

during the cool season. Finally, dense fog 

events are well distributed throughout this 

study area (Fig. 16f), with perhaps some 

preference for the Wilmington-PhiladelphiaTrenton urban corridor. 

3.3 Warm Season Events

3.3.1 Thunderstorm Winds

Measured wind gusts of 50 kts or greater 

associated with thunderstorms are 

considered severe and are routinely entered 

into Storm Data. However, in most cases the 

actual wind speed is not known and the 

severe wind report is based on damage to 

trees and power lines or other serious 

property damage. These reports may include 

downbursts and gustnadoes as well. 

Thunderstorm winds are distinguished from 

high winds (section 3.2.9) by their 

association with deep convection and 

(usually) lightning. 

Figures 17a through 17f show the 

distribution of thunderstorm wind events 

within the WFO PHI study area. As noted 

earlier, records of these events actually 

extend back to 1955, but only those from 

1984 onward were used in this study. The 

year-to-year graph (Fig. 17a) shows 

considerable variability but with a definite 

trend towards more reports. Some of this 

increase is likely due to non-meteorological 

factors such as increasing population, better 

communications, more aggressive 

verification, etc. The monthly distribution 

(Fig. 17b) shows a preference for early- to 

mid-summer (June, July) with a decrease 

into August and especially into September. 

On average, about one in every five days 

(~20 percent) of June and July has at least 

one severe wind report (Fig. 17c). However 

convective wind events can occur in all 

months of the year. These events can also 

occur any time of day or night (Fig. 17d), 

but are most frequent from late afternoon 

through early evening. Over half (464 of 

842) of thunderstorm wind days had three or 

less reports (Fig. 17e), but about one in five 
14

was a “big” day with 10 or more reports. 

The geographic distribution in Fig. 17f does 

not show a clear pattern, although larger 

counties by area tend to have more events. 

About half of the county-to-county 

variability can be explained by county size 

alone. 

3.3.2 Large Hail

Large hail events with hail diameter of 0.75 

inch or greater are routinely entered into 

Storm Data, although hail of smaller 

diameter is also allowed under some 

circumstances. For this study, only hail 

greater than 0.75 inch diameter was 

considered. For most of the study period, 

0.75 inch was the lower limit for “severe” 

hail, i.e., verifying a severe thunderstorm 

warning. In January 2010 the threshold for 

damaging hail was raised to 1 inch

(http://www.nws.noaa.gov/oneinchhail/);

however, hail reports of 0.75 inch or greater 

continued to be entered in Storm Data.

As with thunderstorm winds, the hail 

records extend back to 1955, but only those 

from 1984 on were included here. During 

this time there were 1180 hail events with 

hail diameter greater than 0.75 inch, or 

about one-quarter the number of wind 

reports during the same period. Removing 

hail with diameters less than one inch 

reduces this number by about half to 546 

(546/1180 = 0.463). 

The distribution of hail events is shown in 

Figs. 18a through 18f. The number of events 

has shown an overall increase over the study 

period (Fig. 18a) but especially since 2005. 

The reason for this is not clear except for 

possible factors mentioned above in regard 

to thunderstorm winds. Large hail frequency 

seems to peak in June (Fig. 18b), somewhat 

earlier than winds, and almost vanishes 

during the winter months. However, the 

number of days with large hail actually 

peaks in July (Fig. 18c). The diurnal 

distribution of hail (Fig. 18d) is quite similar 

to thunderstorm winds, with a strong peak in 

the late afternoon/early evening. Most hail 

days have only one or two events (Fig. 18e) 

and there are fewer “big” days (compared to 

wind events) especially for one-inch or 

larger hail size. The geographic distribution 

of large hail (1.00 inch or greater) again 

appears somewhat random (Fig. 18f), 

although there are clearly fewer reports from 

Delmarva and southernmost New Jersey.

3.3.3 Lightning

Thousands of cloud-to-ground lightning 

strikes occur over the United States on many 

days during the warmer months of the year. 

However, only those resulting in deaths, 

injuries or damage are included in Storm 

Data reports. So as with dense fog, the 

results here should not be taken as a general 

lightning climatology. Figures 19a through 

19f show the distribution of these events 

over the WFO PHI study area. 

There has been an overall increase in the 

number of annual lightning events since 

1993 (Fig. 19a). However the two biggest 

years were 2000 and 2001, and since then 

the annual number has generally been steady 

in or near the 40 to 60 event range. 

Lightning events by month (Fig. 19b) show 
15

a steady increase from March through 

August, followed by a precipitous drop in 

September and October with rapid loss of 

solar heating. July and August are 

essentially tied for most lightning event days 

(Fig. 19c) with an average of about one 

event every six days. Lightning is closely 

tied to the convective heating cycle (Fig. 

19d), showing a broad diurnal maximum 

from mid through late afternoon. The great 

majority of days with lightning events had 

three or less events (Fig. 19e), but a handful 

of days had 10 or more. The geographic 

distribution (Fig. 19f) shows no clear 

pattern, but the fact that the three counties 

with the most events are all on the Atlantic 

coast may indicate increased vulnerability to 

lightning in that area. 

3.3.4 Tornadoes

A tornado is “A violently rotating column of 

air, extending to or from a cumuliform 

cloud, or underneath a cumuliform cloud, to 

the ground...” (NWSI 10-1605). 

“Landspouts” and cold-air funnels are also 

classified as tornadoes in Storm Data if they 

reach the ground. However, waterspouts, 

funnel clouds and dust devils are considered 

as separate event types. So-called 

“gustnadoes” are considered to be 

thunderstorm wind events. Only actual 

tornadoes are considered in this study owing 

to the rarity of the other event types 

mentioned above. 

In the Storm Event database the tornado 

record extends back to 1950, a few years 

before the start of the thunderstorm wind 

and hail record. A few of the tornadoes from 

the 1950s and 1960s have suspiciously long 

tracks (e.g., the width of New Jersey) unlike

anything in more recent years. However, it 

was decided to use the entire record since 

tornadoes are relatively rare but significant 

events. 

Figures 20a through 20g show the 

distribution of tornado events in the WFO 

PHI study area. The number of tornadoes 

per year seems to increase through the earlyto mid-1990s and then decrease thereafter 

(Fig. 20a). There is no obvious physical 

cause for such a trend however, so it is 

likely just a random fluctuation. Tornado 

frequency is greatest in July by a wide 

margin (Fig. 20b), although tornadoes have 

been reported in every month except 

December. Interestingly, the month of 

November has had more tornadoes than 

October (20 vs. 19 events, including 11 on 

November 16, 1989), and also more tornado 

days (14 vs. 10). November also showed an 

increase in thunderstorm wind events (Fig. 

17b) suggesting a possible weak secondary 

maximum of severe weather in November. 

As with other convectively driven weather, 

tornadoes are most frequent from midafternoon through early evening (Fig. 20d) 

although they have occurred at all hours. 

The great majority of tornadoes (265/349 = 

75.9 percent) fall into the “weak” EF0 and 

EF1 categories (Fig. 20e), while the 

remainder are mostly EF2s. EF3 tornadoes 

are very rare, and no EF4 or EF5 tornado 

has yet been reported in the WFO PHI area. 

The geographic distribution (Fig. 20f) 

appears to show an axis of higher frequency 

from southeast Pennsylvania southward 

through Delaware. However, this axis 
16

essentially disappears after county size is 

accounted for (Fig. 20g). Finally, out of 231 

tornado days total, three-quarters of them 

produced only one tornado, and 95 percent 

produced three or less (graph not shown).

3.3.5 Funnel Clouds

Funnel clouds often occur under conditions 

that may also produce tornadoes. However 

by definition a funnel cloud does not reach 

the ground and cannot cause direct death or 

injury on the ground. Available funnel cloud 

records for Storm Data go back only to the 

early 1990s, but the total number of reports 

from 1993 through 2013 is similar to that for 

tornadoes (104 vs 115, respectively). The 

monthly, hourly, etc., distributions of funnel 

cloud reports are fairly similar to those for 

tornadoes. Because of their relative 

insignificance, rarity and short record length 

(compared to tornadoes), the distribution 

graphs for funnel clouds are omitted here. 

3.3.6 Flash Flood

A flash flood is a type of flooding 

characterized by rapid rise in water level 

such that it poses a particular threat to life as 

well as property. For Storm Data, a flash 

flood is generally one that occurs within six 

hours of the causative event (heavy rain, 

dam failure, etc.). However, the distinction 

between flooding and flash flooding is not 

always clear, so the preparer of Storm Data 

must sometimes rely on professional 

judgement to distinguish one from the other.

The distribution of flash floods in the WFO 

PHI area is shown in Figs. 21a through 21f. 

There is some suggestion of an upward trend 

(Fig. 21a), especially considering that the 

“biggest” year (1996) was unusual with over 

half of events occurring during the cool 

season (October through March). Due to 

changes in criteria, some of those cool 

season events might now be considered 

floods rather than flash floods. Most flash 

floods occur during the warm season when 

convective rain is dominant (Fig. 21b), 

especially August followed by July, June 

and September. These four months account 

for 901 of the 1157 total events, or about 78 

percent. Flash flood days are far more likely 

in June, July and August than other months 

(Fig. 21c). Flash floods can occur any time 

day or night, but are most frequent from 

mid-afternoon through early evening (Fig. 

21d). This diurnal pattern is similar to other 

convectively driven events, but the 

afternoon/evening maximum is not quite as 

pronounced. Most days with flash flooding 

have only one or two events, with a few 

exceptions (Fig. 21e). Flash flooding is most 

frequent over southeast Pennsylvania and 

adjacent northern Delaware (Fig. 21f), while 

mostly flat terrain along with sandy soil 

limits the frequency of flash flooding over 

the Delmarva peninsula and southern New 

Jersey. It should be noted that certain small 

streams are particularly prone to flash 

flooding, e.g., the Christina River at Coochs’ 

Bridge in northern New Castle County, 

Delaware, which partly accounts for the 

high frequency there. 

3.3.7 Flood

In the most general sense, a flood is any 

inundation by water of a normally dry area; 
17

but for Storm Data it must also pose a threat 

to life and property. Floods, including river 

floods, involve a slower rise in water level 

compared to flash floods, and the time delay 

is generally more than six hours after the 

causative event. 

It is interesting to compare the distribution 

of flash floods with those for floods; the 

latter are shown in Figs. 22a through 22f. 

Over the period of study there were roughly 

equal numbers of events (1239 flood vs. 

1157 flash flood). The distribution by year 

(Fig. 22a) again suggests an increase with 

time. Floods events are more or less evenly 

distributed throughout the year (Fig. 22b; 

compare Fig. 21b). Although classified as 

such in this study (Table 1), it could be 

argued that these slower-acting floods are 

not really warm-season events since they are 

most frequent in March and December. The 

distribution of flood days by month (Fig. 

22c; compare Fig. 21c) also shows a fairly 

random pattern. The start times for flood 

events seem to be rather evenly spread 

around the diurnal cycle (Fig. 22d; compare 

Fig. 21d). The distribution of flood days by 

number of flood events is also quite similar 

to that for flash floods (Fig. 22e; compare 

Fig. 21e). Finally the geographic distribution 

of floods (Fig. 22f) is similar to flash floods, 

except there is a secondary maximum over 

northern New Jersey in Morris and Somerset 

Counties. This is due primarily to frequent 

flooding of the Raritan, Passaic, Millstone 

and other rivers that flow through this area. 

3.3.8 Heavy Rain

A heavy rain event is defined as one not 

associated with a flash flood or flood, but 

causing some kind of damage or other 

economic impact (National Weather Service 

2007). In the WFO PHI Storm Data records, 

heavy rain events seem to be connected 

mainly with urban or poor draining flooding, 

and thus are of lesser significance than 

floods or flash floods. The total number of 

heavy rain events (1221) is similar to that 

for floods, and the event distributions are 

also similar and are not shown here. An 

exception is the geographic distribution (Fig. 

22g) which shows a relatively greater 

proportion of events over the coastal plain of 

southern NJ and the Delmarva region. It is 

generally more difficult for heavy rain to 

produce a flood in that area given the flat 

terrain and sandy soil. 

3.3.9 Drought

Drought is defined as an extended period of 

deficient precipitation resulting in crop 

damage or other adverse impacts on people 

or livestock. The current version of Storm 

Data instructions (National Weather Service 

2007) states that drought events should be 

included for areas classified as D2 or higher 

in the United States Drought Monitor 

(http://droughtmonitor.unl.edu/); however in 

prior years drought conditions have been 

determined by local criteria. Droughts 

typically persist for long periods of time, 

i.e., months, so for any specific zone or 

county there will be only one drought entry 

per month, as long as the drought continues. 

Thus the concept of a “drought day” does 

not apply; one should think of “drought 
18

months” instead when viewing the following 

figures. 

Figures 23a through 23e show the 

distribution of drought events in the WFO 

PHI area. While many years had no drought 

conditions (Fig. 23a), the years 1999 and 

2002 stand out as being especially dry. In 

particular, 2002 had drought conditions over 

almost the entire area during each month 

except December. Drought is most common 

or widespread during the late summer into 

early fall (Fig. 23b) and least likely during 

the spring. Almost half of all September

months (9 of 21) during the study period 

have experienced some drought (Fig. 23c). 

Droughts tend to affect the same areas for 

several months at a time; the area can range 

from a few counties to the entire WFO PHI 

area (Fig. 23d), as in 2002. The geographic 

distribution (Fig. 23e) shows a great deal of 

homogeneity by state, likely because state 

governments often make the official drought 

declarations for their domains.

3.3.10 Wildfire

For purposes of Storm Data, a wildfire is 

considered to be a fire outside of urban areas 

that burns out of control for a period of time, 

and results in one or more fatalities or 

injuries, or causes damage to property. 

Generally speaking, smaller wildfires are not 

included in Storm Data, but the decision to 

include or not depends somewhat on the 

judgement of the person preparing the Storm 

Data entries.

Figures 24a through 24e show the 

distributions of wildfire events over the 

WFO PHI area. There is considerable 

variation from year to year (Fig. 24a), 

ranging from 32 events in 2012 to none in 

2003 and some preceding years. Of course, 

“none” means no events were judged worthy 

of inclusion. There is a strong preference for 

the month of April (Fig. 24b) when stronger 

winds combine with stronger daytime 

heating to produce lower relative humidity, 

drier fuels and more favorable conditions for 

rapid wildfire spread. The months of March 

and April together account for over half of 

all significant wildfires (124 of 208 events). 

Those two months also include about half of 

the total 148 wildfire days (Fig. 24c). Figure 

24d shows the distribution of starting times 

for wildfires, i.e., the time when the fire 

became out of control. (In many cases this is 

likely when the fire was first reported.) 

There is a decided preference for wildfires

during the daytime hours, especially around 

mid-day. Geographically, wildfires occur 

most often in the “pinelands” of southern NJ

(Fig. 24e), with a secondary maximum 

around the Pocono Mountains in northeast 

PA and adjacent northwest NJ. Wildfires 

tend to be fairly isolated events and the vast 

majority of wildfire days (115 of 148) 

involve only one county (graph not shown).

3.3.11 Excessive Heat

Excessive heat is defined as a condition 

where high temperature and humidity 

combine to pose a serious threat to human 

health. This combined effect is measured as 

a Heat Index. These conditions should 

normally be anticipated by an excessive heat 

watch and/or warning. A less extreme 
19

condition, designated simply as “Heat” 

would be anticipated by a heat advisory.

The WFO PHI area is unique in that special 

procedures for assessing heat risk in the 

Philadelphia metropolitan area were 

developed in the mid-1990s by Dr. 

Lawrence Kalkstein (Kalkstein et al. 1996)

at the University of Delaware. These 

procedures have been modified over the 

years since but are still used for Philadelphia 

and nine surrounding counties. Otherwise, a 

heat index threshold of 110°F is used for 

upper Delmarva and southernmost New 

Jersey, while 105°F is used for areas farther 

north in New Jersey and Pennsylvania. 

Before 1996 the term “Heat Wave” was 

used instead of or along with “Excessive 

Heat”. The specific definition of Heat Wave 

is not known for certain, but is probably 

related to the traditional criterion of three or 

more consecutive days with maximum 

temperature reaching or exceeding 90°F 

(American Meteorological Society 2016).

Excessive heat is the most frequent type of 

hazardous weather, i.e., Storm Data

“worthy” event in the WFO PHI area (Fig. 

1). This is partly because heat events tend to 

cover a large area and often last several 

days. For example, if excessive heat occurs 

in ten counties for three days, that would 

count as 30 excessive heat events. 

The breakdown of excessive heat events in 

the WFO PHI area is shown in Figs. 25a to 

25e. The number of events varies widely 

from year to year (Fig. 25a), from none in 

2004 to almost a thousand in 2002, by far 

the greatest number. (Note 2002 was also 

the biggest drought year.) The monthly 

distribution (Fig. 25b) shows 98 percent of 

events occur in the months of June, July and 

August, with almost twice as many in the 

hottest month (July) as in the second hottest 

(August). This pattern holds for excessive 

heat days (Fig. 25c) with about 1 out of 

every 5 days in July experiencing excessive 

heat somewhere in the WFO PHI area. 

Excessive heat days may affect anywhere 

from one county to all 34 counties (Fig. 

25d), but most days affect over half, and 

many have affected all 34. Excessive heat is 

most frequent in the urban heat island of 

Philadelphia (Fig. 25e) and least frequent in 

the higher elevations of the Pocono 

Mountains in northeast Pennsylvania.

4. Summary and Conclusions

This Technical Memorandum has described 

development of a climatology based on 

Storm Data events within the forecast area 

of the National Weather Service Forecast 

Office at Mount Holly, NJ. A total of 23

weather event types were examined, 

including 12 predominantly cool-season 

events and 11 predominantly warm-season 

events. The study area included 34 counties 

covering all or part of four states: New 

Jersey, Pennsylvania, Delaware and 

Maryland. The study period for most event 

types extended from January 1993 through 

March of 2014, or about 21 years. 

Exceptions were tornadoes, thunderstorm 

winds and large hail, for which a longer data 

record was available.

Perhaps the most surprising result from this 

study was the number of excessive heat or 
20

heat-wave events (just over 6000), which 

were by a wide margin the most frequently 

occurring Storm Data event. These heat 

events along with thunderstorm winds 

produced an overall event maximum in July, 

while winter weather and strong (nonconvective) wind events produced a 

secondary maximum in January. Although 

most types of events showed a seasonal 

preference, a few such as areal/river 

flooding were more or less evenly 

distributed throughout the year. 

Most cool season events did not show any 

diurnal preference, although high winds did 

seem to favor the daylight hours to some 

degree while fog was more frequent at night. 

On the other hand, and not unexpectedly, 

convectively driven events such as severe 

weather and flash flooding showed a strong 

diurnal signal with maximum frequency 

from mid- to late-afternoon into early 

evening. These convective events also 

showed some subtle differences in their 

warm-season distribution, with hail most 

frequent in June and flash flooding most 

frequent in August. 

The distribution of winter storm and winter 

weather events across the study area showed 

an expected gradient from south to north, 

while extreme cold events were more evenly 

distributed and high and strong wind events 

showed a preference for coastal areas. 

Convectively driven events such as 

thunderstorm winds, hail and flash flooding 

were more randomly or evenly distributed, 

especially after accounting for county area.

The results of this study should provide 

useful background and familiarization for 

current and future forecasters in the WFO 

PHI forecast area. As part of the data 

analysis, many types of event-days have 

been identified and cataloged; these can be 

the basis for further climate studies, e.g., 

weather patterns associated with severe 

weather days of various magnitudes

(number of events), or a comparison of the 

relative societal impacts of different kinds of 

events. The climatological frequency of 

events can serve as a basis for measuring 

forecast skill (improvement over 

climatology) for those events. Finally it is 

hoped that this study can serve as a model 

for local studies at other forecast offices that 

may wish to take advantage of the growing 

length of record in the Storm Event 

Database. Such studies could help identify 

similarities among and differences between 

neighboring forecast offices.

Acknowledgements

This work would not have been possible 

without the dedicated efforts of the WFO 

PHI staff over the years to collect and record 

information about significant weather events 

in our forecast area. Special credit goes to 

lead forecaster (now retired) Tony Gigi who 

served from 1994 to 2015 as Storm Data 

focal point. During that time he compiled an 

unusually thorough record of these events 

which will serve as a valuable source of 

information for many years to come. Also, 

Brent MacAloney of the NWS Performance 

and Evaluation Branch provided helpful 

information about the history of Storm Data 

methodology and criteria. 
21

References

American Meteorological Society, cited 

2016: “Heat Wave”. Glossary of 

Meteorology. (Available online at http:// 

glossary.ametsoc.org/wiki/Heat_wave]

Hales, J.E., 1987: An examination of the 

National Weather Service severe local storm 

warning program and proposed 

improvements. NOAA Tech. Memo. NWS 

NSSFC-15. 

Kalkstein, L. S., P. F. Jamason, J. S. Greene, 

J. Libby, and L. Robinson, 1996: The 

Philadelphia Hot weather–health watch 

warning system: Development and

application, summer 1995. Bull. Amer. 

Meteor. Soc., 77, 1519–1528.

National Weather Service Instruction 10-

1605, 2007: Storm Data Preparation. 

[Available online from 

http://www.nws.noaa.gov/directives/sym/pd

01016005curr.pdf]

National Weather Service, One Inch Hail 

(2010). Retrieved 25 January 2016, from 

National Weather Service: 

http://www.nws.noaa.gov/oneinchhail/

Pearson, A. D. and C. L. David, 1979: 

Tornado and Severe Thunderstorm Warning 

Verification. Preprints, Eleventh Conference 

on Severe Local Storms, Kansas City, Amer. 

Meteor. Soc., 567-568.
22

TABLE AND FIGURES

Table 1. Storm Data event types included in this study. 

 Predominant County or Period of Total #

Event Type Season Zone-based Records of Reports

Winter Storm Cool Zone Oct 1993 to Mar 2014 1342

Heavy Snow Cool Zone Oct 1993 to Mar 2014 823

Ice Storm Cool Zone Oct 1993 to Mar 2014 147

Blizzard Cool Zone Mar 1993* to Mar 2014 56

Winter Weather** Cool Zone Oct 1994 to Mar 2014 3866

Extreme Cold Cool Zone Oct 1993 to Mar 2014 405

Wind Chill Cool Zone Jan 2005 to Mar 2014 236

High Wind Cool Zone Oct 1993 to Mar 2014 754

Strong Wind Cool Zone Oct 1993 to Mar 2014 3375

Coastal Flood Cool Zone Aug 1993 to Mar 2014 769

Blowout Tide Cool Zone Jan 2004 to Mar 2014 236

Dense Fog Cool Zone Jan 1994 to Mar 2014 592

T-storm Wind Warm County Jan 1984 to Dec 2013 4787

Large Hail*** Warm County Jan 1984 to Dec 2013 1180

Lightning Warm County Apr 1993 to Dec 2013 972

Tornado Warm County Jan 1950 to Dec 2013 349

Funnel Cloud Warm County Apr 1993 to Dec 2013 104

Flash Flood Warm County Apr 1993 to Dec 2013 1157

Flood Warm County Apr 1993 to Dec 2013 1239

Heavy Rain Warm County Jan 1996 to Dec 2013 1221

Drought Warm Zone Sep 1993 to Dec 2013 1033

Wildfire Warm Zone Jan 1994 to Dec 2013 208

Excessive Heat Warm Zone Apr 1993 to Dec 2013 6008

=====================================================================

* The March 1993 blizzard is included to enhance a sparse record.

** Winter weather includes snow, freezing rain and “wintry mix” before 2003.

*** Hail diameter 3⁄4 inch or greater. 
23

Figure 1. Political and geographic features in or near the WFO PHI forecast area. State 

abbreviations are yellow, county names are light blue, county boundaries are black and forecast 

area boundaries are thin white. A portion of the fall line (see text) is in red. The elevation scale at 

the top is in thousands of feet. 
24

Figure 2. Distribution by event type of selected Storm Data events within the WFO PHI forecast area

from 1993-2013(14). Note, thunderstorm winds, hail and tornadoes before 1993 are excluded here.

Figure 3. Storm Data events by year within the WFO PHI forecast area.
25

Figure 4a. All Storm Data events by month for the WFO PHI forecast area. 

Figure 4b. Predominantly cool-season Storm Data events by month and event type. All monthly 

graphs for cool-season events are centered on December and January.
26

Figure 4c. As in Fig. 4b but for predominantly warm-season events. All monthly graphs for 

warm-season events are centered on June and July.
27

Figure 5. Distribution of combined Storm Data events over 34 counties in the WFO PHI forecast 

area. Size and color of each circle corresponds to the number of events in the county. This and 

following maps were created with GPS Visualizer web program (http://www.gpsvisualizer.com). 
28

Figure 6a. Distribution of 1342 total winter storm events over 21 cool seasons in the WFO PHI

forecast area. 

Figure 6b. Distribution of winter storm events by month in the WFO PHI forecast area.
29

Figure 6c. Number and percentage of days per month with at least one winter storm event in the 

WFO PHI forecast area. Total is 181 days.

Figure 6d. Distribution of winter storm days by the number of events per day.
30

Figure 6e. Distribution of winter storm events by county in the WFO PHI forecast area. 
31

Figure 7a. As in Fig. 6a, except for 823 total Heavy Snow events.

Figure 7b. As in Fig. 6b, except for Heavy Snow events.
32

Figure 7c. As in Fig. 6c, except for 87 total days with Heavy Snow events.

Figure 7d. As in Fig. 6d, except for Heavy Snow days.
33

Figure 7e. As in Fig. 6e, except for Heavy Snow Events. 
34

Figure 8a. As in Fig. 6a, except for 147 total Ice Storm events.

Figure 8b. As in Fig. 6b, except for Ice Storm events.
35

Figure 8c. As in Fig. 6c, except for 43 total days with Ice Storm events.

Figure 8d. As in Fig. 6d, except for Ice Storm days.
36

Figure 8e. As in Fig. 6e, except for Ice Storm events. 
37

Figure 9a. As in Fig. 6a, except for all 2368 winter storm verifying events, including heavy 

snow, ice storm and blizzard events.

Figure 9b. As in Fig. 6b, except for all winter storm verifying events.
38

Figure 9c. As in Fig. 6c, except for all 316 total days with winter storm verifying events.

Figure 9d. As in Fig. 6d, except for all winter storm verifying days.
39

Figure 9e. As in Fig. 6e, except for all winter storm verifying events. 
40

Figure 10a. As in Fig. 6a, except for 3866 total winter weather (sub-warning) events.

Figure 10b. As in Fig. 6b, except for winter weather events.
41

Figure 10c. As in Fig. 6c, except for 483 total days with winter weather events.

Figure 10d. As in Fig. 6d, except for winter weather event days.
42

Figure 10e. As in Fig. 6e, except for winter weather events. 
43

Figure 11a. As in Fig. 6a, except for 405 extreme cold events.

Figure 11b. As in Fig. 6b, except for extreme cold events.
44

Figure 11c. As in Fig. 6c, except for 20 total days with extreme cold events.

Figure 11d. As in Fig. 6d, except for extreme cold days.
45

Figure 11e. As in Fig. 6e, except for extreme cold events. 
46

Figure 11f. As in Fig. 6e, except for wind chill events. 
47

Figure 12a. As in Fig. 6a, except for 754 total high wind events.

Figure 12b. As in Fig. 6b, except for high wind events.
48

Figure 12c. As in Fig. 6c, except for 142 total days with high wind events.

Figure 12d. As in Fig. 6d, except for high wind days.
49

Figure 12e. As in Fig. 6e, except for high wind events. 
50

Figure 13a. As in Fig. 6a, except for 3375 total strong wind events.

Figure 13b. As in Fig. 6b, except for strong wind events.
51

Figure 13c. As in Fig. 6c, except for 279 total days with strong wind events. (percentages based 

on 18 seasons)

Figure 13d. As in Fig. 6d, except for strong wind days.
52

Figure 13e. As in Fig. 6e, except for strong wind events. 
53

Figure 14a. As in Fig. 6a, except for 769 total coastal flood events.

Figure 14b. As in Fig. 6b, except for coastal flood events.
54

Figure 14c. As in Fig. 6c, except for 164 total days with coastal flood events.

Figure 14d. As in Fig. 6d, except for coastal flood days.
55

Figure 14e. As in Fig. 6e, except for coastal flood events. 
56

Figure 15a. As in Fig. 6a, except for 236 total blowout tide events.

Figure 15b. As in Fig. 6b, except for blowout tide events.
57

Figure 15c. As in Fig. 6c, except for 32 total days with blowout tide events. (percentages based 

on only 8 seasons)

Figure 15d. As in Fig. 6d, except for blowout tide days.
58

Figure 15e. As in Fig. 6e, except for blowout tide events. 
59

Figure 16a. As in Fig. 6a, except for 592 total dense fog events.

Figure 16b. As in Fig. 6b, except for dense fog events.
60

Figure 16c. As in Fig. 6c, except for 44 total days with dense fog events.

Figure 16d. As in Fig. 6d, except for dense fog days.
61

Figure 16e. Distribution of dense fog events by the starting hour.
62

Figure 16f. As in Fig. 6e, except for dense fog events. 
63

Figure 17a. Distribution by year of 4787 total severe thunderstorm wind events in the WFO PHI 

forecast area. 

Figure 17b. Distribution of severe thunderstorm wind events by month in the WFO PHI forecast 

area.
64

Figure 17c. Number and percentage of days per month with at least one severe thunderstorm 

wind event in the WFO PHI forecast area. Total number of days is 842.

Figure 17d. Distribution of all severe thunderstorm wind events in the WFO PHI forecast area 

by hour of occurrence.
65

Figure 17e. Distribution of severe thunderstorm wind event days by the number of events. (One 

day with 51 reports is not shown here.)
66

Figure 17f. Distribution of severe thunderstorm wind events by county in the WFO PHI forecast 

area. 
67

Figure 18a. As in Fig. 17a, except for 1180 (546) large hail events, with hail size greater than or 

equal to 0.75 inch (1.00 inch).

Figure 18b. As in Fig. 17b, except for large hail events.
68

Figure 18c. As in Fig. 17c, except for 214 total days with large hail events (greater than or equal 

to 1 inch only).

Figure 18d. As in Fig. 17d, except for large hail events (greater than or equal to 1 inch only).
69

Figure 18e. As in Fig. 17e, except for large hail days (greater than or equal to 1 inch only).

.
70

Figure 18f. As in Fig. 17f, except for large hail events (greater than or equal to 1 inch). 
71

Figure 19a. As in Fig. 17a, except for 972 total lightning events.

Figure 19b. As in Fig. 17b, except for lightning events.
72

Figure 19c. As in Fig. 17c, except for 443 total days with lightning events.

Figure 19d. As in Fig. 17d, except for lightning events.
73

Figure 19e. As in Fig. 17e, except for lightning days.
74

Figure 19f. As in Fig. 17f, except for lightning events. 
75

Figure 20a. As in Fig. 17a, except for 349 total tornado events.

Figure 20b. As in Fig. 17b, except for tornado events.
76

Figure 20c. As in Fig. 17c, except for 231 total days with tornado events.

Figure 20d. As in Fig. 17d, except for tornado events.
77

Figure 20e. Distribution by EF-scale of tornado events in the WFO PHI forecast area (F-scale 

before 2008).
78

Figure 20f. As in Fig. 17f, except for tornado events. 
79

Figure 20g. As in Fig. 17f, except weighted for county-area size. 
80

Figure 21a. As in Fig. 17a, except for 1157 total flash flood events.

Figure 21b. As in Fig. 17b, except for flash flood events.
81

Figure 21c. As in Fig. 17c, except for 327 total days with flash flood events.

Figure 21d. As in Fig. 17d, except for flash flood events.
82

Figure 21e. As in Fig. 17e, except for flash flood days. 
83

Figure 21f. As in Fig. 17f, except for flash flood events. 
84

Figure 22a. As in Fig. 17a, except for 1239 total flood events.

Figure 22b. As in Fig. 17b, except for flood events.
85

Figure 22c. As in Fig. 17c, except for 312 total days with flood events.

Figure 22d. As in Fig. 17d, except for flood event starting hour.
86

Figure 22e. As in Fig. 17e, except for flood days.
87

Figure 22f. As in Fig. 17f, except for flood events.
88

Figure 22g. As in Fig. 17f, except for heavy rain events. 
89

Figure 23a. As in Fig. 17a, except for 1033 total drought events. 

Figure 23b. As in Fig. 17b, except for drought events.
90

Figure 23c. Frequency of drought conditions (in one county or more) by month. “Portion” is 

based on 21 total years in the study period, e.g., 21 possible January’s.

Figure 23d. As in Fig. 17e, except for drought months (not days).
91

Figure 23e. As in Fig. 17f, except for drought events (months). 
92

Figure 24a. As in Fig. 17a, except for 208 total wildfire events.

Figure 24b. As in Fig. 17b, except for wildfire events.
93

Figure 24c. As in Fig. 17c, except for 148 total days with wildfire events.

Figure 24d. As in Fig. 17d, except for wildfire events.
94

Figure 24e. As in Fig. 17f, except for wild fire events. 
95

Figure 25a. As in Fig. 17a, except for 6008 total excessive heat events. 

Figure 25b. As in Fig. 17b, except for excessive heat events.
96

Figure 25c. As in Fig. 17c, except for 265 total days with excessive heat events.

Figure 25d. As in Fig. 6d, except for excessive heat days.
97

Figure 25e. As in Fig. 17f, except for excessive heat events.