State of Arizona Exceptional Event Documentation 

for the Event of July 18, 2011, for the 

Phoenix PM10 Nonattainment Area 

Produced by: 

Arizona Department of Environmental Quality 

Maricopa County Air Quality Department 

Maricopa Association of Governments 

Final Report 

January 23, 2013 
EXECUTIVE SUMMARY 

In 2005, Congress identified a need to account for events that result in exceedances of the National 

Ambient Air Quality Standards (NAAQS) that are exceptional in nature1 (e.g., not expected to reoccur or 

caused by acts of nature beyond man-made controls). In response, EPA promulgated the Exceptional 

Events Rule (EER) to address exceptional events in 40 CFR Parts 50 and 51 on March 22, 2007 (72 FR 

13560). On May 2, 2011, in an attempt to clarify this rule, EPA released draft guidance documents on the 

implementation of the EER to State, tribal and local air agencies for review. Based on numerous 

comments EPA received on the May 2011 draft, EPA issued a revised draft guidance document in June 

2012. The EER allows for states and tribes to “flag” air quality monitoring data as an exceptional event 

and exclude those data from use in determinations with respect to exceedances or violations of the 

NAAQS, if EPA concurs with the demonstration submitted by the flagging agency. 

Due to the arid nature of the state, Arizona is highly susceptible to windblown dust events. These events 

are often captured by various air quality monitoring equipment throughout the state, sometimes resulting 

in exceedances or violations of the PM10 NAAQS. In the past, the Arizona Department of 

Environmental Quality (ADEQ) has submitted exceptional event documentation for these events. Due in 

part to issues within the EER, obtaining concurrence on these events has been a difficult task. The ADEQ 

is now taking a new approach and focusing exceptional event documentation on events that are believed 

to be clear-cut exceptional events that are not controllable by human intervention, such as the dust events 

that frequently occurred during the monsoon season of 2011. This new approach was used for the 

exceptional event demonstration submitted in March 2012 by ADEQ for several PM10 events that 

occurred from July 2 – July 8, 2011, in the Phoenix region. In September 2012, EPA concurred that the 

July 2–8, 2011 events were indeed exceptional. As a result, this document is designed to follow the 

format used in the July 2–8, 2011 events submission. 

This demonstration contains detailed information about the windblown dust event that affected the 

Phoenix PM10 nonattainment area on July 18, 2011. On that day, eight exceedances of the PM10 

NAAQS occurred within the nonattainment area. Additional exceedances occurred outside of the Phoenix 

PM10 nonattainment area on this date, and additional documentation may be submitted as a separate 

package at a later time. ADEQ contends that the exceedances that were measured on July 18, 2011, 

within the Phoenix PM10 nonattainment area were the result of natural events that were not reasonably 

controllable or preventable. This assessment report of the July 18, 2011 dust event was a collaborative 

effort involving staff from the Arizona Department of Environmental Quality, Maricopa Association of 

Governments, Maricopa County Air Quality Department, and Pinal County Air Quality Control District. 

 

Section I of this assessment provides a summary of the exceptional event rules and requirements and lays 

out how those rules are met within this specific assessment. 

Section II of this assessment introduces the conceptual model of the thunderstorm events that transpired 

on July 18, 2011, providing a background narrative of the exceptional events. 

Section III of this assessment provides data summaries and time series graphs that help illustrate that the 

events of July 18, 2011, produced PM10 concentrations in excess of normal historical fluctuations. 

 

1

 Section 319 of the Clear Air Act (CAA), as amended by section 6013 of the Safe Accountable Flexible EfficientTransportation Equity Act: A Legacy for Users (SAFE-TEA-LU of 2005), required EPA to propose the Federal Exceptional 

Events Rule (EER) no later than March 1, 2006. 
Section IV of this assessment details the existing area control measures and demonstrates that despite the 

presence and enforcement of these controls, the event on July 18, 2011, was not reasonably controllable 

or preventable. 

Section V of this assessment establishes a clear causal connection between the natural events of July 18, 

2011, and the exceedances of the 24-hour PM10 standard at the monitoring stations. The evidence in this 

section (and the previous section on historical fluctuations) also confirms that the event in question both 

affected air quality and was the result of natural events. 

Section VI of this assessment builds upon the demonstration showing a clear causal connection between 

the natural events and the exceedances and concludes there would have been no exceedances on July 18, 

2011, but for the presence of the natural events. 

Section VII contains conclusions that summarize the exceptional event that occurred on July 18, 2011, 

and relates the requirements in the EER to the information within this document. 

-ii-
Table of Contents 

I. EXCEPTIONAL EVENT RULE (EER) REQUIREMENTS........................ 1

Procedural Requirements.................................................................................................... 1

Public notification that event was occurring (40 CFR 50.14(c)(1(i)) ................................ 1

Place informational flag on data in AQS (40 CFR 50.14(c)(2)(ii)).................................... 1

Notify EPA of intent to flag through submission of initial event description by July 1 of 

calendar year following event (40 CFR 50.14(c)(2)(iii)).................................................... 1

Document that the public comment process was followed for event documentation (40 CFR 

50.14(c)(3)(iv)).................................................................................................................... 2

Submit demonstration supporting exceptional event flag (40 CFR 50.14(a)(1-2))............ 2

Documentation Requirements ............................................................................................ 2

II. CONCEPTUAL MODEL................................................................................. 4

Geographic Setting and Climate......................................................................................... 4

Geographic Setting of Monitors.......................................................................................... 4

Climate ................................................................................................................................ 7

Monsoon Season Description and Event Summary .......................................................... 8

Conclusions......................................................................................................................... 10

III. HISTORICAL FLUCTUATIONS............................................................... 14

Regulatory Measures and Control Programs.................................................................. 18

PM10 Rule Effectiveness................................................................................................... 21

Compliance and Enforcement Activities.......................................................................... 22

Conclusions......................................................................................................................... 24

V. CLEAR CAUSAL RELATIONSHIP............................................................ 25

Introduction........................................................................................................................ 25

Event – July 18, 2011 ......................................................................................................... 25

Visibility Cameras.............................................................................................................. 33

Conclusion........................................................................................................................... 33

VI. “BUT FOR” ANALYSIS .............................................................................. 34

VII. CONCLUSIONS .......................................................................................... 35

Affects Air Quality ............................................................................................................. 35

Natural Event ..................................................................................................................... 35

Summary............................................................................................................................. 36

-iii-
 List of Figures 

Figure 2-1. Phoenix Geographic Setting and PM10 Monitor Locations (source: EPA AQS 

DataMart, NASA MODIS Satellite, Google Earth). PM10 monitor locations are indicated by 

the white markers. ..................................................................................................................................5

Figure 2-2. Drainage System of Phoenix, Arizona. ......................................................................................6

Figure 2-3. Phoenix Monthly Precipitation (top) and Maximum Temperature (bottom) 

Climatology (source: National Weather Service) ..................................................................................7

Figure 2-4. Cross-section of a thunderstorm creating an outflow boundary and haboob (Source: 

Desert Meteorology. Thomas T. Warner. 2004.)...................................................................................9

Figure 2-5. U.S. Drought Monitor analysis released during the time period of the exceedances 

described in this report. ........................................................................................................................11

Figure 2-6. Timeline of PM10 concentrations at monitors in the Phoenix area before, during and 

after the July 18, 2011 event ................................................................................................................12

Figure 3-1. Plot of daily hourly maximum PM10 concentrations (2007 – 2011) at the South 

Phoenix monitoring site. ......................................................................................................................14

Figure 3-2. Plot of 24-hour average PM10 concentrations (2007 – 2011) at the South Phoenix 

monitoring site. ....................................................................................................................................15

Figure 3-3. Plot of daily hourly maximum PM10 concentrations (2007 – 2011) at the Buckeye 

monitoring site. ....................................................................................................................................16

Figure 3-4. Plot of 24-hour average PM10 concentrations (2007 – 2011) at the Buckeye 

monitoring site. ....................................................................................................................................17

Figure 4-1. Timeline of Maricopa County Fugitive Dust Rules and Ordinances. ......................................22

Figure 5-1. Satellite imagery showing strong storms developing and dissipating over Pinal County 

and areas to the southeast of Phoenix. These storms are believed to have been responsible for 

the thunderstorm outflow boundary that caused PM10 exceedances in Maricopa and Pinal 

counties on July 18...............................................................................................................................26

Figure 5-2. Phoenix nonattainment area PM10 monitor readings and Sky Harbor wind speed and 

visibility for July 18, 2011. ..................................................................................................................28

Figure 5-3. Pinal County PM10 monitor readings and Casa Grande wind speed and visibility for 

July 18, 2011. .......................................................................................................................................29

-iv-
List of Tables 

Table 2-1. Summary of Statewide PM10 Measurements for July 18, 2011. ..............................................13

Table 4-1. Rules and Ordinances Regulating Particulate Matter Emissions in Maricopa County .............19

Table 4-2. Pinal County Rules Regulating Existing and New Non-point Sources in Pinal County............20

Table 4-3. Pinal County Rules Regulating Fugitive Dust in Pinal County Portion of Maricopa 

County PM10 Nonattainment Area......................................................................................................20

-v-
-viList of Appendices 

Appendix A – 5-year Historical Fluctuation Graphs for the Phoenix Nonattainment Area PM10 Sites 

Appendix B – ADEQ Forecast Products for Phoenix, Maricopa County, and Pinal County 

Appendix C – Notice of Public Comment Period 

Appendix D – National Weather Service Forecasts, Warnings, and Alerts 
I. EXCEPTIONAL EVENT RULE (EER) REQUIREMENTS 

In addition to the technical requirements that are contained within the EER, procedural requirements must 

be met in order for EPA to concur with the flagged air quality monitoring data. This section of the report 

lays out the requirements of the EER and associated guidance, and discusses how the Arizona Department 

of Environmental Quality (ADEQ) addressed those requirements. 

Procedural Requirements 

This section presents a review of the procedural requirements of the EER as required by 40 CFR 50.14 

(Treatment of Air Quality Monitoring Data Influenced by Exceptional Events) and explains how ADEQ 

fulfills them. The Federal EER requirements include notifying the public that an event was occurring, 

placing informational flags on data in EPA’s Air Quality System (AQS), notifying EPA of the intent to 

flag through submission of initial event description, documenting that the public comment process was 

followed, and submitting a demonstration supporting the exceptional events flag. ADEQ has addressed 

all of these procedural and documentation requirements. 

Public notification that event was occurring (40 CFR 50.14(c)(1(i)) 

ADEQ issued a Dust Control Action Forecast and an Ensemble Forecast for the Greater Phoenix area 

advising citizens of the potential for high winds and blowing dust on July 18, 2011. More information on 

ADEQ’s forecasting program can be found in Section IV. The forecast products that were issued for 

July 18, 2011, are included in Appendix B. 

Place informational flag on data in AQS (40 CFR 50.14(c)(2)(ii)) 

ADEQ and other operating agencies in Arizona submit data into EPA’s AQS. Data from both filter-based 

and continuous monitors operated in Arizona are submitted to AQS. 

When ADEQ and/or another agency operating monitors in Arizona suspects that data may be influenced 

by an exceptional event, ADEQ and/or the other operating agency expedites analysis of the filters 

collected from the potentially affected filter-based air monitoring instruments, quality assures the results, 

and submits the data into AQS. ADEQ and/or other operating agencies also submit data from continuous 

monitors into AQS after quality assurance is complete. 

If ADEQ and/or the operating agency have determined a potential exists that the monitor reading has been 

influenced by an exceptional event, a preliminary flag is submitted for the measurement in the AQS. The 

data are not official until they undergo more thorough quality assurance and quality control, leading to 

certification by May 1 of the year following the calendar year in which the data were collected 

(40 CFR 58.15(a)(2)). The presence of the flag can be confirmed in AQS. 

Notify EPA of intent to flag through submission of initial event description by July 1 of calendar year 

following event (40 CFR 50.14(c)(2)(iii)) 

ADEQ submitted notice to EPA on August 29, 2012, listing all days from calendar year 2011 that ADEQ 

intended to analyze under the Exceptional Events Rule and notifying the EPA of which PM10 data were 

flagged in AQS. Exceedances that occurred on July 18, 2011, within the Phoenix PM10 nonattainment 

area were included on that list. This report serves as the demonstration supporting the flagging of these 

data. 

-1-
On July 18, 2011, eight monitoring sites within the boundaries of the Phoenix PM10 nonattainment area 

exceeded the 24-hour PM10 standard during the high wind event that occurred during the late 

afternoon/evening hours. The PM10 monitors impacted were the Buckeye, Central Phoenix, Durango 

Complex, Dysart, Greenwood, South Phoenix, West 43rd, and West Phoenix monitors operated by 

Maricopa County Air Quality Department (MCAQD). 

Document that the public comment process was followed for event documentation (40 CFR 

50.14(c)(3)(iv)) 

ADEQ posted this assessment report on the ADEQ webpage and placed a hardcopy of the report in the 

ADEQ Records Management Center for public review. ADEQ opened a 30-day public comment period 

on December 3, 2012. A copy of the public notice certification, along with any comments received, will 

be submitted to EPA, consistent with the requirements of 40 CFR 50.14(c)(3)(iv). See Appendix D for a 

copy of the affidavit of public notice. 

Submit demonstration supporting exceptional event flag (40 CFR 50.14(a)(1-2)) 

At the close of the comment period, and after ADEQ has had the opportunity to consider any comments 

submitted on this document, ADEQ will submit this document, the comments received, and ADEQ’s 

responses to those comments to EPA Region IX headquarters in San Francisco, California. The deadline 

for the submittal of this demonstration package is September 30, 2014. 

Documentation Requirements 

Section 50.14(c)(3)(iii) of the EER states that in order to justify excluding air quality monitoring data, 

evidence must be provided for the following elements: 

a. The event satisfies the criteria set forth in 40 CFR 501(j) that: 

(1) the event affected air quality, 

(2) the event was not reasonably controllable or preventable, and 

(3) the event was caused by human activity unlikely to recur in a particular location or was a 

natural event; 

b. There is a clear causal relationship between the measurement under consideration and the event; 

c. The event is associated with a measured concentration in excess of normal historical fluctuations; 

and 

d. There would have been no exceedance or violation but for the event. 

Section II of this assessment introduces the conceptual model of the high wind event that transpired on 

July 18, 2011, providing a background narrative of the exceptional events and an overall explanation on 

how “the event affected air quality.” Further evidence that “the event affected air quality” is provided in 

Section V. Sections II and V also provide evidence that the event was a natural event. 

Section IV of this assessment details the existing area control measures and demonstrates that despite the 

presence and enforcement of these controls, the event on July 18, 2011, was not reasonably controllable 

or preventable. 

Section V of this assessment establishes a clear causal connection between the natural events of July 18, 

2011 and the exceedances of the 24-hour PM10 standard at the monitoring stations. The evidence in this 

section (and the previous section on historical fluctuations) also confirms that the events in question both 

affected air quality and were the result of natural events. 

-2-
Section III of this assessment provides data summaries and time series graphs that help illustrate that the 

event on July 18, 2011, produced PM10 concentrations in excess of normal historical fluctuations. 

Section VI of this assessment builds upon the demonstration showing a clear causal connection between 

the natural events and the exceedances and concludes there would have been no exceedances on July 18, 

2011, but for the presence of the natural events. 

-3-
II. CONCEPTUAL MODEL 

Geographic Setting and Climate

 

This section describes the geographic and climatic setting of the monitors. 

Geographic Setting of Monitors 

Phoenix is located in the Salt River Valley in south-central Arizona. It lies at a mean elevation of 1,090 

feet above mean sea level (msl) in the northeastern part of the Sonoran Desert. Other than the mountains 

in and around the city, the topography of Phoenix is generally flat. The Phoenix area is surrounded by the 

McDowell Mountains (~4,200 ft msl) to the northeast, the foothills of the Bradshaw (~7,900 ft msl) and 

Mazataal (~7,900 ft msl) ranges to the north, the White Tank Mountains (~4,500 ft msl) to the west, the 

Sierra Estrella (~4,450 ft msl) to the southwest, and the Superstition Mountains (~5,000 ft msl) far to the 

east. Within the City are the Phoenix Mountains (~2,600 ft msl) and South Mountain (~2,600 ft msl). 

Current development is pushing north, west, and south into Pinal County. The Phoenix metropolitan area 

contains a fairly dense network of PM10 monitors throughout the area, with a much less dense network of 

monitors located throughout the rest of the state. Figure 2-1 shows the general geographic setting of 

Phoenix, as well as the locations of PM10 monitors throughout the state. It should be noted that some of 

the monitors shown in Figure 2-1 are filter-based monitors; therefore, monitoring data from all locations 

may be available only for select days (i.e., 1-in-6 run days). 

-4-
Figure 2-1. Phoenix Geographic Setting and PM10 Monitor Locations (source: EPA AQS DataMart, 

NASA MODIS Satellite, Google Earth). PM10 monitor locations are indicated by the white markers.

Figure 2-2 depicts the drainage systems or watersheds for the State of Arizona. Many of the rivers that 

form Arizona’s drainage system are dry for most of the year and, consequently, are sources of silt and fine 

soils that become suspended and add to regional PM10 loadings during high wind events. Much of this 

alluvial matter and fine soil is deposited in the low lying areas of central and southern Arizona, with 

larger depositional areas focused in and around the confluences of dry river channels. 

-5-
Figure 2-2. Drainage System of Phoenix, Arizona. 

-6-
Climate 

Phoenix has an arid climate, with very hot summers and temperate winters. The average summer high 

temperature is among the hottest of any populated area in the United States. The temperature reaches or 

exceeds 100oF an average of 110 days during the year and highs top 110oF an average of 18 days during 

the year. Phoenix receives an average of 7.66 inches of rain per year. 

As shown in Figure 2-3, precipitation is sparse during the first part of the summer, but the influx of 

monsoonal moisture, which generally begins in early July and lasts until mid-September, raises humidity 

levels and can cause heavy localized precipitation and flooding. Although thunderstorms are possible at 

any time of the year, they are most common during the monsoon season from July to mid-September as 

humid air is advected from the Gulf of California, Gulf of Mexico, and large thunderstorm complexes that 

form over the Sierra Madre Occidental Mountains in Mexico. This influx of moisture, combined with 

intense solar heating, often creates a very unstable environment that is ripe for thunderstorm development. 

These thunderstorms can bring strong winds and blowing dust, large hail, and heavy rain. Dust storms 

associated with these thunderstorms typically occur in the early part of the monsoon season (July/August) 

before soaking rains help keep soil particles bound to one another. However, depending on the amount of 

precipitation received during the monsoon season, extremely hot temperatures act to dry out the surface 

quickly, and dust storms can occur at any time. During the December through March period, winter 

storms moving inland from the Pacific Ocean can produce significant rains throughout Arizona. This 

December–March time period and the July–August time period are typically the wettest parts of the year. 

Meanwhile, a distinct dry season occurs from April through June for the Phoenix area and the rest of 

Arizona. While these weather patterns describe the general climatology for the Phoenix area over a long 

period of time, Phoenix and the entire state of Arizona are also prone to a high degree of variability in 

these weather patterns from year to year. 

Figure 2-3. Phoenix Monthly Precipitation (top) and Maximum Temperature (bottom) Climatology 

(source: National Weather Service)

-7-
Monsoon Season Description and Event Summary

A multi-day period, including the event day of July 18, 2011, was characterized by monsoonal 

thunderstorm activity throughout Arizona. The North American Monsoon is a shift in wind patterns in 

the summer that occurs as Mexico and the southwest U.S. warm under intense solar heating. As this 

happens, low level moisture is transported primarily from the Gulf of California and eastern Pacific Ocean 

into the southwestern U.S. The Gulf of California, a narrow body of water surrounded by mountains, is 

particularly important for low-level moisture transport into Arizona and Sonora. Mid- and upper-level 

moisture is also transported into the region, mainly from the Gulf of Mexico by easterly winds aloft. This 

combination causes a distinct rainy season over large portions of western North America, which develops 

rather quickly and sometimes dramatically. Rainfall during the monsoon is not continuous and varies 

considerably, depending on a variety of factors. There are usually distinct “burst” periods of heavy rain 

during the monsoon, and “break” periods with little or no rain. Even during active monsoon periods, 

some areas can go without receiving any significant precipitation while other nearby areas experience 

heavy rains and flooding.

The thunderstorm activity in Arizona on July 18 was active in and around the desert areas of central and 

southern Arizona and northern Mexico. These thunderstorms did not result in much precipitation for the 

Phoenix area, but many of the storms were severe in nature and caused many dust-carrying outflow 

boundaries, the most significant of which propagated into the Phoenix area in the late afternoon. 

Downdrafts associated with heavy precipitation from thunderstorms or from collapsing thunderstorms can 

end up producing what are called downbursts, or sometimes more concentrated and severe microbursts, 

which are rapidly descending bursts of air that come through the thunderstorms. These downward bursts 

of air hit the ground and then disperse away from the storms as areas of outflow. These outflow 

boundaries from the thunderstorms can kick up large areas of dust, sometimes called haboobs, and 

transport that dust for long distances from the initiating thunderstorms (see Figure 2-4). 

The active monsoon occurring on July 18 led to numerous thunderstorms and thunderstorm outflows that 

produced blowing dust in many parts of Arizona, including the Phoenix Metropolitan area. The 

windblown dust event produced eight PM10 exceedances in the Phoenix PM10 nonattainment area on 

July 18. The event was primarily driven by strong outflows from severe thunderstorms producing wind 

gusts from one prominent direction. The July 18th event was a widespread dust event with mostly southsoutheasterly winds transporting dust from the desert areas of Pinal County into the central and western 

side of the Phoenix region. Further details on the event can be found in Section V. 

-8-
Figure 2-4. Cross-section of a thunderstorm creating an outflow boundary and haboob (Source: Desert 

Meteorology. Thomas T. Warner. 2004.)

The nature of these monsoonal dust events is such that specific source areas are difficult to determine, as 

outflow from thunderstorms can carry dust over vast distances encompassing many source areas. Because 

of this, it is more appropriate to speak of general source regions for these monsoonal dust storms, which 

typically are identified based upon the locations of the thunderstorms that are believed to have created the 

dust generating and carrying outflow winds. A vast majority of the PM10 impacting the Phoenix area 

from thunderstorm-driven high winds on July 18 originated outside of the Phoenix PM10 nonattainment 

area. The contributing source regions to the dust event were somewhat widespread, but the majority of 

the PM that was transported into Maricopa County likely came from areas within Pinal County to the 

south and southeast of Maricopa County. Other potential source contributors to the PM10 events were 

areas in northern Pima County and southeastern and southern Maricopa County. The exact origin of the 

PM sources is often difficult to determine due to the less dense monitoring networks in the general source 

area. 

Another important factor that led to these significant dust storms was the on-going drought across the 

region. The most recent U.S. Drought Monitor placed the area between Tucson and Phoenix, the primary 

dust source region for winds out of the southeast, in D1 (Moderate) to D3 (Extreme) drought (see 

Figure 2-5). Rainfall in Phoenix since October 1, 2010, had been 55% of normal, and only 44% of 

normal since January 1, 2011 (see the Daily Climate Report for July 18 in Appendix D). For additional 

information pertaining to the 2011 monsoon season in Arizona, see the news media web links provided 

below. 

2011 Monsoon Season Summary and Review  http://bcove.me/krh3qk29

-9-
-10-

This video contains information about the monsoon season, the formation of dust storms in 

Arizona, and the uniqueness of the 2011 monsoon season. 

2011 Monsoon Season Review 2  http://bcove.me/tc6otk0h

This video contains information about the 2011 monsoon season, focusing on the large 

number of dust storms that occurred, and a discussion of why there were so many dust 

storms in 2011. 

The abnormally dry conditions that were in place during the early part of the monsoon season (which 

persisted throughout the duration of the monsoon season) resulted in a large fetch of soils that were 

vulnerable to particulate suspension. The thunderstorm outflow boundaries that affected Maricopa and 

Pinal counties on July 18 resulted in very little rainfall throughout south-central Arizona, but the high 

winds associated with the outflows did transport moderate to very high levels of particulate concentrations 

into the Phoenix area. A timeline summarizing the July 18 event can be seen in Figure 2-6. This figure 

shows hourly PM10 concentrations for a three-day period around, and including, the event day. During 

the 24 hours prior to the event, there were low PM10 concentrations across the Phoenix region for much 

of the day, but late afternoon thunderstorms produced weak outflows and moderate winds that caused a 

one hour spike in PM10 concentrations around 11 p.m., but the spike was not high enough at any 

monitoring sites to cause an exceedance of the 24-hour standard. On the event day, the figure shows low 

PM10 concentrations during the day and then very high PM10 concentrations during the evening hours 

when the thunderstorm outflows traveled through the Phoenix area. Lastly, the figure shows that PM10 

concentrations dropped back down to low levels during the 24 hours after the event, indicating that 

without the impacts of the thunderstorm outflows, PM10 concentrations at all sites in the Phoenix PM10 

nonattainment area July 18, 2011, would have been below the 24-hour average PM10 exceedance 

threshold of 150 μg/m3

. 

Following Figure 2-6, Table 2-1 details the PM10 24-hour average concentrations, the daily maximum 

hourly concentrations and the time at which they occurred, and the flag given to the data in EPA’s AQS 

for all PM10 monitoring sites in Arizona for the event day. 

Conclusions

This Conceptual Model was created to provide a basic description of the general climate and weather 

patterns that led to the active thunderstorm period on July 18 and produced the dust storms and eight 

PM10 exceedances in the Phoenix PM10 nonattainment area during the one-day episode. A more 

detailed analysis of the windblown dust event is included in Section V, where a demonstration of the clear

causal connection between the uncontrollable natural events and PM10 exceedances is presented. 
Figure 2-5. U.S. Drought Monitor analysis released during the time period of the exceedances described in this r

-11-
Figure 2-6. Timeline of PM10 concentrations at monitors in the Phoenix area before, during and after the July 18

-12-
July 18, 2011 

Table 2-1. Summary of Statewide PM10 Measurements for July 18, 2011. 

Monitor 

Monitor 

Type Operator AQS Monitor ID 

24-hr Avg 

PM10 

(μg/m3) 

1-hr Max 

PM10 

(μg/m3) 

Max 

Time 

AQS 

Qualifier 

Flag 

Apache County 

 N/A TEOM WMAT 04-001-1003-81102-1 11 33 1300 

Coconino County 

 N/A TEOM NN 04-005-1237-81102-1 15 31 0200 

Gila County 

 Hayden Old Jail TEOM ADEQ 04-007-1001-81102-3 24 93 1300 

Maricopa County 

 Buckeye TEOM MC 04-013-4011-81102-1 196 1665 1800 RJ 

 Central Phoenix TEOM MC 04-013-3002-81102-4 210 2056 1800 RJ 

 Durango Complex TEOM MC 04-013-9812-81102-1 267 1859 1800 RJ 

 Dysart TEOM MC 04-013-4010-81102-1 163 1008 0000 RJ 

 Fort McDowell/Yuma Frank TEOM FMIR 04-013-5100-8112-1 32 N/A N/A 

 Glendale TEOM MC 04-013-2001-81102-1 141 862 1800 

 Greenwood TEOM MC 04-013-3010-81102-1 208 1549 1800 RJ 

 Higley TEOM MC 04-013-4006-81102-1 104 937 1700 

 JLG Supersite BAM ADEQ 04-013-9997-81102-3 125 826 2100 IJ, EH 

 JLG Supersite TEOM ADEQ 04-013-9997-81102-4 151 1383 1800 IJ 

 South Phoenix TEOM MC 04-013-4003-81102-1 303 2861 1800 RJ 

 West Chandler TEOM MC 04-013-4004-81102-1 126 696 1900 

 West Forty Third TEOM MC 04-013-4009-81102-1 244 1515 1800 RJ 

 West Phoenix TEOM MC 04-013-0019-81102-1 159 908 2100 RJ 

 Zuni Hills TEOM MC 04-013-4016-81102-1 100 441 2300 

Navajo County 

 N/A TEOM WMAT 04-017-1002-81102-1 11 30 1300 

Pima County 

 Ajo TEOM ADEQ 04-019-0001-81102-3 27 95 2300 

 Geronimo BAM PCDEQ 04-019-1113-81102-1 15 57 1400 

 Green Valley BAM PCDEQ 04-019-1030-81102-1 12 47 1300 

 Orange Grove FRM PCDEQ 04-019-0011-81102-2 14 N/A N/A 

 Rillito TEOM ADEQ 04-019-0020-81102-3 43 151 1400 

 South Tucson FRM PCDEQ 04-019-1001-81102-1 18 N/A N/A 

Pinal County 

 Apache Junction Fire Stn. TEOM PCAQCD 04-021-3002-81102-3 N/A N/A N/A 

 Casa Grande Downtown TEOM PCAQCD 04-021-0001-81102-3 219 2919 1600 RJ 

 Combs School TEOM PCAQCD 04-021-3009-81102-3 66 405 1500 

 Cowtown TEOM PCAQCD 04-021-3013-81102-3 1638 24976 1700 RJ 

 Maricopa TEOM PCAQCD 04-021-3010-81102-3 531 9756 1700 RJ 

 Pinal County Housing TEOM PCAQCD 04-021-3011-81102-3 271 3088 1600 RJ 

 Stanfield TEOM PCAQCD 04-021-3008-81102-3 567 9716 1600 RJ 

Santa Cruz County 

 Nogales Post Office BAM ADEQ 04-023-0004-81102-3 13 28 1600 

Yuma County 

 Yuma Supersite TEOM ADEQ 04-027-8011-81102-3 22 73 0700 

Operator Abbreviations: MC – Maricopa County Air Quality Department 

 PCAQCD – Pinal County Air Quality Control District 

 ADEQ – Arizona Department of Environmental Quality 

 FMIR – Fort McDowell Indian Reservation 

 NN – Navajo Nation, AZ, NM, UT 

 PCDEQ – Pima County Department of Environmental Quality 

 SRP-MIC – Salt River Pima-Maricopa Indian Community of Salt River Reservation, AZ 

 WMAT – White Mountain Apache Tribe of Fort Apache Reservation, AZ 

-13-
III. HISTORICAL FLUCTUATIONS 

The PM10 concentrations measured across much of Maricopa County on July 18, 2011, were some of the 

highest hourly and 24-hour averages measured over the last five years. Time series plots of the 24-hour 

PM10 concentrations for the period January 1, 2007, through December 31, 2011, were created for all 

PM10 monitors in the Phoenix PM10 nonattainment area. Additionally, time series plots of the daily 

maximum hourly average PM10 concentrations were created for two monitors in Maricopa County. 

These two additional plots were created to provide a deeper understanding of the magnitude of short-term 

particulate concentrations and their contribution to PM10 exceedances in the Phoenix PM10 

nonattainment area. Time series plots for two monitoring sites (South Phoenix and Buckeye) are included 

within this section, while the remaining 24-hour average plots are available in Appendix A. The South 

Phoenix monitor was selected because of its central location in relation to all monitors in Maricopa 

County and it was the most severely impacted site during the July 18 windblown dust event. The 

Buckeye monitor was selected because it is representative of western Maricopa County, an area of the 

region frequently impacted by dust storms caused by thunderstorm outflow events. The graphs below and 

in Appendix A show that the July 18 event was in the top five most extreme events of the last five years 

for multiple sites. 

Figure 3-1. Plot of daily hourly maximum PM10 concentrations (2007 – 2011) at the South Phoenix 

monitoring site. 

-14-
Figure 3-1 shows the daily maximum hourly averages from the South Phoenix PM10 monitor. The plot 

shows that the July 18th hourly average of 2861 μg/m3

 was the third highest hourly PM10 concentration 

recorded in the last five years. The plot also shows that the maximum hourly average was greater than 

that observed during the July 5, 2011 haboob event which recently received EPA concurrence. The 

hourly average concentrations from this event were surpassed only by two additional dust storms that 

occurred in August and September 2011, another indication of the severity of the 2011 monsoon season. 

It is also clear that the most significant hourly average PM10 concentrations at the South Phoenix monitor 

typically occur during the July through September time frame, a period during which the monsoon season 

is most active and the Phoenix region is frequently impacted by haboob events. 

Figure 3-2 shows the daily 24-hour averages from the South Phoenix PM10 monitor and indicates that the 

July 18th event produced the third highest 24-hour average PM10 concentration measured in the last five 

years. Similar to haboob events that occurred on the same date in 2009, during the first week of July 

2011, and during the last week of August 2011, the July 18 event experienced PM10 concentrations that 

were well above the 24-hour exceedance threshold. 

Figure 3-2. Plot of 24-hour average PM10 concentrations (2007 – 2011) at the South Phoenix monitoring 

site. 

Figures 3-3 and 3-4 provide an historical look at the daily maximum hourly averages and daily 24-hour 

averaged PM10 concentrations from the Buckeye monitor. The plots also show that the maximum hourly 

concentration and exceedance measured at the Buckley monitor on July 18 was among the nine highest 

PM10 concentrations measured over the last five years. As was the case for the South Phoenix monitor, 

-15-
the most significant hourly average PM10 concentrations at the Buckeye monitor typically occur during 

the July through September time frame, a period during which the monsoon season is most active and the 

Phoenix region is frequently impacted by haboob events. 

Figure 3-3. Plot of daily hourly maximum PM10 concentrations (2007 – 2011) at the Buckeye 

monitoring site. 

-16-
Figure 3-4. Plot of 24-hour average PM10 concentrations (2007 – 2011) at the Buckeye monitoring site. 

The plots within this section provide an historical perspective of PM10 concentrations since 2007. These 

plots, along with those in Appendix A, show that the haboob event that occurred on July 18, 2011, 

resulted in one of top ten highest 24-hour average PM10 concentrations seen in the last five years. This 

equates to PM10 concentrations in the 99.5th percentile when compared to historic data. In the case of the 

Greenwood, South Phoenix, and Durango monitoring sites, the 24-hour averaged PM10 concentrations 

measured on July 18 were in the top five highest concentrations observed during the last five years. 

While the PM10 concentrations that were measured on July 18th were extreme in many cases, the plots 

also show that similar events are not uncommon during the monsoon season. As documented in several 

published journals and academic text books, haboobs generated from thunderstorm outflow boundaries 

are a phenomenon that Arizona deals with every year, some years more so than others, namely in 2007, 

2009, and 2011. 

-17-
IV. NOT REASONABLY CONTROLLABLE OR PREVENTABLE

Section 50.1(j) of Title 40 CFR Part 50 requires that an event must be “not reasonably controllable or 

preventable” in order to be defined as an exceptional event. This requirement is met by demonstrating 

that despite reasonable control measures in place within Maricopa County and the Phoenix PM10 

nonattainment area, high wind conditions overwhelmed all reasonably available controls. The event 

occurring on July 18, 2011, was directly related to strong and gusty winds generated by thunderstorm 

outflow boundaries. The strong winds overwhelmed all reasonably available controls, and were also 

responsible for transporting PM into the Phoenix PM10 nonattainment area from areas outside of the 

nonattainment area. As explained in the conceptual model, downdrafts from collapsing thunderstorms 

produce outflow boundaries that contain gusty, turbulent wakes that raise dust from the desert surfaces 

that can be as wide as 100 miles in severe cases. As shown in Section V, the source region for all of the 

dominant thunderstorm outflows and associated transported dust on July 18, 2011, generally came from 

areas outside of the Phoenix PM10 nonattainment area—primarily from the deserts of Pinal, Pima, and 

southern Maricopa counties. While it is likely that some dust was generated within the PM10

nonattainment area as gusts from the thunderstorm outflows passed through the area, the amount of dust 

generated locally was easily overwhelmed by, and largely unnoticeable as compared to, the dust 

transported in from the source regions of the thunderstorm outflows. Strict controls on local sources of 

fugitive dust were in place and enforced on July 18, 2011, but were not capable of controlling transported 

dust and PM10 raised by the gusty and turbulent thunderstorm outflows on these dates. 

The following sections describe the BACM- and MSM-level PM10 control measures in place July 18, 

2011, and the robustness of the programs designed to enforce these measures. Pro-active response and 

analysis of each event as it is occurring verify the uncontrollable nature of each event, while inspections 

of local sources performed before, during, and after the July 18, 2011 event confirmed that no unusual 

anthropogenic PM10-producing activities occurred in Maricopa County, the Phoenix PM10 

nonattainment area, or the local areas surrounding the exceeding monitors. 

Regulatory Measures and Control Programs

ADEQ and MCAQD are responsible for implementing regulatory measures to control emissions from 

agricultural sources, stationary sources, fugitive dust sources, and open burning within Maricopa County. 

Three major programs provide or contribute to air pollution control measures for the Greater Phoenix 

area: 

1. ADEQ’s Agricultural Best Management Program (AgBMP); 

2. Maricopa County’s Inspection and Compliance Program; and 

3. ADEQ’s Air Quality Forecasting Program. 

Specifically, ADEQ is responsible for compliance assistance and enforcement of Agricultural Best 

Management Practices developed by the Governor’s Agricultural Best Management Practices Committee, 

while MCAQD is responsible for compliance assurance for all other significant sources of PM10 

emissions. In addition to routine inspections and inspections driven by complaints, inspections are often 

increased when (1) ADEQ forecasters issue a High Risk for the Maricopa County Dust Control Forecast, 

(2) ADEQ forecasters issue a High Pollution Advisory, or (3) near-real-time monitoring data indicate 

unique activity via high PM concentrations. The forecasting program and inspection/compliance 

programs work together so that resources can be best utilized during days that are of greatest risk for 

elevated PM emissions. 

-18-
On July 25, 2002, EPA took initial action to finalize approval of the Best Available Control Measure 

(BACM) and the Most Stringent Measure (MSM) demonstrations in the Serious Area PM10 plan for the 

Maricopa County portion of the metropolitan Phoenix PM10 nonattainment area (67 FR 48718). These 

BACM and MSM demonstrations were again approved by EPA on July 14, 2006 (71 FR 43979). The 

Agricultural Best Management Practices General Permit rule and related definitions have been approved 

into the Arizona Administrative Code as R18-2-610 and R18-2-611 pursuant to Arizona Revised Statutes 

§ 49-457.2 Maricopa County regulations of PM10 emissions are listed in Table 4-1. 

Table 4-1. Rules and Ordinances Regulating Particulate Matter Emissions in Maricopa County 

Rule/Ordinance Number 

& Title Description 

Rule 300: Visible 

Emissions 

Establishes standards for visible emissions and opacity. 

Rule 310: Fugitive Dust 

from Dust-Generating 

Operations 

Establishes limits for the emissions of particulate matter into the 

ambient air from any property, operations, or activity that may 

serve as a fugitive dust source 

Rule 310.01: Fugitive Dust 

from Non-Traditional 

Sources of Fugitive Dust 

Establishes limits for the emissions of particulate matter into the 

ambient air from open areas, vacant lots, unpaved parking lots, 

and unpaved roadways that are not regulated by Rule 310 and 

that are not required to have either a permit or a dust control 

plan 

Rule 311: Particulate 

Matter from Process 

Industries 

Establishes emission rates based on process weight applicable to 

any affected operations not subject to Rule 316. 

Rule 312: Abrasive 

Blasting 

Establishes limits for particulate emissions from abrasive 

blasting operations 

Rule 314: Open Outdoor 

Fires and Indoor Fireplaces 

at Commercial and 

Institutional Establishments

Establishes limits for the emissions of air contaminants 

produced from open burning 

Rule 316: Nonmetallic 

Mineral Processing 

Establishes limits for the emissions of particulate matter into the 

ambient air from any nonmetallic mining operation or rock 

product processing plant 

Rule 317:

Hospital/Medical/ 

Infectious Waste 

Incinerators 

Establishes limits for the emissions of air pollutants from 

medical waste incinerators 

Rule 322: Power Plant 

Operations 

Establishes limits for the emissions of nitrogen oxides, sulfur 

oxides, carbon monoxide and particulate matter from existing 

power plants and cogeneration plants 

Rule 323: Fuel Burning 

Equipment from 

Industrial/Commercial/ 

Institutional (ICI) Sources 

Establishes limits for the emissions of nitrogen oxides, sulfur 

oxides, carbon monoxide and particulate matter from ICI 

sources 

 

2

 Updates to the AgBMP program in December, 2011, clarified BMPs for crop and added BMPs for animal operations. 

Effective 12/29/2011, R18-2-611 was renumbered to R18-2-610.01 Agricultural PM10 General Permit for Crop 

Operations and R18-2-611.01 Animal Operations PM10 General Permit was added. Definitions for Crop Operations were 

revised at R18-2-610, and new definitions for Animal Operations were added at R18-2-611. 

-19-
Rule/Ordinance Number Description & Title 

Rule 324: Stationary 

Internal Combustion (IC) 

Engines 

Establishes limits for the emissions of carbon monoxide, 

nitrogen oxides, sulfur oxides, volatile organic compounds, and 

particulate matter from stationary internal combustion engines, 

including stationary IC engines used in cogeneration 

Rule 325: Brick and 

Structural Clay Products 

(BSCP) Manufacturing 

Establishes limits for particulate matter emissions from the use 

of tunnel kilns for curing in the brick and structural clay product 

(BSCP) manufacturing processes 

Ordinance P-25: Leaf 

Blower Restriction 

Establishes restrictions for leaf blowers in incorporated and 

unincorporated sections of Area A in Maricopa County 

Ordinance P-26:

Residential Woodburning 

Restriction 

Establishes restrictions for residential woodburning 

Ordinance P-27: Vehicle 

Parking and Use on 

Unstabilized Vacant Lots 

Establishes restrictions for vehicle parking and use on 

unstabilized vacant lots in unincorporated sections of Area A in 

Maricopa County 

Ordinance P-28: OffRoad Vehicle Use in 

Unincorporated Areas of 

Maricopa County 

Establishes restrictions for operating vehicles on unpaved 

property in unincorporated areas of Maricopa County 

Arizona Administrative 

Code R18-2-611 & 610: 

Agricultural PM10 General 

permit 

Establishes a requirement for commercial farmers to implement 

best management practices and maintain a record demonstrating 

compliance 

In addition to the rules and regulations listed in Table 4-1, other PM10 reducing control measures (e.g., 

paving of unpaved roads, PM10 certified street sweepers, controlling unpaved parking lots, etc.) have 

been committed to, and implemented by, local jurisdictions throughout the PM10 nonattainment area, and 

incorporated into the Arizona SIP through PM10 plans such as the Revised MAG 1999 Serious Area 

Particulate Plan for PM10 for the Maricopa County Nonattainment Area. The Pinal County Air Quality 

Control District (PCAQCD) also implements regulatory control measures on emissions from existing and 

new non-point sources within Pinal County (see Table 4-2). Additionally, the PCAQCD implements 

specific nonattainment rules for that part of the Phoenix PM10 nonattainment area that resides in Pinal 

County (see Table 4-3). 

Table 4-2. Pinal County Rules Regulating Existing and New Non-point Sources in Pinal County 

Article Number & Title Description 

Chapter 4, Article 2: Fugitive 

Dust 

Provides a mechanism to reasonably regulate operations which 

periodically may cause fugitive dust emissions into the 

atmosphere 

Chapter 4, Article 3: 

Construction Sites – Fugitive Dust 

Improves the control of excessive fugitive dust emissions that 

have been traditionally associated with construction, earthwork, 

and land development, and thereby minimize nuisance impacts 

Table 4-3. Pinal County Rules Regulating Fugitive Dust in Pinal County Portion of Maricopa 

County PM10 Nonattainment Area 

-20-
Article Number & Title Description 

Chapter 4, Article 4: 

Nonattainment Area Rules; 

Dustproofing for Commercial 

Parking, Drives and Yards 

Establishes rules to avoid violations of the prevailing PM10 

standard and additionally minimize nuisance impacts by 

improving control of excessive fugitive dust emissions from 

unpaved parking lots 

Chapter 4, Article 5: 

Nonattainment Area Rules; 

Stabilization for Residential 

Parking and Drives 

Establishes rules for stabilizing residential properties 

Chapter 4, Article 6: Restrictions 

on Vehicle Parking and Use on 

Vacant Lots

Establishes rules for unpaved or unstabilized vacant lots (valid 

in Pinal County portion of Area A) 

Chapter 4, Article 7: 

Construction Sites in 

Nonattainment Areas – Fugitive 

Dust

Establishes rules to avoid violations of the prevailing PM10 

standard and additionally minimize nuisance impacts by 

improving control of excessive fugitive dust emissions from 

activities associated with construction, earthwork, or land 

development 

Chapter 4, Article 8: 

Nonattainment Area Rules, 

Requirement for Stabilization of 

Disturbed Areas at Vacant Lots

Establishes rules for stabilizing disturbed areas at vacant lots 

PM10 Rule Effectiveness

MCAQD analyzed the effectiveness of its fugitive dust rules (Rules 310, 310.01 and 316) in terms of 

permit compliance rates. This rule effectiveness (RE) study was designed to assess how many sources 

regulated by MCAQD during the subject time period received no PM10 emissions-related violations. As 

a basis for comparison, the percentage of permitted sources in compliance during calendar year 2007 was 

76% for sources subject to Rule 310, 85% for Rule 310.01 sources, and 40% for Rule 316 sources. In 

early 2008, Rules 310, 310.01, and 316 were strengthened, and new ordinances (covering additional 

source categories such as leaf blowers, vacant lots, and off-road vehicles) were adopted. These 

enhancements resulted from MCAQD department’s obligations under such agreements as the 2005 

Revised PM10 State Implementation Plan for the Salt River Area and the Maricopa Association of 

Governments (MAG) 2007 Five Percent Plan for PM10 for the Maricopa County Nonattainment Area to 

reduce PM10 emissions throughout the county. Three major areas that contributed to increased 

compliance were an increase in departmental staffing (especially inspectors), a robust training program, 

and regulatory changes that broadened and strengthened control measures under Rules 310, 310.01, and 

316. 

Source compliance rates were re-assessed for FY 2009 (July 2008–June 2009), a period that allowed time 

for the new and revised regulations to take effect. The results showed significant increases in compliance 

compared with the earlier period: to 90% (from 76%) for Rule 310 sources, to 95% compliance (from 

85%) for Rule 310.01 sources, and to 65% (from 40%) for Rule 316 sources. These improvements 

continued into calendar year 2010 with compliance rates of 94% for Rule 310 sources, 96% for Rule 

310.01, and 73% for Rule 316 sources. The timeline provided in Figure 4-1 illustrates the improvements 

in RE over the last several years, and also points out significant revisions to previous rules, as well as 

newly adopted rules and ordinances. 

-21-
01/27/2010

Rules 310 & 310.01

Revised

01/12/2011

Ordinances

P‐27 & P‐28 Revised

06/01/2005

06/08/2005

Rule 316 Revised

04/07/2004

Rule 310 Revised

02/17/2005

Rule 310.01 Revised

03/26/2008

Rules 310 & 310.01

Revised

March 2008

September 2006

02/20/2008

Ordinances P‐25, P‐27

& P‐28  Adopted

03/12/2008

Rule 316 Revised

Figure 4-1. Timeline of Maricopa County Fugitive Dust Rules and Ordinances. 

Compliance and Enforcement Activities

MCAQD is proactively prepared to respond to high wind events and to protect human health and wellbeing. MCAQD’s approach consists of two primary components: routine proactive inspections and 

surveillance inspections, conducted both during and after significant events. MCAQD proactively 

inspects dust control-permitted sites, and increases the frequency for larger sites of 10 acres or more. 

Rule 316 sources are also proactively inspected multiple times every year. Maricopa County also 

responds to the majority of complaints within 24 hours. 

Maricopa County monitors the ADEQ Five-Day Dust Control Forecast to identify the potential for 

elevated PM10 pollution levels due to high winds or stagnant conditions. When a High Pollution 

Advisory (HPA) is issued for Maricopa County, MCAQD conducts additional surveillance before, during, 

and after the forecast event(s). MCAQD also conducts event surveillance and post-event activities on 

exceedance days that had not been forecast (i.e., those instances in which an HPA had not been issued). 

Pre-event surveillance consists of surveying high-risk areas for any dust-generating activities; educating 

sources of the impending HPA event; and issuing violations for failure to comply with local, state, or 

federal regulations. During the event, MCAQD inspectors survey high-risk areas to confirm that control 

measures are in place, document any violations, and contact other regulatory agencies if necessary. Postevent activities include continued surveys of high-risk areas, re-inspection of sources that had received 

violations within two business days, and an internal MCAQD debriefing of event activities. 

Recently, a total of 12 MCAQD air monitoring sites have been updated with new equipment that allows 

the monitoring sites to report automatically monitored readings at five-minute intervals, where previously 

only hourly data were available. The real-time monitoring data programming includes threshold triggers 

that cause the system to send alerts to MCAQD staff that the PM concentrations are elevated. The system 

allows MCAQD responders to review concentrations at the monitor and to consult the National Weather 

Service website to check for weather event activity. This capability allows the responder to identify 

regional events and monitor specific issues. If necessary, the MCAQD responders can inform nearby 

stakeholders and local governments of the elevated PM10 concentrations. 

-22-
For July 18, 2011, a Maricopa County Dust Control Forecast was issued indicating a moderate risk level 

for unhealthy PM10. The Dust Control Forecast also indicated a potential for strong and gusty winds, and 

a potential for dense blowing and transported dust generated by outflow from thunderstorms. 

An evaluation of inspection reports and compliance records indicate no evidence of unusual 

anthropogenic-based PM10 emissions. During the time period of July 15 through July 21, 2011, MCAQD 

inspectors conducted a total of 165 inspections of permitted facilities, of which 147 were at fugitive dust 

sources. Additionally, MCAQD conducted 91 inspections on vacant lots and unpaved parking lots. 

During this 7-day period, a total of 16 violations were issued county-wide for PM10 and non-PM10-related 

violations. Two violations were issued for PM10 emissions within a 4-mile radius of an exceeding 

monitor; West Phoenix and Central Phoenix. 

On July 15, 2011, a violation was issued to an owner of a vacant lot for failure to stabilize surface areas 

that had been disturbed by horse-riding activity. The vacant lot is located 3.2 miles southwest of the West 

Phoenix monitor, and the total disturbed surface area was 0.11 acres. No horse-riding activity was 

observed during the inspection. The violation was the result of a complaint MCAQD received on July 15. 

The owner was issued a 60-day letter informing them of the stability issue. The owner stabilized the area, 

installed “No Trespassing” signs, and provided the City of Phoenix with the authority to arrest any 

trespassers on the property. 

Additionally on July 15, 2011, a violation was issued to an owner of a residence for parking vehicles on a 

dirt surface that did not meet the control measures requirements of Rule 310.01 Section 302.6(b). The 

residence is located 2.4 miles north of the Central Phoenix monitor. The violation was the result of a 

complaint MCAQD received on July 14, 2011. Though the unpaved parking area did not meet the control 

measure requirements of Rule 310.01, Section 302.6(b), the inspector did not observe unstabilized areas 

during the inspection. The owner of the residence was issued a 60-day letter informing him of the control 

measure requirements. No compliance issues were observed on subsequent inspections. 

MCAQD was prepared for any complaints received due to the high wind event. During the 7 day period 

from July 15 through July 21, 2011, MCAQD received 36 complaints, of which 30 were windblown dustrelated. Each complaint was assigned to and investigated by a MCAQD inspector. A review of all 

records during this period reveals that MCAQD inspectors only observed two PM10 violations of local, 

state, or federal regulations within a 4-mile radius of the exceeding monitors. 

In addition to MCAQD’s efforts in pre-event surveillance and proactive inspections, ADEQ’s 

Agricultural Best Management Practice Program (Ag BMP) inspector monitors the ADEQ Five-Day Dust 

Control Forecast and the MCAQD air monitoring sites that include real-time data. The ADEQ Ag BMP 

inspector uses specific knowledge of seasonal activities and associations with the local growers and 

dairymen to communicate the importance of limiting dust-generating activities, especially during highwind events. Additional outreach is conducted with facility representatives prior to forecasted high-wind 

alert days. Should the PM10 readings at a MCAQD air monitoring site show notable increases, the 

ADEQ Ag BMP inspector is dispatched to contact the owners and operators of agriculture fields in the 

area to discern if their activities are causing negative impacts. The Ag BMP inspector is prepared to 

respond to most agriculture complaints within 24 hours. 

Based on a review of the inspection reports and site visit documentation, there is no evidence to suggest 

that agricultural activities produced unusual PM10 emissions. From July 19 through July 22, 2011, the 

ADEQ Ag BMP inspector received three complaints and performed site visits at one agricultural 

-23-
operation. The agriculture fields visited had established crops of corn, which would not have contributed 

to PM10 emissions. 

Conclusions

The thunderstorm outflow event of July 18, 2011, produced strong gusts and turbulent wakes that 

transported dust and PM10 into the Phoenix PM10 nonattainment area. The source region of the outflows 

that caused the exceedances was largely located in areas outside the Phoenix PM10 nonattainment area, 

primarily the deserts of Pinal, Pima, and southern Maricopa County. The Phoenix area is designated as a 

serious nonattainment area for PM10 and is required to have BACM for all significant sources of PM10. 

BACM-approved control measures on significant anthropogenic sources were in place and enforced 

during the events, and pro-active tracking and response to the events by regulatory agencies and local 

governments confirmed the uncontrollable nature of the dust emissions; therefore, these pre-existing/prior 

approved required controls are adequate for meeting the requirements of an exceptional event and should 

be considered “reasonable” for these purposes. 

Despite the deployment of comprehensive control measures and sophisticated response programs and a 

few localized, low-impact violations of the dust control rules, high wind conditions associated with 

thunderstorms and thunderstorm outflows brought high concentrations of PM10 emissions into, and also 

overwhelmed controls within, the Phoenix PM10 nonattainment area. Strong thunderstorm outflows with 

sustained winds typically ranging from 20-30 mph, and even greater nearest the source regions, were 

enough to overwhelm all available efforts to limit PM10 concentrations from the events. The fact that 

these were natural events involving strong thunderstorm outflow winds that transported PM10 emissions 

into Maricopa County, with a majority of the PM10 emissions recorded by Maricopa County area 

monitors coming from sources outside of the Phoenix PM10 nonattainment area, provides strong evidence 

that the events and exceedances of July 18, 2011, recorded within the Phoenix PM10 nonattainment area, 

were not reasonably controllable or preventable. 

-24-
V. CLEAR CAUSAL RELATIONSHIP 

Introduction

A demonstration of the clear causal connection between uncontrollable natural events or uncontrollable 

emissions associated with thunderstorm outflows and PM10 exceedances on July 18, 2011, is presented in 

the following time series graphs, infrared satellite imagery, and National Weather Service (NWS) data 

tables for selected, representative airports. These various pieces of information contain data on sustained 

wind speed, wind speed gusts, PM10 concentrations, visibility, weather observations, and thunderstorm 

development areas and provide the causal connection between the uncontrollable emissions associated 

with thunderstorm outflows and elevated PM10 concentrations. For reference, Appendix D provides 

additional NWS text products issued preceding and on July 18 containing forecasts and warnings of 

blowing dust due to thunderstorms outflows. A summary of the event day is provided below. 

Event – July 18, 2011

On July 18, 2011, the Phoenix region experienced typical summertime weather conditions with a 

maximum temperature of 111°F, relative humidity of 10-30%, winds at 5-12 miles per hour (mph) from 

various directions, and visibility at 10 miles all day, as reported at the Phoenix Sky Harbor Airport. 

Despite a spike in PM10 concentrations at most of the PM10 monitoring sites in the region late on July 17 

due to windblown dust caused by moderate thunderstorm outflow winds, PM10 concentrations on July 18 

were generally very low all day (20-50 μg/m3

 range). Between 3 p.m. Mountain Standard Time (MST) 

and 5 p.m. MST on July 18, severe thunderstorms developed over portions of Arizona to the southeast of 

the Phoenix region (as shown in Figure 5-1) and by 4 p.m., outflows from the thunderstorms entrained 

dust from desert areas of Pinal County and generated a “haboob” that progressed northwestward into 

Maricopa County, transporting large amounts of PM10. As shown in Figure 5-2, the “haboob” had the 

following impacts in the Phoenix area: southeasterly winds increased to 21-25 mph, with gusts up to 37 

mph; visibility dropped from 10 miles to 1 mile; weather conditions were reported as “haze” and 

“blowing dust”; and hourly PM10 concentrations increased to nearly 3000 μg/m3 at the South Phoenix 

monitoring site and exceeded 1500 μg/m3

 at six sites. Once the winds subsided, visibility remained below 

10 miles and PM10 concentrations remained high for several hours, resulting in 24-hour average 

concentrations above 200 μg/m3

 at five monitoring sites on July 18. In total, eight monitoring sites in the 

region exceeded the PM10 NAAQS and, as was mentioned in the Historical Fluctuation section, most of 

those eight PM10 monitoring sites saw one of their 10 highest 24-hour average PM10 concentrations 

measured in the past five years. Without the existence of the emissions generated by the thunderstorm 

outflows, there would not have been any exceedances of the 24-hour PM10 standard in the Phoenix PM10 

nonattainment area. 

-25-
11:00 a.m. 2:00 p.m. 4:00 p.m. 6:00 p.m. 

1800z Jul 18 2100z Jul 18 2300z Jul 18 0100z Jul 19 

8:00 p.m. 10:00 p.m. 12:00 a.m. 2:00 a.m. 

0300z Jul 19 0500z Jul 19 0700z Jul 19 0900z Jul 19 

Figure 5-1. Satellite imagery showing strong storms developing and dissipating over Pinal County and 

areas to the southeast of Phoenix. These storms are believed to have been responsible for the 

thunderstorm outflow boundary that caused PM10 exceedances in Maricopa and Pinal counties on July 

18. 

Figure 5-2 depicts the dust event that occurred in Maricopa County impacting the Phoenix area on July 

18. Hourly averaged PM10 concentrations at monitors throughout Maricopa County are shown along 

with visibility readings (blue diamonds) and wind speeds reported at the Phoenix Sky Harbor Airport. 

Measurements at Sky Harbor were used due to its central location amongst most of the monitors that 

experienced exceedances on July 18. As can be seen in the graph, visibility was reduced down to 1 mile 

at the Sky Harbor Airport during the windblown dust event, and this visibility reduction coincided with 

rapidly increasing PM10 concentrations and sustained wind speeds up to 25 mph in the Phoenix area. 

The NWS data tables after Figure 5-3 also show reports of haze and blowing dust during the 6 p.m. ho

when PM10 concentrations were the highest. 

ur, 

A review of the 5-minute wind and PM10 data at several of the Maricopa County monitoring sites showed 

that within 15 minutes, rapid wind speed increases and significant wind direction changes associated with 

the arrival of the haboob produced very rapid spikes in PM10 concentrations. For example, the 

Greenwood site, near the intersection of highways I10 and I17, had 5-minute average winds at 8 mph out 

of the east-southeast and a PM10 concentration of 68 μg/m3

 at 5:40 p.m. By 5:55 p.m., the 5-minute wind 

speed average had increased to over 18 mph out of the south and PM10 concentrations reached 4704 

μg/m3

. Similarly, as the haboob raced to the northwest, the Dysart monitoring site had 5-minute average 

winds at 2 mph out of the southwest and a PM10 concentration of 20 μg/m3

 at 6:00 p.m.; however, by 

6:15 p.m., the 5-minute average winds had exceeded 26 mph out of the southeast and PM10 

concentrations reached 2964 μg/m3. It was clearly evident that a thunderstorm outflow boundary had 

passed through the Phoenix region and the direction of flow caused the greatest increases in PM10 

concentrations in the central and western portions of the Phoenix PM10 nonattainment area. 

-26-
As shown in Figure 5-3, several monitoring sites in the upwind, dust source region of Pinal County 

measured 24-hour average PM10 exceedances, with the peak site—Cowtown—having a 24-hour average 

of 1,638 μg/m3

 and an extremely high maximum 1-hour average during the 5 p.m. hour of 24,976 μg/m3. 

Two other monitoring sites also measured concentrations of nearly 10,000 μg/m3

. These spikes preceded 

the PM10 concentration spikes at the monitoring sites in Maricopa by one to two hours, which is 

consistent with wind speeds and directions observed at the Phoenix Sky Harbor Airport in Maricopa 

County and the Casa Grande Airport in Pinal County for the propagation of the haboob across the region. 

Only monitoring sites in Maricopa and Pinal counties exceeded the NAAQS due to thunderstorm 

outflows on this day, but the broad range of sites and the extreme hourly- and daily-averaged PM10 

concentrations reflect the magnitude of the “haboob.” Additionally, the NWS airport meteorological data 

tables for the Phoenix Sky Harbor and Casa Grande airports show that the strongest winds associated with 

the haboob were out of the southeast. These winds, combined with the high PM10 concentrations in Pinal 

County, indicate that PM10 was already entrained in the atmosphere outside of the Phoenix PM10 

nonattainment area and then transported into the region by the strong winds. 

A visualization of this dust event was created using Phoenix visibility camera images that can be viewed 

as an animation here: 

Looking south toward Phoenix, with South Mountain in the distance: 

www.phoenixvis.net/videos/mpeg4/SOMT_07182011.mp4

The video clearly shows the wall of dust blowing across the Phoenix region in the late afternoon/early 

evening of July 18, 2011. The snapshot on the cover of this report was taken from the 5:50 p.m. mark of 

the video and coincides exactly with the PM10 concentration spikes seen in the 5-minute data from the 

PM10 monitoring sites in the central Phoenix area. Videos showing other views across the Valley on this 

date are also available (see Visibility Cameras section). 

-27-
Figure 5-2. Phoenix nonattainment area PM10 monitor readings and Sky Harbor wind speed and 

visibility for July 18, 2011.

-28-
Figure 5-3. Pinal County PM10 monitor readings and Casa Grande wind speed and visibility for July 18, 

2011.

-29-
-30-

U.S. Department of Commerce National Oceanic & Atmospheric Administration QUALITY CONTROLLED LOCAL CLIMATOLOGICAL DATA (final) 

HOURLY OBSERVATIONS TABLE 

PHOENIX SKY HARBOR INTL AIRPORT 

(23183) PHOENIX, AZ (07/2011)

Elevation: 1107 ft. above sea level Latitude: 33.427 Longitude: -112.003 Data Version: VER3

Dry 

Bulb 

Temp

Wet 

Bulb 

Temp

Dew 

Point

Temp

Date

Time 

(LST)

Station 

Type

Sky 

Conditions

Visibility

(SM)

Weather 

Type (F) (C) (F) (C) (F) (C)

Rel 

Humd 

%

Wind 

Speed (MPH)

Wind

Dir

Wind

Gusts

(MPH)

Station

Pressure

(in. hg)

Press

Tend

Ne

3-h

Ch

(m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18

0051 0151 0251 0351 0451 0551 0651 0751 0851 0951 1051 1151 1251 1351 1451 1551 1651 1739 1751 1816 1830 1831 1851 1901 1922 1951 2014 2051 2151 2251 2351

11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11

SCT150 BKN200 SCT150 BKN200 SCT150 BKN200 SCT150 BKN200 SCT150 BKN200 SCT150 SCT200 FEW180 BKN210 FEW180 SCT210 FEW180 SCT210 FEW180 SCT210 FEW095 SCT180 SCT210 FEW100 SCT210 SCT250 FEW110 SCT210 SCT250 FEW120 SCT210 SCT250 FEW120 SCT210 SCT250 FEW120 SCT210 SCT250 FEW120 SCT210 BKN250 FEW006 SCT120 SCT190 SCT009 SCT029 SCT120 BKN022 BKN120 BKN180 BKN022 BKN120 BKN250 BKN022 BKN120 BKN250 BKN020 BKN120 OVC250 BKN030 BKN120 BKN250 BKN030 BKN120 BKN250 SCT030 BKN120 BKN160 SCT045 BKN120 BKN160 SCT090 BKN140 BKN230 SCT090 BKN140 BKN230 SCT090 BKN140 BKN230 SCT090 BKN140 BKN230

10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 6.00 5.00 2.00 1.00 1.00 1.00 2.00 2.00 3.00 3.00 10.00 10.00 10.00

HZ BLDU HZ BLDU HZ BLDU HZ BLDU HZ BLDU HZ 

959393929190929495991001041061071081111081081031009999979795959595959493

35.033.933.933.332.832.233.334.435.037.237.840.041.141.742.243.942.242.039.438.037.037.036.136.035.035.035.035.035.034.433.9

70717170707071717172727272727172737272727171717170707071707071

21.321.521.821.321.221.021.621.621.822.122.022.422.222.321.822.122.522.022.022.021.921.921.621.621.321.321.321.521.321.121.5

57596059595960595958575654545150545255575757575757575758575759

13.915.015.615.015.015.015.615.015.014.413.913.312.212.210.610.012.211.012.814.014.014.013.914.014.013.914.014.413.913.915.0

28 32 33 33 34 35 34 31 30 26 24 20 18 17 15 13 17 16 20 24 25 25 26 26 28 28 28 29 28 29 32

8 10 6 7 7 6 8 8 3 8 6 8 10 0 5 10 13 21 25 23 16 16 13 13 15 13 8 11 11 9 8

150140120160160160140150150100VR 160100000170190120130140190180190210170160140150110110110130

16 17 20 30 37 30 28 28 18 

28.65 28.65 28.65 28.65 28.66 28.66 28.68 28.69 28.69 28.69 28.68 28.66 28.64 28.61 28.59 28.58 28.57 28.57 28.58 28.62 28.64 28.65 28.66 28.66 28.66 28.65 28.64 28.65 28.66 28.68 28.69

0 3 1 8 8 6 0 3

004001012003024015027010

Dynamically generated Wed Oct 31 17:28:33 EDT 2012 via http://cdo.ncdc.noaa.gov/qclcd/QCLCD
-31-

U.S. Department of Commerce National Oceanic & Atmospheric Administration QUALITY CONTROLLED LOCAL CLIMATOLOGICAL DATA (final) 

HOURLY OBSERVATIONS TABLE 

CASA GRANDE MUNICIPAL ARPT (03914)

CASA GRANDE, AZ (07/2011)

Elevation: 1462 ft. above sea level Latitude: 32.95 Longitude: -111.766 Data Version: VER2

Dry 

Bulb 

Temp

Wet 

Bulb 

Temp

Dew 

Point 

Temp

Date

Time 

(LST)

Station 

Type

Sky 

Conditions

Visibility 

(SM)

Weather 

Type (F) (C) (F) (C) (F) (C)

Rel

Humd

%

Wind 

Speed 

(MPH)

Wind

Dir

Wind

Gusts

(MPH)

Station

Pressure

(in. hg)

Press

Tend

Net

3-hr

Chg

(mb)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 

0015 0035 0055 0115 0135 0155 0215 0235 0255 0315 0335 0355 0415 0435 0455 0515 0535 0555 0615 0635 0655 0715 0735 0755 0815 0835 0855 0915 0935 0955 1015 1035 1055 1115 1135 1155 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR 

10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 

919191919090909090888888888886868684888890909090919193959597999799100

102100

33.033.033.033.032.032.032.032.032.031.031.031.031.031.030.030.030.029.031.031.032.032.032.032.033.033.034.035.035.036.037.036.037.038.039.038.0

696968696970706969686969696869696967696970707071717171717171717271727272

20.620.620.020.620.421.021.020.420.420.120.620.620.620.120.320.320.319.420.620.621.021.021.021.521.721.721.421.821.821.521.822.121.822.022.322.0

575755575759595757575959595759595957595959595961616159595957575957575757

14.0

14.013.014.014.015.015.014.014.014.015.015.015.014.015.015.015.014.015.015.015.015.015.016.016.016.015.015.015.014.014.015.014.014.014.014.0

32 32 30 32 33 35 35 33 33 35 38 38 38 35 40 40 40 40 38 38 35 35 35 38 37 37 32 30 30 26 25 28 25 24 23 24 

11 14 13 8 9 9 5 0 6 0 0 0 0 0 0 0 6 5 7 7 9 8 10 6 6 6 7 11 8 5 7 3 0 7 0 3 

110 100 110 130 130 120 120 000 110 000 000 000 000 000 000 000 050 070 050 080 090 100 110 100 110 100 120 120 100 130 170 070 000 100 000 150 

22 

28.34 

28.34 28.32 28.32 28.32 28.32 28.31 28.30 28.31 28.32 28.32 28.32 28.32 28.32 28.32 28.33 28.32 28.33 28.34 28.34 28.34 28.35 28.35 28.35 28.35 28.35 28.35 28.35 28.35 28.35 28.35 28.35 28.34 28.34 28.34 28.33 

2 4 7 

003000003
-32-

Dry 

Bulb 

Temp

Wet 

Bulb 

Temp

Dew 

Point 

Temp

Date

Time 

(LST)

Station 

Type

Sky 

Conditions

Visibility 

(SM)

Weather 

Type (F) (C) (F) (C) (F) (C)

Rel

Humd

%

Wind 

Speed 

(MPH)

Wind

Dir

Wind

Gusts

(MPH)

Station

Pressure

(in. hg)

Press

Tend

Net

3-hr

Chg

(mb)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18

1215 1235 1255 1315 1335 1355 1415 1435 1455 1515 1535 1555 1615 1635 1655 1715 1735 1755 1815 1835 1855 1915 1935 1955 2015 2035 2055 2115 2135 2155 2215 2235 2255 2315 2335 2355

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR BKN011 FEW011 CLR CLR BKN006 BKN011 CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR

10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 1.50 8.00 10.00 4.00 1.00 9.00 5.00 10.00 10.00 3.00 2.50 5.00 7.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

VCTS VCTS 

10210210210210410610410610810810610210610495979391959191939191919191919090908888868684

39.039.039.039.040.041.040.041.042.042.041.039.041.040.035.036.034.033.035.033.033.034.033.033.033.033.033.033.032.032.032.031.031.030.030.029.0

727271727372727273737372737372717070697069697069696969696969696970707069

22.322.321.822.322.622.422.122.422.722.722.822.322.822.622.321.520.921.120.721.120.620.421.120.620.620.620.620.620.420.420.420.621.220.920.920.5

575755575755555555555757575761575759555957555957575757575757575961616161

14.014.013.014.014.013.013.013.013.013.014.014.014.014.016.014.014.015.013.015.014.013.015.014.014.014.014.014.014.014.014.015.016.016.016.016.0

23 23 21 23 21 19 20 19 17 17 20 23 20 21 32 26 30 34 26 34 32 28 34 32 32 32 32 32 33 33 33 38 40 43 43 46

5 9 6 3 0 5 0 8 11 6 30s 21 20 26 29 28 29 28 18 18 16 13 9 10 0 8 5 5 10 0 6 7 5 6 5 0

110 060 110 100 000 070 000 190 140 290 070 060 080 110 120 120 150 150 150 170 160 160 210 170 000 150 150 090 100 000 110 130 130 120 140 000

18 34 33 26 34 38 37 37 29 25 21 

28.32 28.32 28.31 28.29 28.28 28.27 28.27 28.27 28.27 28.25 28.25 28.25 28.25 28.27 28.28 28.27 28.28 28.28 28.28 28.28 28.28 28.28 28.30 28.30 28.32 28.32 28.32 28.33 28.33 28.34 28.34 28.34 28.35 28.35 28.36 28.36

7 2 2 

024003007

Dynamically generated Wed Oct 31 15:26:23 EDT 2012 via http://cdo.ncdc.noaa.gov/qclcd/QCLCD
Visibility Cameras

Visibility camera images were compiled into time lapse photography videos in order to visually 

show the dust impacts on portions of Maricopa County on July 18. The animations are available 

online for each event and for each of the visibility camera sites in Maricopa County. These 

views can be accessed using the camera site codes SOMT (South Mountain), CAME (Camelback 

Mountains), SUPM (Superstition Mountains), ESMO (Estrella Mountains), and WHTM (White 

Tank Mountains). It should be noted that these time lapse photography videos may require the 

installation of a mp4 viewer. 

 

The video files can be accessed as follows: 

 

http://www.phoenixvis.net/videos/mpeg4/xxxx_mmddyyyy.mp4

 

xxxx = 4-character site code (ie. SOMT) 

mm = 2-character month (ie. 07) 

dd = 2-character day (ie. 18) 

yyyy = 4-character year (ie. 2011) 

Conclusion

The information presented within this section has adequately demonstrated a clear causal 

relationship between the emissions generated by uncontrollable natural events and the 

exceedances measured at the monitors. The satellite images, time series graphs, and 

meteorological data tables provided in this section show the temporal progression of the dust 

events from the development of the thunderstorms, to the increase in wind speeds, and to the rise 

in PM10 concentrations. The combination of the PM10 and wind data from Maricopa and Pinal 

counties shows the transport of particulate matter from the south through Pinal County and into 

the Phoenix PM10 nonattainment area. This information supports the conclusion that the events 

were primarily drawing from emission sources outside of Maricopa County and were being 

transported into the Phoenix area. 

-33-
VI. “BUT FOR” ANALYSIS 

Section 50.14(c)(3)(iv)(D) in 40 CFR part 50 requires that an exceptional event demonstration 

must satisfy that “[t]here would have been no exceedance or violation but for the event.” The 

prior sections of this submittal have provided detailed information that the exceedances on July 

18, 2011, were not reasonably controllable or preventable and there is a clear causal relationship 

between transported PM10 from thunderstorm outflows originating in the desert areas outside of 

the Phoenix PM10 nonattainment area and the measured exceedances within the nonattainment 

area. The weight of evidence in these sections demonstrates that but for the existence of 

emissions generated by these thunderstorm outflows and associated transported PM10, there 

would have been no exceedances of the 24-hour PM10 standard. 

As detailed in Section IV, all reasonable control measures were in place and actively enforced 

before, during, and after the exceedances of July 18, 2011. Inspection and compliance data of 

local fugitive dust sources during this time period revealed that PM10 from anthropogenic 

activities was well controlled and constant. Local regulatory agencies, industry, and the general 

public were alerted to the possibility of dust storms due to thunderstorm activity through daily 

forecasts and media reports. Heightened, real-time surveillance of PM10 monitoring stations 

during the events established a clear link between rapidly rising PM10 concentrations and the 

arrival of the thunderstorm outflows. On-the-ground observations recorded during the events 

consistently identify transported or re-entrained PM10 as the cause of the elevated concentrations 

near the exceeding monitors. 

As shown in Section V, detailed, time series data establish a clear causal relationship between 

the arrival of emissions generated by thunderstorm outflows and elevated PM10 concentrations 

at the monitors. Multiple, independent measurements of wind speed, wind direction, and 

visibility all point to the presence of thunderstorm outflows as the delivery vehicle for 

transported PM10 into the nonattainment area. Any local PM10 emissions generated during the 

arrival of the thunderstorm outflows are linked to the turbulent and gusty outflow front of the 

storm and pale in comparison to the amount of transported PM10 originating outside the 

nonattainment area. The source regions for the thunderstorm outflows and transported PM10 are 

clearly identified as the desert areas outside of the Phoenix PM10 nonattainment area. 

The body of evidence presented in this submittal provides no alternative that could tie the 

exceedances of July 18, 2011, to any other causal source but transported and re-entrained PM10 

generated from thunderstorm outflows, confirming that there would have been no exceedances 

but for the presence of these uncontrollable natural events. 

-34-
VII. CONCLUSIONS 

The exceedances that occurred on July 18, 2011, satisfy the following criteria of 40 CFR 50.1(j) 

and meet the definition of an exceptional event: 

 The event affects air quality; 

 The event is not reasonably controllable or preventable; and 

 The event is unlikely to reoccur at a particular location or [is] a natural event. 

Affects Air Quality

As stated in the preamble to the Exceptional Events Rule, the event in question is considered to 

have affected air quality if it can be shown that there is a clear causal relationship between the 

monitored exceedance and the event, and that the event is associated with a measured 

concentration in excess of normal historical fluctuations. Given the information presented in 

Sections 2, 3, 4, and 5, we can reasonably conclude that the event in question affected air quality. 

Not Reasonably Controllable or Preventable

Section 50.1(j) of Title 40 CFR Part 50 requires that an event must be “not reasonably 

controllable or preventable” in order to be defined as an exceptional event. This requirement is 

met by demonstrating that, despite reasonable control measures in place within Maricopa County 

and the Phoenix PM10 nonattainment area, high wind conditions overwhelmed all reasonably 

available controls. Despite the deployment of comprehensive control measures and sophisticated 

response programs, high wind conditions associated with thunderstorms and thunderstorm 

outflows brought high concentrations of PM10 emissions into, and also overwhelmed controls 

within, the Phoenix PM10 nonattainment area. The event discussed in this document that caused 

the exceedances in this request (see Sections II and V) were caused by thunderstorm-driven 

outflow winds that transported dust into Maricopa County from areas largely outside of the 

Phoenix PM10 nonattainment area. The fact that these were natural events involving strong 

thunderstorm outflow winds that transported PM10 emissions into Maricopa County, with a 

majority of the PM10 emissions recorded by Maricopa County area monitors coming from 

sources outside of the Phoenix PM10 nonattainment area, provides strong evidence that the event 

and exceedances of July 18, 2011 recorded within the nonattainment area were not reasonably 

controllable or preventable. 

Natural Event

As discussed above, the events shown to cause these exceedances were emissions of PM10 

driven by high winds caused by thunderstorm activity and related outflow boundaries on July 18, 

2011. The events therefore qualify as natural events. 

-35-
-36-

Summary

In summary, the exceedances of the federal 24-hour PM10 standard on July 18, 2011, would not 

have occurred but for the monsoonal thunderstorm-driven high winds and windblown dust 

transport from areas largely outside the Phoenix PM10 nonattainment area, based on the weight 

of evidence summarized below. 

 Graphs of PM10 at various Phoenix area monitors in Section V and the tables containing 

meteorological data for representative NWS airport sites show that the timing of elevated 

PM10 events was consistent with decreased visibility and high winds at the NWS sites. 

 Visibility camera imagery discussed in Section V indicates that large quantities of PM10

were transported into the Phoenix Metro area during the July 18th event. The timing of 

the dust storms depicted in the visibility camera imagery is consistent with the PM10

concentration measurements, elevated winds, and reduced visibility reported during the 

event. 

 Historical Fluctuation analyses and graphs in Section III and Appendix A showing five 

years of 24-hour average data for each of the Phoenix PM10 nonattainment area monitors 

depict numerous atypically high PM10 concentrations during the third quarter of 2011, 

including the day of July 18. The sudden increase in the frequency of elevated PM10

concentrations during this time period was exceptional from an historical perspective. 

 Each of the exceedances of the PM10 standard recorded on July 18 is tied to 

thunderstorm activity and thunderstorm generated outflow winds, as can be seen in data 

and analyses in Section V. 

 Figures and tables in Section V show that the timing of thunderstorm-generated outflow 

boundary passage and increases in wind speeds at monitoring locations and National 

Weather Service stations during each of the events during this period is consistent with 

the timing of elevated PM10 concentrations recorded at the monitoring locations in the 

Phoenix PM10 nonattainment area. 

 Wind directions, thunderstorm-generated outflow boundary propagation, and 

concentration patterns showing elevated levels of PM10 in Pinal County prior to levels 

increasing in Maricopa County, all depicted in Section V, help show that a vast majority 

of the dust that impacted Phoenix PM10 nonattainment area monitors originated in Pinal 

County and other areas located generally south of the Phoenix PM10 nonattainment area. 

 Section IV discusses rules that are in place in the Phoenix PM10 nonattainment area as 

well as inspections that were conducted in the area to verify compliance with those rules 

in order to show that the events are not reasonably controllable or preventable. 

Additionally, the visibility webcam time lapse photography videos referenced in Section 

II help illustrate the magnitude and scale of these events, which supports the claim that 

the exceedances recorded during this period were not reasonably controllable or 

preventable. 
APPENDIX A

5-YEAR HISTORICAL FLUCTUATION GRAPHS FOR 

THE PHOENIX PM10 NONATTAINMENT AREA MONITORING SITES
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Buckeye 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Central Phoenix 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

West Phoenix 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Greenwood 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500550

PM10 Concentration (μg/m

3

)

Date

JLG Supersite 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

West 43rd 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

South Phoenix 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Durango 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

West Chandler 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Higley 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Glendale 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Dysart 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Zuni Hills 5-Year Historical Fluctuation - 24 Hour Averages
0

50

100150200250300350400450500

PM10 Concentration (μg/m

3

)

Date

Apache Junction 5-Year Historical Fluctuation - 24 Hour Averages
APPENDIX B

ADEQ FORECAST PRODUCTS FOR PHOENIX, 

MARICOPA COUNTY, AND PINAL COUNTY
MARICOPA COUNTY DUST CONTROL FORECAST 

ISSUED SUNDAY, JULY 17, 2011

Five-day weather outlook:

DURING ACTIVE SUMMER MONSOON EPISODES STRONG GUSTY WINDS AND DENSE BLOWING DUST ARE 

POSSIBLE EVEN FROM DISTANT THUNDERSTORMS

The summer monsoon circulation pattern has resumed and for the Phoenix metro area this means a slight chance for thunderstorms 

each day thru this forecast period. Gradient winds will be relatively light. Since last week’s rainfall has reduced but not eliminated 

the risk for periods of dense blowing and/or transported dust generated by outflow boundaries, there will be a moderate risk for 

unhealthy PM-10 levels. 

RISK FACTORS

WINDS STAGNATION

UNHEALTHY PM-10

RISK LEVEL

Day 1: Mon 07/18/2011

No significant winds 

expected except strong 

and gusty due to 

outflow from 

thunderstorms. 

+ 

No significant 

stagnation expected. = MODERATE 

Day 2: Tue 07/19/2011

No significant winds 

expected except strong 

and gusty due to 

outflow from 

thunderstorms. 

+ 

No significant 

stagnation expected. = MODERATE

Day 3: Wed 07/20/2011

No significant winds 

expected except strong 

and gusty due to 

outflow from 

thunderstorms. 

+ 

No significant 

stagnation expected. = MODERATE

EXTENDED OUTLOOK 

Day 4: Thu 07/21/2011

No significant winds 

expected except strong 

and gusty due to 

outflow from 

thunderstorms. 

+ 

No significant 

stagnation expected. = MODERATE

Day 5: Fri 07/22/2011

No significant winds 

expected except strong 

and gusty due to 

outflow from 

thunderstorms. 

 

+ No significant 

stagnation expected. = MODERATE 

The Maricopa County Dust Control Action Forecast is issued to assist in the planning of work activities to 

help reduce dust pollution. A recorded message of this forecast can be accessed at 602-771-2368. To review 

the complete air quality forecast for the Phoenix metropolitan area, as well as the health impacts and 

reduction methods for different air pollutants, call 602-771-2367 for recorded forecast information or click on 

ADEQ's Air Quality Forecast at http://www.azdeq.gov/environ/air/ozone/ensemble.pdf.

CKR 04/28/2011
VERY UNHEALTHY (201-300) 

UNHEALTHY (151-200)

UNHEALTHY FOR SENSITIVE GROUPS (101-150) 

MODERATE (51-100)

GOOD (0-50) 

 For more information visit: 

http://www.airnow.gov/index.cfm?action=aqibasics.aqi

 

NEW!!! CLICK HERE FOR UPDATED OZONE SEASON STATS NEW!!! 

AIR QUALITY FORECAST FOR MONDAY, JULY 18, 2011

This report is updated by 1:00 p.m. Sunday thru Friday and is valid 

for areas within and bordering Maricopa County in Arizona

 

* O3 = Ozone CO = Carbon Monoxide PM-10 = Particles 10 microns & smaller PM-2.5 = Particles smaller than 2.5 microns

*“Ozone Health Watch” means that the highest concentration of OZONE may approach the federal health standard. 

“PM-10 or PM-2.5 Health Watch” means that the highest concentration of PM-10 or PM-2.5 may approach the federal health standard. 

“High Pollution Advisory” means that the highest concentration of OZONE, PM-10, or PM-2.5 may exceed the federal health standard. 

“DUST” means that short periods of high PM-10 concentrations caused by outflow from thunderstorms are possible.

FORECAST 

DATE

NOTICES 

(*SEE BELOW 

FOR DETAILS) 

AIR POLLUTANT 

YESTERDAY 

SAT 07/16/2011

OZONE HEALTH 

WATCH 

 

DUST 

Highest AQI Reading/Site 

(Preliminary data only) 

TODAY 

SUN 07/17/2011

OZONE HEALTH 

WATCH 

DUST 

TOMORROW 

MON 07/18/2011

OZONE HEALTH 

WATCH 

DUST 

EXTENDED 

TUE 07/19/2011

OZONE HEALTH 

WATCH 

LIKELY/ 

OZONE HIGH 

 POLLUTION 

ADVISORY 

POSSIBLE 

DUST 

O3* 

87 

TONTO NATIONAL 

MONUMENT 

90 

MODERATE 

97 

MODERATE 

100 

MODERATE 

CO* 09 

GREENWOOD 

06 

GOOD 

07 

GOOD

08 

GOOD 

PM-10* 35 

BUCKEYE 

70 

MODERATE

75 

MODERATE

75 

MODERATE 

PM-2.5* 30 

PHOENIX SUPERSITE 

48 

GOOD

42 

GOOD

44 

GOOD
Health message for Sunday, July 17: Unusually sensitive people should consider 

reducing prolonged or heavy exertion outdoors. 

Health message for Monday, July 18: Unusually sensitive people should consider 

reducing prolonged or heavy exertion outdoors. 

Synopsis and Discussion

OZONE: TODAY’S OZONE HEALTH WATCH HAS BEEN EXTENDED THRU MONDAY JULY 18

The synoptic weather pattern for today thru Tuesday will feature a strong upper level high situated to the northeast of 

Arizona. This location often yields a low-level easterly wind flow that tends to inhibit local ozone dispersion. At the 

same time, afternoon high temperatures near 110 degrees F – forecasted to occur under mostly sunny skies – will 

contribute to very efficient ozone production. Thus, ozone concentrations near or at unhealthy levels are expected the 

next few days and as a result the Ozone Health Watch currently in effect has been extended thru at least Monday. 

PARTICLES: The summer monsoon circulation pattern has resumed and for the Phoenix metro area this means a 

slight chance for thunderstorms each day thru this forecast period. Gradient winds will be relatively light. Since last 

week’s rainfall has reduced but not eliminated the risk for periods of dense blowing and/or transported dust generated 

by outflow boundaries, there will be a moderate risk for unhealthy PM-10 levels. 

MONITORING SITE MAPS: STATIC MAP - http://www.azdeq.gov/environ/air/monitoring/images/map.jpg

 INTERACTIVE MAPS - http://aqwww.maricopa.gov/AirMonitoring/SitePollutionMap.aspx

 http://www.airnow.gov/ 

POLLUTION MONITOR READINGS FOR SATURDAY, JULY 16, 2011 

O3 (OZONE) 

Info on current 8-hour ozone standard: http://www.epa.gov/air/ozonepollution/pdfs/2008_03_aqi_changes.pdf

For archived AQI maps go to: http://www.airnow.gov/index.cfm?action=airnow.maps

 SITE NAME MAX 8-HR VALUE (PPB) MAX AQI AQI COLOR CODE 

Alamo Lake (La Paz County) 53 45 

Apache Junction (Pinal County) 65 67 

Blue Point 63 61 

Buckeye 48 41 

Casa Grande (Pinal County) 55 47 

Cave Creek 60 51 

Central Phoenix 62 58 

Dysart 51 43 

Falcon Field 64 64 

Fountain Hills 63 61 

Glendale 59 50 

Humboldt Mountain 57 48 

North Phoenix 66 71 

Phoenix Supersite 65 67 

Pinal Air Park (Pinal County) 54 46 

Pinnacle Peak 57 48 

Queen Valley (Pinal County) 64 64 

Rio Verde 59 50 

South Phoenix 61 54 

South Scottsdale 66 71 

Tempe 60 51 

Tonto Nat’l Mon. (Gila County) 71 87 

West Chandler 62 58 

West Phoenix 62 58 

Yuma (Yuma County) 53 45 
CO (CARBON MONOXIDE) 

 SITE NAME MAX 8-HR VALUE (PPM) MAX AQI AQI COLOR CODE 

Central Phoenix 0.7 08 

Greenwood 0.8 09 

Phoenix Supersite NOT AVBL NOT AVBL NOT AVBL 

West Phoenix 0.4 05 

PM-10 (PARTICLES)

 SITE NAME MAX 24-HR VALUE (ug/m3) MAX AQI AQI COLOR CODE 

Buckeye 38 35 

Central Phoenix 26 24 

Combs School(Pinal County NOT AVBL NOT AVBL NOT AVBL 

Durango 26 24 

Dysart 18 17 

Glendale 21 19 

Greenwood 28 26 

Higley 33 31 

Maricopa (Pinal County) NOT AVBL NOT AVBL NOT AVBL 

Phoenix Supersite 25 23 

South Phoenix 35 32 

West Chandler 27 25 

West Forty Third 38 35 

West Phoenix 34 31 

Zuni Hills 18 17 

PM-2.5 (PARTICLES) 

(Some data derived from light-scattering equipment)

For maps go to: http://www.airnow.gov/

 SITE NAME MAX 24-HR VALUE (ug/m3) MAX AQI AQI COLOR CODE 

Durango 7.0 23 

Dysart 3.1 10 

Estrella Mountain Park 5.4 18 

Glendale 4.6 15 

Phoenix Supersite 9.3 30 

South Phoenix 4.6 15 

Vehicle Emissions Lab 3.7 12 

West Phoenix 5.3 17 

 
LOCAL AIR POLLUTANTS IN DETAIL 

O3 (OZONE): 

Description – This is a secondary pollutant that is formed by the reaction of other primary 

pollutants (precursors) such as VOCs (volatile organic compounds) and NOx (Nitrogen Oxides) 

in the presence of heat and sunlight. 

Sources – VOCs are emitted from motor vehicles, chemical plants, refineries, factories, and 

other industrial sources. NOx is emitted from motor vehicles, power plants, and other sources of 

combustion. 

Potential health impacts – Exposure to ozone can make people more susceptible to respiratory 

infection, result in lung inflammation, and aggravate pre-existing respiratory diseases such as 

asthma. Other effects include decrease in lung function, chest pain, and cough. 

 Unit of measurement – Parts per billion (ppb). 

 Averaging interval – Highest eight-hour period within a 24-hour period (midnight to midnight). 

Reduction tips – Curtail daytime driving, refuel cars and use gasoline-powered equipment as late 

in the day as possible. 

CO (CARBON MONOXIDE): 

Description – A colorless, odorless, poisonous gas formed when carbon in fuels is not burned 

completely. 

Sources – In cities, as much as 95 percent of all CO emissions emanate from automobile exhaust. 

Other sources include industrial processes, non-transportation fuel combustion, and natural 

sources such as wildfires. Peak concentrations occur in colder winter months. 

Potential health impacts – Reduces oxygen delivery to the body’s organs and tissues. The health 

threat is most serious for those who suffer from cardiovascular disease. 

 Unit of measurement – Parts per million (ppm). 

 Averaging interval – Highest eight-hour period within a 24-hour period (midnight to midnight) 

 Reduction tips – Keep motor vehicle tuned properly and minimize nighttime driving. 

 

PM-10 & PM-2.5 (PARTICLES): 

Description – The term “particulate matter” (PM) includes both solid particles and liquid droplets 

found in air. Many manmade and natural sources emit PM directly or emit other pollutants that 

react in the atmosphere to form PM. Particles less than 10 micrometers in diameter tend to pose 

the greatest health concern because they can be inhaled into and accumulate in the respiratory 

system. Particles less than 2.5 micrometers in diameter are referred to as “fine” particles and are 

responsible for many visibility degradations such as the “Valley Brown Cloud” (see 

http://www.phoenixvis.net/). Particles with diameters between 2.5 and 10 micrometers are referred 

to as “coarse”. 

Sources – Fine = All types of combustion (motor vehicles, power plants, wood burning, etc.) and 

some industrial processes. Coarse = crushing or grinding operations and dust from paved or 

unpaved roads. 

Potential health impacts – PM can increase susceptibility to respiratory infections and can 

aggravate existing respiratory diseases, such as asthma and chronic bronchitis. 

 Units of measurement – Micrograms per cubic meter (ug/m3) 

 Averaging interval – 24 hours (midnight to midnight). 

 Reduction tips – Stabilize loose soils, slow down on dirt roads, carpool, and use public transit. 

 {Updated 03/23/2010} 
APPENDIX C

NOTICE OF PUBLIC COMMENT PERIOD
Request for Public Comments on Exceptional Events in the Greater Phoenix Area

In 2005, Congress identified a need to account for events that result in exceedances of the 

National Ambient Air Quality Standards (NAAQS) that are exceptional in nature (e.g., not 

expected to reoccur or caused by acts of nature beyond man-made controls.) In response, EPA 

promulgated the Exceptional Events Rule (EER) to address exceptional events in 40 CFR Parts 

50 and 51 on March 22, 2007 (72 FR 13560). On May 2, 2011, EPA released draft guidance 

documents on the implementation of the EER to State, tribal and local air agencies for review. 

The EER allows for states and tribes to “flag” air quality monitoring data as an exceptional 

event. If flagged, these data can be excluded from consideration in air quality planning if EPA 

concurs with the demonstration submitted by the flagging agency documenting that all 

procedural and technical requirements have been met. 

Pursuant to 40 CFR 50.14(c)(3)(i), the Arizona Department of Environmental Quality (ADEQ) is 

soliciting comments on its final demonstrations of events that have caused elevated 

concentrations of PM10 in the Greater Phoenix area on February 19; July 18; August 3; August 

18; August 25 through 28; September 2; October 4; November 4, 2011; January 21 – 22 and 

February 27, 2012. ADEQ has decided to flag these episodes based on these analyses. Copies of 

the demonstrations are available for review beginning Monday, December 3, 2012, on the ADEQ 

website at www.azdeq.gov/environ/air/plan/. Interested parties can submit written comments 

throughout the comment period which will end at 5:00 p.m. on Tuesday, January 1, 2013. Any 

comments received will be responded to and forwarded to EPA with the final demonstrations. 

Written comments should be addressed, faxed, or e-mailed to: 

Andra Juniel, Air Assessment Section, Arizona Department of Environmental Quality, 1110 W. 

Washington Street, 3415-A, Phoenix, AZ 85007, PHONE: (602) 771-4417; FAX: (602) 771-

2366, E-mail: juniel.andra@azdeq.gov. 

In addition to being available on-line, copies of the analyses are available for review, Monday 

through Friday, 8:30 a.m. to 4:30 p.m., at the ADEQ Records Center, 1110 W. Washington St., 

Phoenix, AZ, 85007, Attn: David Olivo, (602) 771-4380, email: olivo.david@azdeq.gov. 

Persons with a disability may request reasonable accommodations by contacting Linda Morrison 

at (602) 771-4793 or 1-800-234-5677 ext. 771-4793. This document is available in alternative 

formats by contacting ADEQ TDD phone number at (602) 771-4829. 
APPENDIX D

NATIONAL WEATHER SERVICE FORECASTS, 

WARNINGS, AND ALERTS
378 

CDUS45 KPSR 190712

CLIPHX

CLIMATE REPORT

NATIONAL WEATHER SERVICE PHOENIX AZ

1211 AM MST TUE JUL 19 2011

...................................

...THE PHOENIX AZ CLIMATE SUMMARY FOR JULY 18 2011...

CLIMATE NORMAL PERIOD 1971 TO 2000

CLIMATE RECORD PERIOD 1895 TO 2011

WEATHER ITEM OBSERVED TIME RECORD YEAR NORMAL DEPARTURE LAST

 VALUE (LST) VALUE VALUE FROM YEAR

 NORMAL

..................................................................

TEMPERATURE (F)

YESTERDAY

 MAXIMUM 112 301 PM 115 1989 107 5 109

 MINIMUM 90 627 AM 68 1946 84 6 89

 1908

1899

 AVERAGE 101 95 6

PRECIPITATION (IN)

 YESTERDAY 0.00 1.04 1946 0.03 -0.03 T

 MONTH TO DATE 0.58 0.48 0.10 T

 SINCE OCT 1 3.36 6.09 -2.73 5.40

 SINCE JAN 1 1.62 3.65 -2.03 4.92

SNOWFALL (IN)

 YESTERDAY 0.0

 MONTH TO DATE 0.0

 SINCE JAN 1 0.0

 SINCE JUL 1 0.0

 SNOW DEPTH 0

DEGREE DAYS

HEATING

 YESTERDAY 0 0 0 0

 MONTH TO DATE 0 0 0 0

 SINCE JUN 1 0 0 0 0

 SINCE JUL 1 0 0 0 0

COOLING

 YESTERDAY 36 30 6 34

 MONTH TO DATE 540 524 16 611

 SINCE JUN 1 1323 1269 54 1405

 SINCE JAN 1 2187 2064 123 2057

..................................................................
WIND (MPH)

 HIGHEST WIND SPEED 31 HIGHEST WIND DIRECTION SE (140)

 HIGHEST GUST SPEED 39 HIGHEST GUST DIRECTION SE (140)

 AVERAGE WIND SPEED 8.9

SKY COVER

 POSSIBLE SUNSHINE MM

 AVERAGE SKY COVER 0.6

WEATHER CONDITIONS

THE FOLLOWING WEATHER WAS RECORDED YESTERDAY.

 HAZE

 SANDSTORM

RELATIVE HUMIDITY (PERCENT)

HIGHEST 37 600 AM

LOWEST 14 400 PM

AVERAGE 26

..........................................................

THE PHOENIX AZ CLIMATE NORMALS FOR TODAY

 NORMAL RECORD YEAR

MAXIMUM TEMPERATURE (F) 107 116 1989

MINIMUM TEMPERATURE (F) 84 68 1913

 1899

SUNRISE AND SUNSET

JULY 19 2011..........SUNRISE 532 AM MST SUNSET 737 PM MST

JULY 20 2011..........SUNRISE 532 AM MST SUNSET 736 PM MST

- INDICATES NEGATIVE NUMBERS.

R INDICATES RECORD WAS SET OR TIED.

MM INDICATES DATA IS MISSING.

T INDICATES TRACE AMOUNT.

&&

.....................................................................

DEW POINT

 HIGHEST DEW POINT MM

 LOWEST DEW POINT MM

 AVERAGE DEW POINT MM

.....................................................................

$$

855 

FXUS65 KPSR 180200

AFDPSR
AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE PHOENIX AZ

700 PM MST SUN JUL 17 2011

.SYNOPSIS...

A FLOW ALOFT FROM THE SOUTHEAST IN THE CLOCKWISE FLOW OF AIR AROUND 

STRONG HIGH PRESSURE ALOFT OVER THE CENTRAL UNITED STATES WILL BRING 

SOMEWHAT HIGHER HUMIDITY TO OUR AREA TONIGHT AND MONDAY. THIS MEANS 

POSSIBLE DESERT BLOWING DUST AND A SLIGHT CHANCE OF THUNDERSTORMS IN 

SOUTH-CENTRAL AND SOUTHWEST ARIZONA MAINLY IN THE LATE AFTERNOON AND 

NIGHTTIME HOURS THROUGH TUESDAY. TEMPORARY DRYING IS EXPECTED FOR 

MOST OF THE AREA LATE WEDNESDAY THROUGH THURSDAY INTO FRIDAY... 

FOLLOWED BY MORE MOISTURE AND AN INCREASING CHANCE OF SHOWERS AND 

THUNDERSTORMS NEXT WEEKEND.

&&

.DISCUSSION...

UPDATED THE HIGH AND LOW RESOLUTION FORECAST PRODUCTS TO BETTER 

REFLECT THE EXPECTED WEATHER DURING THE REMAINDER OF THIS EVENING 

AND THROUGH THE OVERNIGHT HOURS. 18.00Z KPSR SOUNDING IS VERY 

STABLE...EVIDENT FROM THE AREA OF CONVECTION CROSSING PINAL COUNTY 

THAT SEEMED TO HIT A BRICK WALL AS NEARED CASA GRANDE. SCATTERED 

CONVECTION CONTINUES ACROSS FAR EASTERN ARIZONA...AND LOOKS TO HAVE 

INCREASED A BIT IN COVERAGE THE PAST TWO HOURS. THIS IS BEING FORCED 

BY TWO MID-LEVEL LOWS /LARGER ONE OVER SONORA MX WITH ANOTHER NEAR 

THE WHITE MOUNTAINS/. EXPECTING THE SONORA LOW TO LIFT NNW THROUGH 

THE OVERNIGHT HOURS...WITH THE POSITIVE VORTICITY ADVECTION AND 

MOISTURE ADVECTION AROUND 500 MB WE SHOULD SEE SOME 

ISOLATED/SCATTERED CONVECTION DEVELOP AND MOVE THROUGH OUR CWA 

THROUGH THE EARLY MORNING HOURS ON MONDAY.

PREVIOUS DISCUSSION...

ALTHOUGH MOST RAINFALL AMOUNTS ARE EXPECTED TO BE LESS THAN A 

QUARTER OF AN INCH TONIGHT...SOME AMOUNTS OVER A QUARTER OF AN INCH 

ARE EXPECTED IN SOUTH-CENTRAL ARIZONA WITH THE BEST CHANCE 

SOUTHEAST...EAST AND NORTHEAST OF PHOENIX. THERE IS ALSO A SLIGHT 

CHANCE OF SHOWERS AND THUNDERSTORMS ACROSS SOUTHWEST ARIZONA TONIGHT 

THROUGH MONDAY NIGHT...AND IN SOUTHEAST CALIFORNIA MAINLY LATE 

TONIGHT AND MONDAY MORNING.

DRIER AIR FROM THE SOUTHWEST IS EXPECTED TO TEMPORARILY MOVE OVER 

MOST OF THE AREA INCLUDING THE GREATER PHOENIX AREA LATE WEDNESDAY 

THROUGH THURSDAY INTO FRIDAY. THIS IS EXPECTED TO HAPPEN AS THE WEST 

COAST LOW PRESSURE SYSTEM MOVES INLAND. THUS...HAVE CONFINED THE 

POSSIBILITY OF ANY SHOWERS AND THUNDERSTORMS TO THE HIGHER TERRAIN 

EAST OF THE GREATER PHOENIX AREA WEDNESDAY THROUGH DAYTIME FRIDAY.

IT IS STILL TOO EARLY TO TELL YET HOW CLOSE A TROPICAL STORM OR 

POSSIBLY STRONGER WILL GET TO THE SOUTHERN TIP OF BAJA CALIFORNIA 

LATE THIS COMING WEEK...FRIDAY OR SATURDAY. THIS SYSTEM BEARS 

WATCHING IN TERMS OF THE AMOUNT OF MOISTURE THAT MAY MOVE INTO THE 

SOUTHWESTERN UNITED STATES INCLUDING OUR AREA NEXT WEEKEND FOR A 

SIGNIFICANT INCREASE IN SHOWERS AND THUNDERSTORMS.

FOR THE TIME BEING...HAVE BROUGHT BACK THE POSSIBILITY OF SHOWERS 
AND THUNDERSTORMS TO ALL OF SOUTH-CENTRAL ARIZONA FRIDAY NIGHT...THEN

SPREAD THAT POSSIBILITY FROM SOUTH-CENTRAL ARIZONA WEST TO THE LOWER 

COLORADO RIVER VALLEY BY SATURDAY NIGHT...AND THE ENTIRE AREA FROM 

SOUTH-CENTRAL ARIZONA ACROSS SOUTHWEST ARIZONA AND SOUTHEAST 

CALIFORNIA SUNDAY. AS WE GET CLOSER TO NEXT WEEKEND AND EARLY NEXT 

WEEK...WE WILL MOST LIKELY HAVE TO INCREASE THE CHANCE OF SHOWERS 

AND THUNDERSTORMS FOR AT LEAST ONE OF THE LATE AFTERNOON AND 

NIGHTTIME PERIODS...DURING THE WEEKEND AND EARLY NEXT WEEK.

&&

.AVIATION...

SOUTH-CENTRAL ARIZONA INCLUDING KPHX AND KIWA...

DRY/STABLE ENVIRONMENT OVER PHOENIX THIS EVENING WHICH IS CRUSHING 

CONVECTION AS IT APPROACHES FROM THE SOUTHEAST. OUTFLOW FROM THE SE 

IS WASHING OUT RIGHT NOW...LIKELY NOT TO MAKE IT TO KPHX. BETTER 

CONVECTION FURTHER EAST SHOULD HOLD SOMEWHAT CAUSING AN INCREASE IN 

CLOUDS AND SOME -SHRA DURING THE LATE EVENING THROUGH EARLY MORNING 

HOURS. COULD SEE BETTER COVERAGE OF /WEAK/ CONVECTION ON MONDAY.

SOUTHEAST CALIFORNIA INCLUDING KIPL AND KBLH...

GENERALLY LIGHT WINDS WITH INCREASING MID/HIGH CLOUDS HEADING INTO 

MONDAY MORNING. A STRAY SHOWER MAY WANDER TOWARD KBLH EARLY MONDAY 

MORNING.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. 

&&

.FIRE WEATHER...

WEDNESDAY THROUGH SUNDAY...

HIGH PRESSURE CENTERED OVER THE CENTRAL UNITED STATES WILL SKIRT 

FURTHER EAST IN RESPONSE TO AN UPPER LEVEL TROUGH APPROACHING THE 

PACIFIC COAST. THIS WILL HELP TO SET UP A RETURN FLOW OF MONSOONAL 

MOISTURE FROM THE GULF OF CALIFORNIA INTO ARIZONA...INCREASING THE 

PROBABILITY OF MAINLY AFTERNOON AND EVENING SHOWERS. THE GREATEST 

THREAT FOR STORMS LOOKS TO BE ON FRIDAY AND SATURDAY...WITH THE 

BIGGEST THREATS BEING HEAVY RAINFALL...GUSTY SURFACE WINDS...AND 

BLOWING DUST.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

DISCUSSION...INIGUEZ/ELLIS
AVIATION...INIGUEZ

FIRE WEATHER...MEYERS/MCLANE

339 

FXUS65 KPSR 181003

AFDPSR

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE PHOENIX AZ

300 AM MST MON JUL 18 2011

.SYNOPSIS...

MEXICAN MONSOON MOISTURE HAS STARTED TO MOVE DEEPER INTO CENTRAL AND 

EASTERN ARIZONA. A COUPLE OF WEAK WEATHER DISTURBANCES MOVING INTO 

ARIZONA FROM MEXICO WILL INCREASE THE THREAT FOR AFTERNOON AND 

EVENING SHOWERS AND THUNDERSTORMS MAINLY OVER CENTRAL AND EASTERN 

ARIZONA TODAY AND TUESDAY. A PACIFIC LOW PRESSURE SYSTEM MOVING 

THROUGH THE WESTERN STATES NORTH OF ARIZONA WILL BEGIN TO PUSH THE 

MONSOON MOISTURE EAST...INTO EASTERN ARIZONA WHERE MOST OF THE 

THUNDERSTORM ACTIVITY WILL DEVELOP WEDNESDAY THROUGH FRIDAY. MONSOON 

MOISTURE WILL PUSH BACK DEEPER INTO ARIZONA THIS WEEKEND FOR MORE 

WIDESPREAD THUNDERSTORM COVERAGE THIS WEEKEND.

&&

DISCUSSION...

MONSOON MOISTURE CONTINUED TO INCREASE FROM THE SOUTH...AND HAD 

BECOME A LITTLE THICKER OVER SOUTHERN AZ DURING THE PREVIOUS 24 

HOURS. AT THE SAME TIME...AN INVERTED TROF LOCATED SOUTHWEST OF EL 

PASO TEXAS AT 07Z MON...SEEN THE BEST IN WATER VAPOR IMAGERY... 

CONTINUED TO MOVE TOWARD AZ. A VERY NOTICEABLE UPPER LEVEL 

DEFORMATION ZONE AHEAD OF THIS FEATURE...AND DEVELOPING OVER SOUTH 

CENTRAL AZ...WILL CONTINUE TO PRODUCE A SLIGHT CHANCE OF SHOWERS 

INTO THE MORNING HOURS. AT 08Z ONLY SPOTTY LIGHT SHOWERS WERE 

DETECTED BY RADAR VICINITY PHOENIX...CASA GRANDE...AND OVER ZONE 24. 

NO LIGHTNING. SOUTHEAST CA AND THE COLORADO RIVER VALLEY HAD MOSTLY 

CLEAR SKIES.

THERE WAS ABSOLUTELY NO UPPER AIR DATA OVER A LARGE PORTION OF 

NORTHWEST MEXICO...I.E. NO CHIHUAHUA NOR GUAYMAS SOUNDINGS LAST 

EVENING...AND IS PERHAPS THE REASON WHY MODELS DIDNT INITIALIZE THE 

INVERTED TROF SOUTHWEST OF EL PASO VERY WELL. THE EUROPEAN MODEL 

CAME CLOSEST AND WE WILL PRETTY MUCH FOLLOW ITS KINEMATIC FLOW 

REGIME AT AND ABOVE 500 MB WITH A SLIGHT MODIFICATION. 

THE MEXICAN INVERTED TROF WILL CONTINUE TO MOVE TOWARD THE NORTHWEST 

AND INTO SOUTHERN AZ THIS AFTERNOON BUT IN A MUCH WEAKENED STATE. 

WEAKNESS ALOFT COMBINED WITH INCREASED MOISTURE...AND FAIRLY STIFF 

SOUTHEASTERLY MID LEVEL CONVECTIVE STEERING CURRENTS /10-15 KTS/... 

CERTAINLY BODE WELL FOR AN ELEVATED THUNDERSTORM THREAT THIS 

AFTERNOON AND EVENING. SINCE WE TALK IN PROBABILITY TERMS...THE 

ABOVE ATMOSPHERIC PARAMETERS TODAY CONVEY AN ATYPICAL MONSOON DAY 

AND WARRANT THE ELEVATED CONVECTIVE THREAT...REPLETE WITH GUSTY 

OUTFLOW WINDS AND A CHANCE OF DENSE BLOWING DUST ON THE DESERTS. 

SOUTHEAST CA AND THE FAR SOUTHWEST AZ DESERTS VICINITY THE COLORADO 
RIVER VALLEY WILL REMAIN DRY AND MORE STABLE ALOFT.

TUESDAY...

ANOTHER BUT VERY WEAK UPPER LEVEL PERTURBATION WILL FLOW INTO 

CENTRAL AND SOUTHERN AZ FROM THE SOUTHWEST DIRECTION. MID LEVEL 

CONVECTIVE STEERING CURRENTS OVER THE CENTRAL AZ DESERTS WILL 

DECREASE TO LIGHT AND VARIABLE WINDS...BUT DEEP MONSOON MOISTURE

REMAINS. IT APPEARS MOST CONVECTION TUESDAY AFTERNOON/EVENING WILL 

DEVELOP OVER THE MOUNTAINS EAST OF PHOENIX AND IN SOUTHEAST AZ... 

HOWEVER SINCE THERE IS SOME WEAKNESS ALOFT...CONVECTIVE OUTFLOW 

BOUNDARIES AND POSSIBLE BOUNDARY COLLISIONS COULD GENERATE EVENING 

CONVECTION ON THE CENTRAL DESERTS BETWEEN CASA GRANDE AND PHOENIX.

AGAIN...SOUTHEAST CA AND THE FAR SOUTHWEST AZ DESERTS VICINITY THE 

COLORADO RIVER VALLEY WILL REMAIN DRIER AND MORE DYNAMICALLY STABLE 

ALOFT.

WEDNESDAY THROUGH FRIDAY...

A PACIFIC TROF PASSING THROUGH THE WESTERN STATES NORTH OF AZ WILL 

BEGIN TO TREND THE WINDS ALOFT TO A MORE STABLE AND DRIER WEST AND 

SOUTHWESTERLY DIRECTION. ONLY A SLIGHT CHANCE OF AFTERNOON AND 

EVENING SHOWERS WILL EXIST EAST OF PHOENIX...BUT CONCENTRATED OVER 

EASTERN AZ. 

SATURDAY THROUGH MONDAY...

A COUPLE OF WELL DEFINED INVERTED TROFS IN THE MEXICAN EASTERLIES... 

WILL BRUSH SOUTHEAST AZ THIS PERIOD. THIS WILL ENABLE MONSOON 

MOISTURE TO PUSH BACK INTO AZ FOR AN ELEVATED CHANCE OF AFTERNOON 

AND EVENING THUNDERSTORMS OVER CENTRAL AND EASTERN AZ.

&&

.AVIATION...

SOUTH-CENTRAL ARIZONA INCLUDING KPHX AND KIWA...

DRY/STABLE ENVIRONMENT OVER PHOENIX THIS EVENING WHICH IS CRUSHING 

CONVECTION AS IT APPROACHES FROM THE SOUTHEAST. OUTFLOW FROM THE SE 

IS WASHING OUT RIGHT NOW...LIKELY NOT TO MAKE IT TO KPHX. BETTER 

CONVECTION FURTHER EAST SHOULD HOLD SOMEWHAT CAUSING AN INCREASE IN 

CLOUDS AND SOME -SHRA DURING THE LATE EVENING THROUGH EARLY MORNING 

HOURS. COULD SEE BETTER COVERAGE OF /WEAK/ CONVECTION ON MONDAY.

SOUTHEAST CALIFORNIA INCLUDING KIPL AND KBLH...

GENERALLY LIGHT WINDS WITH INCREASING MID/HIGH CLOUDS HEADING INTO 

MONDAY MORNING. A STRAY SHOWER MAY WANDER TOWARD KBLH EARLY MONDAY 

MORNING.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. 

&&

.FIRE WEATHER...

WEDNESDAY THROUGH SUNDAY...

HIGH PRESSURE CENTERED OVER THE CENTRAL UNITED STATES WILL SKIRT 

FURTHER EAST IN RESPONSE TO AN UPPER LEVEL TROUGH APPROACHING THE 

PACIFIC COAST. THIS WILL HELP TO SET UP A RETURN FLOW OF MONSOONAL 

MOISTURE FROM THE GULF OF CALIFORNIA INTO ARIZONA...INCREASING THE 

PROBABILITY OF MAINLY AFTERNOON AND EVENING SHOWERS. THE GREATEST 
THREAT FOR STORMS LOOKS TO BE ON FRIDAY AND SATURDAY...WITH THE 

BIGGEST THREATS BEING HEAVY RAINFALL...GUSTY SURFACE WINDS...AND 

BLOWING DUST.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

DISCUSSION...VASQUEZ

AVIATION...INIGUEZ

FIRE WEATHER...MEYERS/MCLANE

176 

FXUS65 KPSR 182252

AFDPSR

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE PHOENIX AZ

340 PM MST MON JUL 18 2011

.SYNOPSIS...

A FLOW ALOFT FROM THE SOUTHEAST IN THE CLOCKWISE FLOW OF AIR AROUND 

STRONG HIGH PRESSURE ALOFT CENTERED OVER THE NATIONS MIDSECTION IS 

BRINGING SOMEWHAT HIGHER HUMIDITY TO OUR AREA. THIS MEANS POSSIBLE 

DESERT BLOWING DUST AND AT LEAST A SLIGHT CHANCE OF THUNDERSTORMS IN 

SOUTH-CENTRAL AND SOUTHWEST ARIZONA MAINLY IN THE LATE AFTERNOONS AND

EVENINGS THROUGH WEDNESDAY. TEMPORARY DRYING IS EXPECTED FOR MOST OF 

THE AREA BY THURSDAY INTO FRIDAY...FOLLOWED BY A RETURN OF SOME 

HUMIDITY AND AT LEAST A SLIGHT CHANCE OF THUNDERSTORMS MAINLY IN THE 

LATE AFTERNOON AND NIGHTTIME HOURS THIS COMING WEEKEND AND EARLY NEXT

WEEK.

&&

.DISCUSSION...

A LINE OF THUNDERSTORMS WAS MOVING FROM SOUTHERN GILA COUNTY WEST 

OVER NORTH-CENTRAL PINAL COUNTY AND NORTHEAST MARICOPA COUNTY AT MID 

AFTERNOON TODAY. IT WAS MOSTLY SUNNY WITH SOME CLOUDS WEST OF THERE 

ACROSS THE REST OF SOUTH-CENTRAL AND SOUTHWEST ARIZONA...AND SUNNY 

OVER SOUTHEAST CALIFORNIA.

TEMPERATURES WERE GENERALLY CLOSE TO THE SAME TO A FEW DEGREE WARMER 

TODAY COMPARED WITH THE SAME TIME YESTERDAY. IT WAS MORE HUMID TODAY 

WITH DEW POINT TEMPERATURES GENERALLY 10 TO 20 DEGREES HIGHER EARLY 
THIS AFTERNOON COMPARED WITH THE SAME TIME YESTERDAY. WIND WAS 

GENERALLY FROM THE SOUTH AND SOUTHWEST 10 TO 20 MPH...EXCEPT STRONG 

GUSTY AND VARIABLE NEAR THE THUNDERSTORMS.

THE CENTER OF VERY STRONG HIGH PRESSURE ALOFT CONTINUED OVER THE 

NATIONS MIDSECTION TODAY. THE SOUTHWEST EXTENSION OF THIS HIGH 

PRESSURE SYSTEM RESULTED IN ANOTHER HOT DAY IN OUR AREA TODAY WITH A 

HIGH OF 112 DEGREES AT SKY HARBOR IN PHOENIX AS OF 301 PM MST. THIS 

HIGH PRESSURE CENTER IS EXPECTED TO WEAKEN SOMEWHAT AND MOVE TO THE 

SOUTHEASTERN UNITED STATES BY THURSDAY THROUGH SATURDAY.

A FLOW ALOFT FROM THE SOUTHEAST IN THE CLOCKWISE FLOW OF AIR AROUND 

THE HIGH PRESSURE SYSTEM IS BRINGING SOMEWHAT HIGHER HUMIDITY MAINLY 

TO SOUTH-CENTRAL ARIZONA. LOW PRESSURE SYSTEMS IN THIS FLOW MEAN 

POSSIBLE DESERT BLOWING DUST AND AT LEAST A SLIGHT CHANCE OF 

THUNDERSTORMS IN SOUTH-CENTRAL AND SOUTHWEST ARIZONA IN THE LATE 

AFTERNOON AND EVENING HOURS AT LEAST THROUGH TUESDAY INTO WEDNESDAY. 

ONE SUCH HIGH LEVEL LOW PRESSURE SYSTEM WAS CENTERED OVER FAR 

SOUTHEAST ARIZONA AND MOVING NORTH NEAR THE NEW MEXICO BORDER TODAY. 

HOWEVER...THERE ARE OTHER LESS DISCERNIBLE SYSTEMS IN THE FLOW.

MOST OF WHAT THUNDERSTORMS ARE ABLE TO MOVE FROM THE HIGHER TERRAIN 

INTO THE LOWER DESERT OF SOUTH-CENTRAL ARIZONA THIS EVENING ARE 

EXPECTED TO BE SOUTHEAST AND EAST OF PHOENIX. OTHERWISE...SOME GUSTY 

OUTFLOW WINDS AND BLOWING DUST ARE EXPECTED TO BE THE MAIN RESULTS 

OF THE THUNDERSTORMS ON THE LOWER SOUTH-CENTRAL ARIZONA DESERT 

INCLUDING THE GREATER PHOENIX AREA INTO THIS EVENING.

ANOTHER LOW PRESSURE SYSTEM CURRENTLY OVER THE SOUTHERN GULF OF 

CALIFORNIA IS EXPECTED TO MOVE NORTH OVER SOUTHWEST AND 

SOUTH-CENTRAL ARIZONA TUESDAY NIGHT AND WEDNESDAY. THIS WILL BRING A 

BETTER POSSIBILITY OF SHOWERS AND THUNDERSTORMS TO SOUTHWEST ARIZONA 

TUESDAY NIGHT AND WEDNESDAY AS WELL AS SOUTH-CENTRAL ARIZONA.

DRIER AIR FROM THE SOUTHWEST IS EXPECTED TO TEMPORARILY MOVE OVER 

MOST OF THE AREA INCLUDING THE GREATER PHOENIX AREA THURSDAY INTO 

FRIDAY. THIS IS EXPECTED TO HAPPEN AS A LOW PRESSURE SYSTEM FROM THE 

GULF OF ALASKA MOVES INLAND OVER THE WESTERN UNITED STATES. THUS...

THE POSSIBILITY OF ANY SHOWERS AND THUNDERSTORMS IS CONFINED TO THE 

HIGHER TERRAIN EAST OF THE GREATER PHOENIX AREA THURSDAY THROUGH 

DAYTIME FRIDAY.

A RETURN OF SOME HUMIDITY AND AT LEAST A SLIGHT CHANCE OF 

THUNDERSTORMS MAINLY IN THE LATE AFTERNOON AND NIGHTTIME HOURS IS 

EXPECTED THIS COMING WEEKEND AND EARLY NEXT WEEK. THIS IS EXPECTED 

TO HAPPEN AS HIGH PRESSURE ALOFT BUILDS NEAR THE 4 CORNERS WITH A 

FLOW ALOFT FROM THE SOUTHEAST BRINGING SOME MOISTURE FROM MEXICO 

OVER OUR AREA IN THE CLOCKWISE FLOW AROUND THE HIGH PRESSURE SYSTEM. 

&&

.AVIATION...

SOUTH-CENTRAL ARIZONA INCLUDING KPHX AND KIWA...

ISOLATED SHOWERS AND STORMS DEVELOPING THIS AFTERNOON AND EVENING 

ACROSS SOUTH-CENTRAL ARIZONA. MOST ACTIVITY IS EXPECTED TO REMAIN 

OVER THE HIGHER TERRAIN NORTH AND EAST OF THE PHOENIX METROPOLITAN 

AREA WITH A SOME STORMS MOVING INTO THE LOWER DESERTS. OUTFLOW 
BOUNDARIES FROM ALL SHOWERS AND STORMS COULD RESULT IN GUSTY SURFACE 

WINDS AND BLOWING DUST. 

SOUTHEAST CALIFORNIA INCLUDING KIPL AND KBLH...

LIGHT WINDS WITH ONLY A FEW CLOUDS WILL EXTEND THROUGH THE PERIOD.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. 

&&

.FIRE WEATHER...

THURSDAY THROUGH MONDAY...

HIGH PRESSURE MOVING TOWARDS THE EAST WILL ALLOW MONSOON MOISTURE TO 

ENTER INTO THE SOUTHWEST...INCREASING THE PROBABILITY OF AFTERNOON 

AND EVENING SHOWERS. RESULTING IN GUSTY WINDS...LOCALIZED DUST...AND 

EXTENSIVE AREAS OF LIGHTNING.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

DISCUSSION...ELLIS

AVIATION...WATERS

FIRE WEATHER...WATERS/MCLANE

694 

FXUS65 KPSR 190230

AFDPSR

AREA FORECAST DISCUSSION...UPDATED

NATIONAL WEATHER SERVICE PHOENIX AZ

730 PM MST MON JUL 18 2011

.SYNOPSIS...

A FLOW ALOFT FROM THE SOUTHEAST IN THE CLOCKWISE FLOW OF AIR AROUND 

STRONG HIGH PRESSURE ALOFT CENTERED OVER THE NATIONS MIDSECTION IS 

BRINGING SOMEWHAT HIGHER HUMIDITY TO OUR AREA. THIS MEANS POSSIBLE 

DESERT BLOWING DUST AND AT LEAST A SLIGHT CHANCE OF THUNDERSTORMS IN 

SOUTH-CENTRAL AND SOUTHWEST ARIZONA MAINLY IN THE LATE AFTERNOONS AND

EVENINGS THROUGH WEDNESDAY. TEMPORARY DRYING IS EXPECTED FOR MOST OF 

THE AREA BY THURSDAY INTO FRIDAY...FOLLOWED BY A RETURN OF SOME 

HUMIDITY AND AT LEAST A SLIGHT CHANCE OF THUNDERSTORMS MAINLY IN THE 

LATE AFTERNOON AND NIGHTTIME HOURS THIS COMING WEEKEND AND EARLY NEXT

WEEK.
&&

.DISCUSSION...

A NUMBER OF OUTFLOW BOUNDARIES MOVED THROUGH MARICOPA AND PINAL 

COUNTIES THIS EVENING RESULTING IN SEVERAL DUST STORMS. A NUMBER OF 

REPORTS WERE RECEIVED WITH VISIBILITY WELL BELOW CRITERIA /QUARTER 

MILE OR LESS/. A CLUSTER OF STRONG STORMS DID DEVELOP OVER THE WEST 

VALLEY AS A COUPLE OF THE BOUNDARIES COLLIDED...PRODUCING LOCALIZED 

STRONG WINDS AND HEAVY RAINFALL /HIGHEST MEASURED ON FLOOD CONTROL 

DISTRICTS NETWORK WAS 0.91 INCHES/. THE VAST MAJORITY OF THE VALLEY 

DID NOT GET RAIN THOUGH MOST AREAS DID GET VARYING DEGREES OF DUST.

REDUCED RAIN CHANCES FOR THE REST OF THE EVENING AS ACTIVITY IS 

DECREASING AND WITHOUT THE OUTFLOW BOUNDARIES/LIFTING MECHANISMS THE 

100 J/KG OF CIN WILL NOT BE OVERCOME TO RELEASE THE 300-500 J/KG OF 

CAPE EVIDENT IN THE 19.00Z KPSR SOUNDING. HRRR MODEL HAS BEEN DOING 

WELL SO FAR TODAY WITH THE CONVECTION AND THE LATEST RUN AGREES WITH 

T-STORM COVERAGE DECREASING WITH SHOWERS STRAGGLING INTO THE EARLY 

MORNING HOURS.

PREVIOUS DISCUSSION...

THE CENTER OF VERY STRONG HIGH PRESSURE ALOFT CONTINUED OVER THE 

NATIONS MIDSECTION TODAY. THE SOUTHWEST EXTENSION OF THIS HIGH 

PRESSURE SYSTEM RESULTED IN ANOTHER HOT DAY IN OUR AREA TODAY WITH A 

HIGH OF 112 DEGREES AT SKY HARBOR IN PHOENIX AS OF 301 PM MST. THIS 

HIGH PRESSURE CENTER IS EXPECTED TO WEAKEN SOMEWHAT AND MOVE TO THE 

SOUTHEASTERN UNITED STATES BY THURSDAY THROUGH SATURDAY.

ANOTHER LOW PRESSURE SYSTEM CURRENTLY OVER THE SOUTHERN GULF OF 

CALIFORNIA IS EXPECTED TO MOVE NORTH OVER SOUTHWEST AND 

SOUTH-CENTRAL ARIZONA TUESDAY NIGHT AND WEDNESDAY. THIS WILL BRING A 

BETTER POSSIBILITY OF SHOWERS AND THUNDERSTORMS TO SOUTHWEST ARIZONA 

TUESDAY NIGHT AND WEDNESDAY AS WELL AS SOUTH-CENTRAL ARIZONA.

DRIER AIR FROM THE SOUTHWEST IS EXPECTED TO TEMPORARILY MOVE OVER 

MOST OF THE AREA INCLUDING THE GREATER PHOENIX AREA THURSDAY INTO 

FRIDAY. THIS IS EXPECTED TO HAPPEN AS A LOW PRESSURE SYSTEM FROM THE 

GULF OF ALASKA MOVES INLAND OVER THE WESTERN UNITED STATES. THUS...

THE POSSIBILITY OF ANY SHOWERS AND THUNDERSTORMS IS CONFINED TO THE 

HIGHER TERRAIN EAST OF THE GREATER PHOENIX AREA THURSDAY THROUGH 

DAYTIME FRIDAY.

A RETURN OF SOME HUMIDITY AND AT LEAST A SLIGHT CHANCE OF 

THUNDERSTORMS MAINLY IN THE LATE AFTERNOON AND NIGHTTIME HOURS IS 

EXPECTED THIS COMING WEEKEND AND EARLY NEXT WEEK. THIS IS EXPECTED 

TO HAPPEN AS HIGH PRESSURE ALOFT BUILDS NEAR THE 4 CORNERS WITH A 

FLOW ALOFT FROM THE SOUTHEAST BRINGING SOME MOISTURE FROM MEXICO 

OVER OUR AREA IN THE CLOCKWISE FLOW AROUND THE HIGH PRESSURE SYSTEM. 

&&

.AVIATION...

SOUTH-CENTRAL ARIZONA INCLUDING KPHX AND KIWA...

DUST WILL CONTINUE TO SETTLE AROUND THE PHX AREA DURING THE NEXT TWO 

HOURS LEADING TO GRADUAL IMPROVEMENT IN VISIBILITY. TSTM ACTIVITY 

FOR THE MOST PART IS OVER FOR THIS EVENING/TONIGHT THOUGH A SHOWER 
OR TWO WILL BE POSSIBLE OVERNIGHT. SOUTHEAST WINDS WILL GRADUAL 

DECAY AND BECOME LIGHT BY 04Z/9 PM MST.

SOUTHEAST CALIFORNIA INCLUDING KIPL AND KBLH...

LIGHT WINDS WITH ONLY A FEW CLOUDS WILL EXTEND THROUGH THE PERIOD.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. 

&&

.FIRE WEATHER...

THURSDAY THROUGH MONDAY...

HIGH PRESSURE MOVING TOWARDS THE EAST WILL ALLOW MONSOON MOISTURE TO 

ENTER INTO THE SOUTHWEST...INCREASING THE PROBABILITY OF AFTERNOON 

AND EVENING SHOWERS. RESULTING IN GUSTY WINDS...LOCALIZED DUST...AND 

EXTENSIVE AREAS OF LIGHTNING.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

DISCUSSION...INIGUEZ/ELLIS

AVIATION...INIGUEZ

FIRE WEATHER...WATERS/MCLANE

813 

FPUS75 KPSR 182143

NOWPSR

SHORT TERM FORECAST

NATIONAL WEATHER SERVICE PHOENIX AZ

243 PM MST MON JUL 18 2011

AZZ023-182345-

GREATER PHOENIX AREAINCLUDING THE CITIES OF...BUCKEYE...CAREFREE...CAVE CREEK...

CHANDLER...FOUNTAIN HILLS...GILBERT...GLENDALE...MESA...PEORIA...

PHOENIX...SCOTTSDALE...SUN CITY...AND TEMPE

243 PM MST MON JUL 18 2011

.NOW...

STRONG WINDS FROM THUNDERSTORMS OVER PINAL COUNTY WILL ENTER

SOUTHEAST PORTIONS OF THE PHOENIX METRO AREA AFTER 3 PM. WINDS OF

30 TO 40 MPH ALONG WITH BLOWING DUST WILL AFFECT QUEEN CREEK AND EAST
MESA. LOCAL VISIBILITIES BELOW 1 MILE CAN BE EXPECTED WITH LARGER AREAS

OF VISIBILITIES BELOW 3 MILES. MOTORISTS ARE ADVISED TO USE CAUTION

DUE TO SUDDENLY REDUCED VISIBILITIES. 

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

AJ

805 

WUUS55 KPSR 182205

SVRPSR

AZC021-182300-

/O.NEW.KPSR.SV.W.0014.110718T2205Z-110718T2300Z/

BULLETIN - EAS ACTIVATION REQUESTED

SEVERE THUNDERSTORM WARNING

NATIONAL WEATHER SERVICE PHOENIX AZ

305 PM MST MON JUL 18 2011

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

* SEVERE THUNDERSTORM WARNING FOR...

 CENTRAL PINAL COUNTY IN SOUTH CENTRAL ARIZONA...

 THIS INCLUDES THE CITY OF FLORENCE...

* UNTIL 400 PM MST

* AT 302 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A

 SEVERE THUNDERSTORM CAPABLE OF PRODUCING DAMAGING WINDS IN EXCESS

 OF 60 MPH. IN ADDITION...DENSE BLOWING DUST MAY ACCOMPANY THIS

 SEVERE STORM. THIS STORM WAS LOCATED 7 MILES SOUTH OF FLORENCE...

 AND MOVING NORTHWEST AT 15 MPH.

* OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO SAN

 TAN VALLEY AND MAGMA

LAT...LON 3289 11146 3320 11160 3326 11139 3307 11120

 3304 11123

TIME...MOT...LOC 2205Z 149DEG 12KT 3295 11134

$$

LEINS

280 

WUUS55 KPSR 190106

SVRPSR

AZC013-190145-

/O.NEW.KPSR.SV.W.0015.110719T0106Z-110719T0145Z/

BULLETIN - EAS ACTIVATION REQUESTED
SEVERE THUNDERSTORM WARNING

NATIONAL WEATHER SERVICE PHOENIX AZ

606 PM MST MON JUL 18 2011

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

* SEVERE THUNDERSTORM WARNING FOR...

 CENTRAL MARICOPA COUNTY IN SOUTH CENTRAL ARIZONA...

 THIS INCLUDES THE CITIES OF...GLENDALE...YOUNGTOWN...WADDELL...

 SURPRISE...SUN CITY WEST...SUN CITY...PEORIA...EL MIRAGE...

* UNTIL 645 PM MST

* AT 605 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A

 SEVERE THUNDERSTORM CAPABLE OF PRODUCING DAMAGING WINDS IN EXCESS

 OF 60 MPH. THIS STORM WAS LOCATED NEAR GLENDALE SPORTS COMPLEX...

 OR NEAR YOUNGTOWN...AND MOVING NORTH AT 15 MPH.

* THE SEVERE THUNDERSTORM WILL CONTINUE TO MOVE THROUGH THE WEST 

 SIDE OF THE PHOENIX METRO AREA.

LAT...LON 3365 11247 3373 11215 3350 11214 3348 11230

TIME...MOT...LOC 0106Z 165DEG 13KT 3355 11227

$$

LEINS

891 

WWUS55 KPSR 190112

SVSPSR

SEVERE WEATHER STATEMENT

NATIONAL WEATHER SERVICE PHOENIX AZ

612 PM MST MON JUL 18 2011

AZC013-190145-

/O.CON.KPSR.SV.W.0015.000000T0000Z-110719T0145Z/

MARICOPA AZ612 PM MST MON JUL 18 2011

...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 645 PM MST

FOR CENTRAL MARICOPA COUNTY...

AT 611 PM MST...A SEVERE THUNDERSTORM WAS REPORTED CAPABLE OF

PRODUCING DAMAGING WINDS IN EXCESS OF 60 MPH. THIS STORM WAS LOCATED

NEAR YOUNGTOWN...MOVING NORTH AT 15 MPH. SPOTTERS HAVE REPORTED 

WINDS OF 50 MPH WITH THIS STORM ALONG WITH MINOR TREE DAMAGE.

OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO EL

MIRAGE...WADDELL...SUN CITY...SURPRISE...PEORIA AND SUN CITY WEST

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REPORT SEVERE WEATHER TO THE NEAREST LAW ENFORCEMENT AGENCY. THEY

WILL RELAY YOUR REPORT TO THE NATIONAL WEATHER SERVICE FORECAST

OFFICE.
&&

LAT...LON 3365 11247 3373 11215 3350 11214 3348 11230

TIME...MOT...LOC 0112Z 165DEG 13KT 3357 11228

$$

LEINS

792 

WWUS55 KPSR 190120

SVSPSR

SEVERE WEATHER STATEMENT

NATIONAL WEATHER SERVICE PHOENIX AZ

620 PM MST MON JUL 18 2011

AZC013-190145-

/O.CON.KPSR.SV.W.0015.000000T0000Z-110719T0145Z/

MARICOPA AZ620 PM MST MON JUL 18 2011

...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 645 PM MST

FOR CENTRAL MARICOPA COUNTY...

AT 618 PM MST...A SEVERE THUNDERSTORM WAS REPORTED CAPABLE OF

PRODUCING DAMAGING WINDS IN EXCESS OF 60 MPH. THIS STORM WAS LOCATED

NEAR YOUNGTOWN...MOVING NORTH AT 15 MPH. SPOTTERS CONTINUE TO REPORT 

PEA SIZED HAIL AND GUSTY WINDS OVER 50 MPH. 

OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO EL

MIRAGE...WADDELL...SUN CITY...SURPRISE...PEORIA AND SUN CITY WEST

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REPORT SEVERE WEATHER TO THE NEAREST LAW ENFORCEMENT AGENCY. THEY

WILL RELAY YOUR REPORT TO THE NATIONAL WEATHER SERVICE FORECAST

OFFICE.

&&

LAT...LON 3365 11247 3373 11215 3350 11214 3348 11230

TIME...MOT...LOC 0119Z 165DEG 13KT 3360 11228

$$

LEINS

710 

WWUS55 KPSR 190144

SVSPSR

SEVERE WEATHER STATEMENT

NATIONAL WEATHER SERVICE PHOENIX AZ

644 PM MST MON JUL 18 2011
AZC013-190154-

/O.EXP.KPSR.SV.W.0015.000000T0000Z-110719T0145Z/

MARICOPA AZ644 PM MST MON JUL 18 2011

...THE SEVERE THUNDERSTORM WARNING FOR CENTRAL MARICOPA COUNTY WILL

EXPIRE AT 645 PM MST...

THE STORM NEAR YOUNGTOWN WILL CONTINUE TO WEAKEN...HOWEVER GUSTY 

WINDS MAY CONTINUE THROUGH 7 PM.

REPORT SEVERE WEATHER TO THE NEAREST LAW ENFORCEMENT AGENCY. THEY

WILL RELAY YOUR REPORT TO THE NATIONAL WEATHER SERVICE FORECAST

OFFICE.

LAT...LON 3365 11247 3373 11215 3350 11214 3348 11230

TIME...MOT...LOC 0144Z 165DEG 13KT 3369 11231

$$

LEINS

058 

WWUS75 KPSR 182129

NPWPSR

URGENT - WEATHER MESSAGE

NATIONAL WEATHER SERVICE PHOENIX AZ

229 PM MST MON JUL 18 2011

AZZ028-182300-

/O.NEW.KPSR.DS.W.0006.110718T2129Z-110718T2300Z/

NORTHWEST AND NORTH CENTRAL PINAL COUNTYINCLUDING THE CITIES OF...APACHE JUNCTION...CASA GRANDE...

FLORENCE

229 PM MST MON JUL 18 2011

...DUST STORM WARNING IN EFFECT UNTIL 4 PM MST THIS AFTERNOON...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A DUST STORM

WARNING...WHICH IS IN EFFECT UNTIL 4 PM MST THIS AFTERNOON. 

* AFFECTED AREA: NORTHERN PINAL COUNTY.

* LOCATIONS INCLUDE: QUEEN VALLEY...SAN TAN VALLEY...APACHE

 JUNCTION...COOLIDGE...AND FLORENCE.

* TIMING: STRONG WINDS MOVING WESTWARD WILL CREATE DENSE BLOWING DUST

 THROUGH AT LEAST 3 PM WITH LINGERING POOR VISIBILITIES UNTIL ABOUT

 4 PM.

* WINDS: 35 TO 45 MPH...MOSTLY FROM THE EAST.
* VISIBILITY: LOCAL VISIBILITIES AT OR BELOW ONE QUARTER MILE WITH

 LARGE AREAS BELOW 1 MILE. 

* IMPACTS: POOR VISIBILITIES WILL CREATE HAZARDOUS DRIVING

 CONDITIONS. AFFECTED ROADWAYS INCLUDE...BUT ARE NOT LIMITED TO...HIGHWAY

 60...HIGHWAY 79...AND HIGHWAY 287.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

IF YOU ENCOUNTER BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR SEE

IT APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT YOUR

VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP YOUR FOOT

OFF THE BRAKE PEDAL.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

765 

WWUS75 KPSR 182339

NPWPSR

URGENT - WEATHER MESSAGE

NATIONAL WEATHER SERVICE PHOENIX AZ

439 PM MST MON JUL 18 2011

.ANOTHER ROUND OF DENSE BLOWING DUST WILL MOVE ACROSS NORTHERN PINAL

COUNTY AND INTO THE PHOENIX AREA THIS AFTERNOON.

AZZ023-028-190100-

/O.NEW.KPSR.DS.W.0007.110718T2339Z-110719T0100Z/

GREATER PHOENIX AREA-NORTHWEST AND NORTH CENTRAL PINAL COUNTYINCLUDING THE CITIES OF...MESA...PHOENIX...APACHE JUNCTION...

CASA GRANDE...FLORENCE

439 PM MST MON JUL 18 2011

...DUST STORM WARNING IN EFFECT UNTIL 6 PM MST THIS EVENING...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A DUST STORM

WARNING...WHICH IS IN EFFECT UNTIL 6 PM MST THIS EVENING. 

* AFFECTED AREA: NORTHERN PINAL COUNTY AND EASTERN PORTIONS OF THE

 PHOENIX METRO AREA.

* LOCATIONS INCLUDE: CASA GRANDE...COOLIDGE...FLORENCE...QUEEN

 CREEK...MESA...GILBERT...CHANDLER...TEMPE...INTERSTATE 10...HIGHWAY

 79...HUNT HIGHWAY.

* TIMING: BLOWING DUST WILL CONTINUE TO OVERSPREAD NORTHERN PINAL

 COUNTY THROUGH 5 PM WITH LINGERING POOR VISIBILITIES UNTIL ABOUT 6
 PM. FOR THE PHOENIX AREA...DENSE BLOWING DUST WILL ENTER THE

 SOUTHEAST VALLEY AT ABOUT 445 PM AND SPREAD OVER MAINLY EASTERN

 PORTIONS OF THE METRO AREA UNTIL ABOUT 6 PM.

* WINDS: SOUTHEAST 30 TO 40 MPH. 

* VISIBILITY: ONE QUARTER MILE OR LESS.

* IMPACTS: DANGEROUS DRIVING CONDITIONS DUE TO SUDDENLY POOR

 VISIBILITIES. 

PRECAUTIONARY/PREPAREDNESS ACTIONS...

IF YOU ENCOUNTER BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR SEE

IT APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT YOUR

VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP YOUR FOOT

OFF THE BRAKE PEDAL.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

360 

WWUS75 KPSR 190000

NPWPSR

URGENT - WEATHER MESSAGE

NATIONAL WEATHER SERVICE PHOENIX AZ

500 PM MST MON JUL 18 2011

.ANOTHER ROUND OF DENSE BLOWING DUST WILL MOVE ACROSS NORTHERN

PINAL COUNTY AND INTO MARICOPA COUNTY...INCLUDING THE PHOENIX AREA...THIS

AFTERNOON.

AZZ027-190200-

/O.EXB.KPSR.DS.W.0007.000000T0000Z-110719T0200Z/

SOUTHWEST MARICOPA COUNTYINCLUDING THE CITY OF...GILA BEND

500 PM MST MON JUL 18 2011

...DUST STORM WARNING IN EFFECT UNTIL 7 PM MST THIS EVENING...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A DUST STORM

WARNING...WHICH IS IN EFFECT UNTIL 7 PM MST THIS EVENING. 

* AFFECTED AREA: SOUTHWEST MARICOPA COUNTY 

* LOCATIONS INCLUDE: GILA BEND...INTERSTATE 8...HIGHWAY 85.

* TIMING: BLOWING DUST WILL OVERSPREAD SOUTHWEST MARICOPA COUNTY
 THROUGH 630 PM WITH LINGERING POOR VISIBILITIES UNTIL ABOUT 7 PM.

* WINDS: EAST 25 TO 35 MPH.

* VISIBILITY: ONE QUARTER MILE OR LESS.

* IMPACTS: DANGEROUS DRIVING CONDITIONS DUE TO SUDDENLY POOR 

 VISIBILITIES.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

IF YOU ENCOUNTER BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR

SEE IT APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT

YOUR VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP

YOUR FOOT OFF THE BRAKE PEDAL.

&&

$$

AZZ023-028-190100-

/O.CON.KPSR.DS.W.0007.000000T0000Z-110719T0100Z/

GREATER PHOENIX AREA-NORTHWEST AND NORTH CENTRAL PINAL COUNTYINCLUDING THE CITIES OF...MESA...PHOENIX...APACHE JUNCTION...

CASA GRANDE...FLORENCE

500 PM MST MON JUL 18 2011

...DUST STORM WARNING REMAINS IN EFFECT UNTIL 6 PM MST THIS

EVENING...

A DUST STORM WARNING REMAINS IN EFFECT UNTIL 6 PM MST THIS

EVENING. 

* AFFECTED AREA: NORTHERN PINAL COUNTY AND EASTERN PORTIONS OF 

 THE PHOENIX METRO AREA. 

* LOCATIONS INCLUDE: MESA...PHOENIX...APACHE JUNCTION...CASA 

 GRANDE...FLORENCE 

* TIMING: BLOWING DUST WILL CONTINUE TO OVERSPREAD NORTHERN PINAL

 COUNTY AND THE PHOENIX AREA THROUGH 530 PM WITH LINGERING POOR

 VISIBILITIES UNTIL ABOUT 6 PM. 

* WINDS: SOUTHEAST 30 TO 40 MPH.

* VISIBILITY: ONE QUARTER MILE OR LESS.

* IMPACTS: DANGEROUS DRIVING CONDITIONS DUE TO SUDDENLY POOR 

 VISIBILITIES.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

IF YOU ENCOUNTER BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR

SEE IT APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT

YOUR VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP

YOUR FOOT OFF THE BRAKE PEDAL.
&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

800 

WWUS75 KPSR 190100

NPWPSR

URGENT - WEATHER MESSAGE

NATIONAL WEATHER SERVICE PHOENIX AZ

600 PM MST MON JUL 18 2011

.ANOTHER ROUND OF DENSE BLOWING DUST WILL MOVE ACROSS NORTHERN

PINAL COUNTY AND INTO MARICOPA COUNTY...INCLUDING THE PHOENIX

AREA...THIS AFTERNOON.

AZZ028-190200-

/O.EXP.KPSR.DS.W.0007.000000T0000Z-110719T0100Z/

NORTHWEST AND NORTH CENTRAL PINAL COUNTYINCLUDING THE CITIES OF...APACHE JUNCTION...CASA GRANDE...

FLORENCE

600 PM MST MON JUL 18 2011

...DUST STORM WARNING WILL EXPIRE AT 6 PM MST THIS EVENING...

THE DUST STORM WARNING WILL EXPIRE AT 6 PM MST THIS EVENING FOR

NORTHWEST PINAL COUNTY.

$$

AZZ022-190200-

/O.EXB.KPSR.DS.W.0007.110719T0100Z-110719T0200Z/

NORTHWEST MARICOPA COUNTYINCLUDING THE CITIES OF...BUCKEYE...NEW RIVER...WICKENBURG

600 PM MST MON JUL 18 2011

...DUST STORM WARNING IN EFFECT UNTIL 7 PM MST THIS EVENING...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A DUST STORM

WARNING...WHICH IS IN EFFECT UNTIL 7 PM MST THIS EVENING. 

* AFFECTED AREA: NORTHWEST MARICOPA COUNTY

* LOCATIONS INCLUDE: BUCKEYE...NEW RIVER...WICKENBURG

* TIMING: THROUGH 7 PM.

* WINDS: EAST 25 TO 35 MPH.

* VISIBILITY: ONE QUARTER MILE OR LESS.

* IMPACTS: DANGEROUS DRIVING CONDITIONS DUE TO SUDDENLY POOR 
 VISIBILITIES.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

IF YOU ENCOUNTER BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR

SEE IT APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT

YOUR VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP

YOUR FOOT OFF THE BRAKE PEDAL.

&&

$$

AZZ023-190200-

/O.EXT.KPSR.DS.W.0007.000000T0000Z-110719T0200Z/

GREATER PHOENIX AREAINCLUDING THE CITIES OF...BUCKEYE...MESA...PHOENIX

600 PM MST MON JUL 18 2011

...DUST STORM WARNING NOW IN EFFECT UNTIL 7 PM MST THIS

EVENING...

THE DUST STORM WARNING IS NOW IN EFFECT UNTIL 7 PM MST THIS

EVENING. 

* AFFECTED AREA: WESTERN PORTIONS OF THE PHOENIX METRO AREA.

* LOCATIONS INCLUDE: BUCKEYE...MESA...PHOENIX

* TIMING: BLOWING DUST WILL CONTINUE TO OVERSPREAD THE PHOENIX AREA

 THROUGH 7 PM.

* WINDS: SOUTHEAST 30 TO 40 MPH.

* VISIBILITY: ONE QUARTER MILE OR LESS.

* IMPACTS: DANGEROUS DRIVING CONDITIONS DUE TO SUDDENLY POOR 

 VISIBILITIES.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

IF YOU ENCOUNTER BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR

SEE IT APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT

YOUR VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP

YOUR FOOT OFF THE BRAKE PEDAL.

&&

$$

AZZ027-190200-

/O.CON.KPSR.DS.W.0007.000000T0000Z-110719T0200Z/

SOUTHWEST MARICOPA COUNTYINCLUDING THE CITY OF...GILA BEND

600 PM MST MON JUL 18 2011

...DUST STORM WARNING REMAINS IN EFFECT UNTIL 7 PM MST THIS
EVENING...

A DUST STORM WARNING REMAINS IN EFFECT UNTIL 7 PM MST THIS

EVENING. 

* AFFECTED AREA: SOUTHWEST MARICOPA COUNTY

* LOCATIONS INCLUDE: GILA BEND

* TIMING: BLOWING DUST WILL OVERSPREAD SOUTHWEST MARICOPA 

 COUNTY THROUGH 630 PM WITH LINGERING POOR VISIBILITIES UNTIL 

 ABOUT 7 PM.

* WINDS: EAST 25 TO 35 MPH.

* VISIBILITY: ONE QUARTER MILE OR LESS.

* IMPACTS: DANGEROUS DRIVING CONDITIONS DUE TO SUDDENLY POOR 

 VISIBILITIES.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

IF YOU ENCOUNTER BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR

SEE IT APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT

YOUR VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP

YOUR FOOT OFF THE BRAKE PEDAL.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

897 

WWUS85 KPSR 182138

SPSPSR

SPECIAL WEATHER STATEMENT

NATIONAL WEATHER SERVICE PHOENIX AZ

238 PM MST MON JUL 18 2011

AZZ024-028-182215-

PINAL AZ238 PM MST MON JUL 18 2011

...SIGNIFICANT WEATHER ADVISORY...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

SIGNIFICANT WEATHER ADVISORY FOR...

 NORTH CENTRAL PINAL COUNTY IN SOUTH CENTRAL ARIZONA

UNTIL 315 PM MST.
AT 234 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED 

STRONG THUNDERSTORMS NEAR FLORENCE JUNCTION AND QUEEN VALLEY. THE 

STORMS WERE MOVING SLOWLY WESTWARD.

WIND GUSTS UP TO 55 MPH ARE EXPECTED WITH THIS STORM.

LOCATIONS IMPACTED INCLUDE...

 FLORENCE...QUEEN VALLEY...AND RURAL NORTHERN PINAL COUNTY...

IN ADDITION TO VERY STRONG WINDS...DENSE BLOWING DUST IS POSSIBLE 

ALONG WITH LOCALIZED BRIEF HEAVY RAINFALL. TAKE SHELTER INSIDE OF A 

STURDY BUILDING UNTIL THE STORMS PASS.

LAT...LON 3298 11131 3295 11136 3295 11140 3346 11140

 3335 11103 3318 11103

TIME...MOT...LOC 2135Z 180DEG 0KT 3316 11125

$$

AJ

531 

WWUS85 KPSR 182212

SPSPSR

SPECIAL WEATHER STATEMENT

NATIONAL WEATHER SERVICE PHOENIX AZ

312 PM MST MON JUL 18 2011

AZZ023-024-028-182300-

GILA AZ-MARICOPA AZ-PINAL AZ312 PM MST MON JUL 18 2011

...SIGNIFICANT WEATHER ADVISORY...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

SIGNIFICANT WEATHER ADVISORY FOR...

 WEST CENTRAL GILA COUNTY IN EAST CENTRAL ARIZONA

 NORTH CENTRAL PINAL COUNTY IN SOUTH CENTRAL ARIZONA

 EAST CENTRAL MARICOPA COUNTY IN SOUTH CENTRAL ARIZONA

UNTIL 400 PM MST

AT 302 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A

STRONG THUNDERSTORM 8 MILES NORTH OF QUEEN VALLEY...MOVING WEST AT 20

MPH.

WIND GUSTS UP TO 50 MPH ARE EXPECTED WITH THIS STORM...ALONG WITH

FREQUENT DANGEROUS LIGHTNING.

LOCATIONS IMPACTED INCLUDE...

 KINGS RANCH...

 APACHE JUNCTION...

 EAST MESA...

 HIGLEY...
 QUEEN CREEK...

SOME UNSECURED OBJECTS WILL BE BLOWN AROUND...TREE LIMBS COULD BE

SNAPPED OFF...AND ISOLATED POWER OUTAGES WILL BE POSSIBLE. SEEK

SHELTER INDOORS UNTIL THE STORM PASSES.

LOCALLY DENSE BLOWING DUST IS POSSIBLE. IF YOU ENCOUNTER BLOWING DUST

WHILE DRIVING...PULL OVER AS FAR OFF THE ROADWAY AS POSSIBLE AND

PARK. TURN OFF YOUR HEADLIGHTS AND KEEP YOUR FOOT OFF THE BRAKE.

LAT...LON 3335 11175 3385 11142 3347 11105 3311 11126

TIME...MOT...LOC 2212Z 111DEG 16KT 3343 11131

$$

WATERS

485 

WWUS85 KPSR 182304

SPSPSR

SPECIAL WEATHER STATEMENT

NATIONAL WEATHER SERVICE PHOENIX AZ

404 PM MST MON JUL 18 2011

AZZ022>024-028-190000-

GREATER PHOENIX AREA-NORTHWEST AND NORTH CENTRAL PINAL

COUNTY-NORTHWEST MARICOPA COUNTY-SOUTHERN GILA/TONTO NF FOOTHILLS404 PM MST MON JUL 18 2011

...AREAS OF BLOWING DUST POSSIBLE THROUGH RUSH HOUR...

AT 400 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED 

GUSTY WINDS FROM DISSIPATING THUNDERSTORMS. THESE WINDS WILL RESULT 

IN AREAS OF BLOWING DUST 

$$

494 

WWUS85 KPSR 182306 CCA

SPSPSR

SPECIAL WEATHER STATEMENT...CORRECTED

NATIONAL WEATHER SERVICE PHOENIX AZ

406 PM MST MON JUL 18 2011

AZZ022>024-028-190000-

GREATER PHOENIX AREA-NORTHWEST AND NORTH CENTRAL PINAL

COUNTY-NORTHWEST MARICOPA COUNTY-SOUTHERN GILA/TONTO NF FOOTHILLS406 PM MST MON JUL 18 2011

...AREAS OF BLOWING DUST POSSIBLE THROUGH RUSH HOUR...

AT 400 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED 
GUSTY WINDS FROM DISSIPATING THUNDERSTORMS. THESE WINDS WILL RESULT 

IN AREAS OF BLOWING DUST WHICH WILL REDUCE VISIBILITIES TO 2 MILES. 

SIGNIFICANT REDUCTIONS IN VISIBILITIES ARE NOT EXPECTED...BUT USE 

CAUTION IF TRAVELING DURING RUSH HOUR AND IF YOU ENCOUNTER BLOWING 

DUST.

$$

LEINS

725 

WWUS85 KPSR 190022

AWWPHX

AZZ023-190130-

AIRPORT WEATHER WARNING FOR SKY HARBOR AIRPORT

NATIONAL WEATHER SERVICE PHOENIX AZ

522 PM MST MON JUL 18 2011

...AIRPORT WEATHER WARNING FOR SKY HARBOR AIRPORT IN EFFECT UNTIL

630 PM MST...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED AN AIRPORT WEATHER

WARNING FOR SKY HARBOR AIRPORT FOR STRONG GUSTY WINDS.

A DUST STORM IS MOVING TOWARD THE AIRPORT...WHICH WILL DROP

VISIBILITY TO A HALF MILE OR LESS AND CAUSE WINDS TO GUST INTO THE 30

TO 40 KNOT RANGE FROM THE SOUTH.

$$

INIGUEZ

129 

WWUS85 KPSR 190054

SPSPSR

SPECIAL WEATHER STATEMENT

NATIONAL WEATHER SERVICE PHOENIX AZ

554 PM MST MON JUL 18 2011

AZZ023-190145-

MARICOPA AZ554 PM MST MON JUL 18 2011

...SIGNIFICANT WEATHER ADVISORY...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

SIGNIFICANT WEATHER ADVISORY FOR...

 CENTRAL MARICOPA COUNTY IN SOUTH CENTRAL ARIZONA

UNTIL 645 PM MST

AT 553 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A

STRONG THUNDERSTORM NEAR GLENDALE...MOVING NORTH AT 15 MPH.
WIND GUSTS UP TO 50 MPH ARE EXPECTED WITH THIS STORM.

LOCATIONS IMPACTED INCLUDE...

 LUKE AFB...

 YOUNGTOWN...

 SUN CITY... EL MIRAGE AND WADDELL...

 SURPRISE...

 SUN CITY WEST...

LAT...LON 3367 11239 3369 11210 3348 11210 3337 11240

TIME...MOT...LOC 0054Z 165DEG 13KT 3353 11221

$$

LEINS

602 

WWUS85 KPSR 190129

SPSPSR

SPECIAL WEATHER STATEMENT

NATIONAL WEATHER SERVICE PHOENIX AZ

629 PM MST MON JUL 18 2011

AZZ022-023-190215-

MARICOPA AZ629 PM MST MON JUL 18 2011

...SIGNIFICANT WEATHER ADVISORY...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

SIGNIFICANT WEATHER ADVISORY FOR...

 NORTH CENTRAL MARICOPA COUNTY IN SOUTH CENTRAL ARIZONA

UNTIL 715 PM MST

AT 626 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A 

STRONG THUNDERSTORM 9 MILES NORTH OF PHOENIX. THIS STORM WAS 

DISSIPATING AND PRODUCING GUSTY OUTFLOW WINDS AROUND 40 MPH. 

LOCATIONS IMPACTED INCLUDE...

 WITTMANN...

 ANTHEM...

 NEW RIVER...

SOME UNSECURED OBJECTS COULD BE BLOWN AROUND. SEEK SHELTER INDOORS

UNTIL THE STORM PASSES.

LOCALLY DENSE BLOWING DUST IS POSSIBLE. IF YOU ENCOUNTER BLOWING DUST

WHILE DRIVING...PULL OVER AS FAR OFF THE ROADWAY AS POSSIBLE AND

PARK. TURN OFF YOUR HEADLIGHTS AND KEEP YOUR FOOT OFF THE BRAKE.

LAT...LON 3371 11204 3380 11217 3373 11242 3353 11254

 3398 11265 3398 11261 3390 11228 3392 11224
 3396 11224 3404 11217 3405 11209

TIME...MOT...LOC 0128Z 180DEG 0KT 3367 11205

$$

LEINS