State of Arizona Exceptional Event 

Documentation for the Event of July 3, 2011, 

for the Yuma County PM10

Nonattainment Area 

Final Report Prepared for 

Arizona Department of Environmental Quality 

Phoenix, AZ 

January 23, 2013
This PDF document contains blank pages to accommodate two-sided printing. 
State of Arizona Exceptional Event Documentation 

for the Event of July 3, 2011, for the Yuma County 

PM10 Nonattainment Area 

Final Report 

STI-912033-5503-DR 

Prepared by 

Daniel Alrick 

ShihMing Huang 

Hilary Minor 

Hilary Hafner 

Sonoma Technology, Inc. 

1455 N. McDowell Blvd., Suite D 

Petaluma, CA 94954-6503 

Ph 707.665.9900 | F 707.665.9800 

sonomatech.com 

Prepared for 

Theresa Rigney 

Air Quality Assessment Section Manager 

1110 W. Washington Street 

Phoenix, AZ 85007 

602.771.2274

Janaury 23, 2013 

Cover graphic illustrates hourly wind speeds and PM2.5 concentrations in Yuma, AZ, during the July 3, 2011, event. 

See Section 3 for more information.

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Acknowledgments 

Acknowledgments 

The authors would like to acknowledge the Arizona Department of Environmental Quality 

and the Environmental Protection Agency – Region 9 for helpful discussion and feedback during 

the preparation of this document. 

iii

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Table of Contents 

Table of Contents 

Section Page

List of Figures .............................................................................................................................. vi

List of Tables................................................................................................................................vii

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

1.1 Exceptional Event Rule Requirements.......................................................................1

1.2 Report Contents .........................................................................................................3

2. Conceptual Model................................................................................................................1

2.1 Geographic Setting ....................................................................................................1

2.2 Climate, Monsoon, and Thunderstorms .....................................................................2

2.3 Event Day Summary ..................................................................................................4

3. Causal Relationship.............................................................................................................1

4. Historical Norm ....................................................................................................................1

5. Not Reasonably Controllable or Preventable ......................................................................1

5.1 Background................................................................................................................1

5.2 Forecasts and Warnings ............................................................................................4

5.3 Wind Observations.....................................................................................................4

5.4 Summary....................................................................................................................5

6. But-For Analysis ..................................................................................................................1

7. Conclusions .........................................................................................................................1

7.1 Affects Air Quality.......................................................................................................1

7.2 Not Reasonably Controllable or Preventable .............................................................1

7.3 Natural Event .............................................................................................................1

7.4 Summary....................................................................................................................1

Appendix A: Historical Fluctuation Graphs for Yuma County .................................................. A-1

Appendix B: ADEQ and NWS Forecast Products.................................................................... B-1

v
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 List of Figures 

List of Figures 

Figure Page

2-1. Location of air quality and meteorological monitors and relevant geographical 

features in the Yuma area.................................................................................................2

2-2. Average monthly temperatures and precipitation at KNYL,1981-2010. ............................3

2-3. Cross-section of a thunderstorm creating an outflow boundary and haboob....................4

2-4. Outflow boundaries from thunderstorms over northwest Mexico moved 

northwestward and transported dust to the Yuma area. ...................................................5

3-1. Infrared satellite image from 8:00 p.m. MST on July 2, 2011 (GOES-West).....................1

3-2. Yuma and Imperial County meteorological monitors and wind direction data...................2

3-3. Hourly PM concentrations and wind speeds at the Yuma Supersite monitor on 

July 2 and 3, 2011.............................................................................................................3

10

3-4. Base radial velocity at 12:07 a.m. on July 3, 2011, from NWS Doppler radar in 

Yuma, AZ. .........................................................................................................................4

3-5. Hourly PM concentrations at the Yuma Supersite monitor and visibility at Yuma 

MCAS................................................................................................................................4

10

3-6. Hourly PM concentrations and wind speeds at the Brawley and Niland AQS 

monitors in Imperial County, CA, on July 2 and 3, 2011. ..................................................5

10

4-1. 24-hr average PM10 concentrations at the Yuma Supersite monitor (2006-2011). ...........2

4-2. Daily maximum 1-hr PM concentrations at the Yuma Supersite monitor (2006-

2011). ................................................................................................................................3

10

4-3. 24-hr average PM10 concentrations at the Yuma Supersite monitor for 2006-2011. ........3

4-4. Daily maximum 1-hr PM concentrations at the Yuma Supersite monitor for 2006-

2011. .................................................................................................................................4

10

vi
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 List of Tables 

List of Tables 

Table Page 

2-1. Yuma Supersite monitor information and PM10 measurements on July 3, 2011. .............5

3-1. Observed wind speeds and wind gusts at Yuma and Imperial County monitors on 

July 3, 2011.......................................................................................................................3

5-1. Control measures implemented in Yuma nonattainment area 1994-2001........................1

5-2. Yuma-area-implemented control measures, 2000-2004...................................................2

vii

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Introduction

1

1. Introduction 

On July 3, 2011, the Yuma Supersite monitor recorded a 24-hr average PM10

concentration of 225 μg/m3

. This value is in exceedance of the National Ambient Air Quality 

Standard (NAAQS) for 24-hr PM10 of 150 μg/m3

. The purpose of this report is to demonstrate 

that this exceedance was due to naturally occurring windblown dust, was not reasonably 

controllable or preventable, was historically unusual, and would not have occurred “but-for” the 

windblown dust and, therefore, is an exceptional event. 

1.1 Exceptional Event Rule Requirements 

In addition to the technical requirements that are contained within the Exceptional Event 

Rule (EER), procedural requirements must also be met in order for the U.S. Environmental 

Protection Agency (EPA) to concur with the flagged air quality monitoring data. This section of 

the report contains the requirements of the EER and associated guidance, and discusses how 

Arizona Department of Environmental Quality (ADEQ) addressed those requirements. 

1.1.1 Public Notification That the Event Was Occurring (40 CFR 50.14(c)(1)(i)) 

ADEQ issued Dust Control Action Forecasts for the Greater Yuma area advising citizens 

of the potential for high wind dust events during the July 2-3, 2011, time frame. More 

information on ADEQ’s forecasting program can be found in Section 6 of this report. The 

forecast products that were issued during the period July 2-3, 2011, are included in Appendix B. 

1.1.2 Place Informal Flag on Data in AQS (40 CFR 50.14(c)(2)(ii)) 

ADEQ and other operating air quality agencies in Arizona submit data into EPA’s Air 

Quality System (AQS), the official repository of ambient air quality data. This data submittal to 

AQS includes PM data from both filter-based and continuous monitors operated in Arizona. 

When ADEQ and/or another agency operating monitors in Arizona suspect 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 other operating air quality agencies have determined that a potential 

exists that a monitor’s reading(s) have been influenced by an exceptional event, a preliminary 

flag is submitted for the measurement in AQS. The data are not official until they undergo more 

thorough quality assurance and quality control, leading to certification by May 1st 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. 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Introduction

2

1.1.3 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 intends to analyze under the Exceptional Events Rule. The PM10 exceedance 

that occurred at one monitor on July 3, 2011, in the Yuma PM10 nonattainment area was 

included on this list. This assessment report serves as demonstration supporting the flagging of 

these data. 

1.1.4 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. 

1.1.5 Submit Demonstration Supporting Exceptional Event Flag (40 CFR 

50.14(a)(1-2)) 

At the close of the public 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 package is 

September 30, 2014. 

1.1.6 Documentation Requirements (40 CFR 50.14(c)(3)(iii)) 

The EER states that in order to justify the exclusion of air quality monitoring data, 

evidence must be provided for the following elements: 

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

a. the event affected air quality, 

b. the event was not reasonably controllable or preventable, and 

c. the event was caused by human activity unlikely to recur in a particular 

location or was a natural event; 

2. There is a clear causal relationship between the measurement(s) under 

consideration and the event; 

3. The event is associated with a measured concentration(s) in excess of normal 

historical fluctuations; and 

4. There would have been no exceedance or violation but for the event. 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Introduction 

3

1.2 Report Contents 

Section 2 of this assessment contains a conceptual model of the wind-blown dust event 

that transpired on July 3, 2011, providing a background narrative of the exceptional event and 

an overall explanation that the event affected air quality. Section 2 also provides evidence that 

the event was a natural event. 

Section 3 of this assessment establishes a clear causal connection between the natural 

event on July 3, 2011, and the exceedance of the 24-hr PM10 standard at the monitoring station. 

The evidence in this section also confirms that the event in question both affected air quality and 

was the result of natural events. 

Section 4 of this assessment contains data summaries and time-series graphs which 

help illustrate that the event of July 3, 2011, produced PM10 concentrations in excess of 

historical normal fluctuations. 

Section 5 of this assessment details the existing dust control measures and 

demonstrates that despite the presence and enforcement of these controls, the event of July 3, 

2011, was not reasonably controllable or preventable. 

Section 6 of this assessment builds upon the demonstration, showing a clear causal 

connection between the natural event and the exceedance, and concludes that the exceedance 

of the 24-hr PM10 standard on July 3, 2011, would not have occurred but for the event. 

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Conceptual Model

1 

2. Conceptual Model 

This section provides a narrative background and summarizes the meteorological and 

air quality conditions in place on July 3, 2011, in Yuma. Elements described in this section 

include: 

 A description and map of geographic setting of the air quality and meteorological 

monitors. 

 A description of Yuma’s climate. 

 An overall description of meteorological and air quality conditions on the event day. 

2.1 Geographic Setting 

Yuma is located in the Sonoran Desert and Lower Colorado River Valley in extreme 

southwestern Arizona at an elevation of 138 feet above sea level. The Yuma Metropolitan 

Statistical Area is defined as Yuma County, which had a population of 195,751 as of the 2010 

census. Yuma County is bordered by Imperial County, California, to the north and northwest 

and by Mexican state of Baja California to the west and south (Figure 2-1). Yuma lies just west 

of the confluence of the Colorado and Gila Rivers. Most of Yuma is located in the Colorado 

River Floodplain, commonly known as the Yuma Valley. The Yuma Valley follows the course of 

the Colorado River southward to the Sea of Cortez. Part of Yuma is built on the Yuma Mesa, a 

prominent land feature extending to the east of Yuma. The Gila Mountains, located roughly 15 

to 20 miles east and southeast of Yuma, are a range of mountains with a peak elevation of 

3,156 feet. 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Conceptual Model

2 

G G

G

G

G

G

Yuma MCAS

Yuma South

Yuma Valley Yuma Supersite

Yuma North Gila

Yuma Courthouse

Yuma

Fortuna Foothills Fortuna Foothills

San Luis San Luis

Somerton Somerton

Yuma McAs/Yuma Intl

Yuma

Imperial

§ ̈¦8

tu95

tu95

UV34

UV24

UV186 UV24

UV34

UV24

St

A

C

8th

S24

24th

4 E

Somer ot n

Frontage

9 E

18 E

Laguna

4th

44th

York

Imperial Dam

Mehring

20 E

24 E

Colby

52nd

16 E

20 E

St

St

Frontage

St

St

9 E

St

St

St

St

Yuma Proving Ground Yuma Proving Ground

All American Canal

Main Drain

/ Yuma Area Monitors

G AQS

AZMET

G NWS

0 5 10 15 20 2.5

Miles

G

Figure 2-1. Location of air quality and meteorological monitors and relevant 

geographical features in the Yuma area. AQS monitors measure air quality and 

meteorological data; AZMET and NWS monitors measure only meteorological data. 

The PM10 exceedance on July 3, 2011, was recorded at the Yuma Supersite monitor, 

which is located in central Yuma and has been operational since January 1, 2010. The Yuma 

Courthouse monitor shown in Figure 2-1 is inactive but measured PM10 prior to January 1, 

2010. Data from the Yuma Courthouse monitor was used to supplement the Yuma Supersite 

data record for the Historical Norm section of this demonstration. Three Arizona Meteorological 

Network (AZMET) sites are in operation in the Yuma area, located northeast, west, and 

southwest of the city. A National Weather Service monitor is located at the Yuma Marine Corps 

Air Station (MCAS). All of the above sites measure wind speed and direction. 

2.2 Climate, Monsoon, and Thunderstorms 

Yuma is one of the hottest cities of any size in the United States, with average high 

temperatures around 107°F in July and around 70°F in January (Figure 2-2). Yuma receives 

roughly 90% of possible sunshine each year. Yuma is one of the driest cities in the United 

States, with an average annual rainfall of just over 3 inches. The bulk of this rain usually falls 

during the December through March and July through August time periods. During the 

December through March period, winter storms originating from the Pacific Ocean can produce 

significant rains in southwestern Arizona. During the July and August time period, monsoonal 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Conceptual Model

3 

moisture originating from the Gulf of California, Gulf of Mexico, and large thunderstorm 

complexes over the Sierra Madre Occidental Mountains in Mexico move northward into Arizona. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

20

40

60

80

100

120

Precipitation (inches)

Temperature (°F)

Precipitation

Max Temp

Min Temp

Mean Temp

Figure 2-2. Average monthly temperatures and precipitation at KNYL,1981-2010. 

The influx of moisture associated with the monsoon, combined with strong solar heating, 

can result in unstable atmospheric conditions that are favorable for the development of 

thunderstorms. Heavy precipitation associated with thunderstorms, and the eventual collapse 

or dissipation of thunderstorms, can generate what are known as downbursts (Figure 2-3). 

Downbursts are the rapid descent of rain-cooled air in a thunderstorm. Upon reaching the 

surface, this air rapidly disperses horizontally away from the storm as outflow boundaries (also 

called gust fronts). The high winds associated with outflow boundaries can efficiently loft dust 

into the air and transport the dust over long distances, resulting in dust storms (also called 

haboobs) with high PM10 concentrations and low visibilities. 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Conceptual Model

4 

Figure 2-3. Cross-section of a thunderstorm creating an outflow boundary and haboob 

(Source: Desert Meteorology. Thomas T. Warner. 2004.) 

Dust storms associated with these thunderstorms typically occur in the early part of the 

monsoon season (July) before subsequent rains moisten the soil and limit potential lofting of soil 

into the air. However, depending on the amount and frequency of precipitation received during 

the monsoon season, extremely hot temperatures act to dry the surface soils very quickly; thus, 

dust storms can occur at any time during the year. Specific PM10 source regions are difficult to 

determine during thunderstorm-driven dust storms because the thunderstorm outflow can carry 

dust over long distances that encompass many possible sources of dust. Instead, we consider 

general PM10 source regions, which are typically identified based on the locations of the 

thunderstorms that are believed to have generated the dust-laden outflow winds. 

2.3 Event Day Summary 

On the evening of July 2, 2011, strong winds generated by thunderstorms over 

northwestern Mexico transported dust northwestward into southern Arizona and southeastern 

California, impacting the Yuma area (Figure 2-4). The windblown dust resulted in a 24-hr 

average PM10 concentration of 225 μg/m3

 at the Yuma Supersite monitor (Table 2-1); this value 

is in exceedance of the NAAQS. The hourly and 24-hr average PM10 concentrations measured 

at the Yuma Supersite monitor were in excess of normal historical fluctuations. The dust was 

naturally occurring and likely originated over undeveloped lands of northwestern Mexico and 

southeastern Arizona outside the city of Yuma, and wind gusts in excess of 40 mph would have 

likely overwhelmed reasonable dust control measures. PM10 monitors in southeastern 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Conceptual Model

5 

California also recorded high PM10 concentrations as the dust storm moved through that region, 

illustrating the regional nature of this event. 

Region of

thunderstorm

development

Figure 2-4. Outflow boundaries from thunderstorms over northwest Mexico moved 

northwestward and transported dust to the Yuma area. 

Table 2-1. Yuma Supersite monitor information and PM10 measurements on July 3, 

2011. 

Monitor Monitor 

Type Operator AQS Monitor ID 

24-hr Avg 

PM10

(μg/m3) 

1-hr Max 

PM10

μg/m3) 

Time of 

Max 1-hr 

PM10

AQS 

Qualifier 

Flag 

Yuma 

Supersite TEOM1 ADEQ 04-027-8011-81102-3 225 3089 00:00 RJ2

 

1

 Tapered element oscillating microbalance monitor. 

2

 RJ is the qualifier flag for High Winds. 

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Causal Relationship

3. Causal Relationship 

Meteorological and air quality observations indicate that dust carried by thunderstorm 

outflow was directly responsible for the high PM10 concentrations observed in Yuma on July 3, 

2011. On the evening of July 2, intense thunderstorms developed over the higher terrain of 

northwestern Mexico, just south of the Arizona border (Figure 3-1). These thunderstorms 

expanded northwestward and weakened as they moved into south-central Arizona, but the 

thunderstorms generated dust-carrying outflow boundaries that propagated northwestward 

toward the Yuma area. As stated in Section 2.2, thunderstorms associated with the summer 

monsoon season can generate strong winds and blowing dust across Arizona. The likely 

source regions for PM10 during the July 3, 2011, event were the deserts of far northwestern 

Mexico and along the path of the outflow boundary from far northwestern Mexico northwestward 

into western Pima County and southern Yuma County. This region largely consists of natural, 

undisturbed desert. The last time Yuma recorded any measurable rainfall leading up to the 

July 3, 2011, high wind event was on April 8, 2011, when showers associated with a cold front 

produced 0.08 inches of rain at the Yuma MCAS. This combination of geography and lack of 

rainfall preceding the event resulted in a large fetch of soils that were particularly vulnerable to 

particulate suspension. 

Figure 3-1. Infrared satellite image from 8:00 p.m. MST on July 2, 2011 (GOES-West). 

Colder temperatures (blues, purples, and white) are indicative of tall, convective 

(thunderstorm) clouds. Thunderstorms over northwestern Mexico collapsed and 

generated outflow boundaries that carried dust northwestward into Arizona. 

1 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Causal Relationship

Figure 3-2 and Table 3-1 summarize the progression of the thunderstorm outflow and 

windblown dust as it moved through southwestern Arizona. The dust-laden outflow first arrived 

in the Yuma area around midnight on July 3. PM10 concentrations at the Yuma Supersite 

monitor increased sharply over this time period, with a 1-hr concentration of 3,089 μg/m3

recorded at midnight on July 3 (Figure 3-3). Meteorological monitors in the Yuma area 

measured sustained winds of over 20 mph and wind gusts in excess of 40 mph coincident with 

the sharp increase in PM10 concentrations (Figure 3-3 and Appendix A). The strong 

southeasterly winds and dust were evident on the National Weather Service Doppler radar in 

Yuma (Figure 3-4). Visibility at the Yuma MCAS also decreased significantly with the arrival of 

the dust (Figure 3-5), prompting the National Weather Service office in Phoenix, AZ, to issue a 

Dust Storm Warning for Yuma County (see Appendix B). It is also important to note that before 

the abrupt increase in PM10 in Yuma, winds were light (at or less than 5 mph) and PM10

concentrations were much lower, illustrating the relationship between the high winds and the 

dust. 

Figure 3-2. Yuma and Imperial County meteorological monitors and wind direction data. 

Wind speed and wind gust data for each monitor are shown in Table 3-1. Gusty 

southeasterly to south-southeasterly winds transported dust across Yuma and Imperial 

Counties on early July 3, 2011. 

2 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Causal Relationship

Table 3-1. Observed wind speeds and wind gusts at Yuma and Imperial County 

monitors on July 3, 2011. The Yuma Supersite monitor reported a PM10 concentration of 

3089 μg/m3

 at 00:00 on July 3, 2011, coincident with the peak wind speed and wind gust 

reported at that monitor. 

0

5

10

15

20

25

30

35

40

45

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Wind Speed (mph)

Concentration (μg/m3)

July 2, 2011                          Time of Day (MST)                        July 3, 2011

Yuma Supersite Wind Speed

Yuma Supersite Wind Gust

Yuma Supersite PM10

Figure 3-3. Hourly PM10 concentrations and wind speeds at the Yuma Supersite monitor 

on July 2 and 3, 2011. PM10 concentrations and wind speeds sharply increased at 

midnight on July 3, 2011, indicating the arrival of windblown dust. 

3 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Causal Relationship

Figure 3-4. Base radial velocity at 12:07 a.m. on July 3, 2011, from NWS Doppler radar 

in Yuma, AZ. Blue and green indicate flow toward the radar; red and yellow indicate flow 

away from the radar. Strong southeasterly winds associated with thunderstorm outflow 

carried dust into the Yuma area. 

0

2

4

6

8

10

0

500

1000

1500

2000

2500

3000

3500

4000 12

Visibility (miles)

Concentration (μg/m3)

July 2, 2011                          Time of Day (MST)                        July 3, 2011

Yuma MCAS Visbility

Yuma Supersite PM10

Figure 3-5. Hourly PM10 concentrations at the Yuma Supersite monitor and visibility at 

Yuma MCAS. Visibility was greatly reduced starting at 12:00 a.m. MST on July 3, 

coincident with the sharp increase in PM10 concentrations at the Yuma Supersite monitor, 

indicating the arrival of windblown dust. 

PM10 concentrations decreased after 2:00 a.m. MST as the dust storm propagated 

northwestward out of Arizona and into southeastern California. Air quality monitors in 

neighboring Imperial County, CA, also reported gusty southeasterly winds and sharp increases 

4 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Causal Relationship 

in PM10 concentrations, illustrating the widespread nature of this dust storm event (Figure 3-6). 

No rainfall was reported across southwestern Arizona and southeastern California with the 

passage of this outflow boundary and the associated dust. 

0

5

10

15

20

25

30

35

40

45

0

100

200

300

400

500

600

700

800

900

1000

Wind Speed (mph)

Concentration (μg/m3)

July 2, 2011                          Time of Day (MST)                        July 3, 2011

Niland Wind Speed

Niland PM10

Brawley PM10

Figure 3-6. Hourly PM10 concentrations and wind speeds at the Brawley and Niland 

AQS monitors in Imperial County, CA, on July 2 and 3, 2011. PM10 concentrations and 

wind speeds sharply increased at midnight on July 3, 2011, indicating the arrival of 

windblown dust. 

5 

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Historical Norm

1 

4. Historical Norm 

PM10 concentrations measured at the Yuma Supersite monitor on July 3, 2011, were 

unusual and in excess of normal historical fluctuations. The PM10 concentrations measured on 

July 3, 2011, were some of the highest hourly and 24-hr averages measured over the last five 

years, with hourly concentrations exceeding 3,000 μg/m3

. To establish the severity of this 

event, PM10 concentrations measured on July 3, 2011, were compared to a historical 2006-

2011 5-year annual data set. Time-series plots of the 24-hr average PM10 concentrations for 

the period January 1, 2006, through December 31, 2011, provide a historical perspective of 

PM10 concentrations (Figures 4-1 and 4-2). The 24-hr average PM10 concentration on July 3, 

2011, is the 6th highest daily average in the last 5 years and the highest daily average in 2011. 

Additionally, time-series plots of the daily maximum 1-hr PM10 concentrations were created to 

provide a deeper understanding of the frequency with which short-term particulate 

concentrations affect the Yuma area. The daily maximum 1-hr PM10 concentration on July 3, 

2011, is the second highest observation in the last 5 years. 

Historical daily cumulative distributions of the daily maximum 1-hr and 24-hr average 

PM10 concentrations were created for the Yuma county monitor for the 2006-2011 period to 

provide additional evidence in establishing the severity of this event. Figures 4-3 and 4-4 show 

histograms of daily maximum 1-hr PM10 concentrations and 24-hr average PM10 concentrations 

at the Yuma county monitor and the corresponding 95th percentile. The daily maximum 1-hr 

PM10 concentrations and the 24-hr average PM10 concentration on July 3, 2011, were above the 

95th percentile at the Yuma Supersite monitor. Concentrations in excess of the 95th percentile 

are considered to be unusual.3

 Given the recorded values and using similar methodology to the 

one accepted by EPA, it is clear that the PM10 levels on July 3, 2011, were outside of normal 

historical fluctuations. This analysis provides evidence that the event affected air quality on a 

historic scale. 

 

3 Excluding days on which concentrations caused by exceptional events exceed the 95th percentile threshold employs 

a general test of statistical significance and has the effect of ensuring that such concentrations would clearly fall 

beyond the range of normal expectations for air quality during a particular time of year. Source: “The treatment of 

Data Influenced by Exceptional Events,” 71 FR 12598.
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Historical Norm

0

50

100

150

200

250

300

350

400

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

24‐hour Average PM10 (μg/m3)

2006

2007

2008

2009

2010

2011

July 3, 2011

Figure 4-1. 24-hr average PM10 concentrations at the Yuma Supersite monitor 

(2006-2011). The 24-hr average PM10 concentration on July 3, 2011, is highlighted by 

the red arrow. 

2 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Historical Norm

0

500

1000

1500

2000

2500

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3500

4000

4500

5000

5500

JanJan

Feb

MarMar

Apr

MayMay

Jun

JulJul

Aug

SepSep

Oct

Nov

DecDec

Daily Maximum 1‐hour PM10 (μg/m3)

2006

2007

2008

2009

2010

2011

July 3, 2011

Figure 4-2. Daily maximum 1-hr PM10 concentrations at the Yuma Supersite monitor 

(2006-2011). The daily maximum 1-hr PM10 concentration on July 3, 2011, is highlighted 

by the red arrow. 

0

50

100

150

200

250

300

1520253035404550556065707580859095

100110120130140150160170180190200250300350400

Number of Days

24‐hour Average PM10 (μg/m3)

July 3, 2011

95th Percentile

Figure 4-3. 24-hr average PM10 concentrations at the Yuma Supersite monitor for 

2006-2011. The 24-hr average PM10 concentration on July 3, 2011, was in excess of the 

95th percentile.

3 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Historical Norm

0

20

40

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15305065758595

110130150170190225500

100030005000

Number of Days

Daily Maximum 1‐hour PM10 (μg/m3)

95th percentile

July 3, 2011

Figure 4-4. Daily maximum 1-hr PM10 concentrations at the Yuma Supersite monitor for 

2006-2011. The daily maximum 1-hr PM10 concentration on July 3, 2011, was in excess 

of the 95th percentile. 

4 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Not Reasonably Controllable or Preventable

1 

5. Not Reasonably Controllable or Preventable 

5.1 Background 

Yuma was designated as a moderate PM10 nonattainment area by operation of the 1990 

Clean Air Act. The nonattainment area is defined in 40 CFR §481.303. ADEQ completed a 

state implementation plan (SIP) for the area in 1991; however, the plan was found incomplete 

and in 1994 ADEQ updated the plan, identifying additional reasonably available control 

measures (RACM). In 2001, due to several years of “clean data” and the existence of 

permanent and enforceable measures, ADEQ began the development of a maintenance plan 

and request for redesignation of the area to attainment. The maintenance plan was submitted 

to EPA in August 2006. 

5.1.1 Control Measures 

Details of the control measures implemented from 1994-2001 are in Appendix G of the 

2006 Yuma PM10 Maintenance Plan. A list of the measures is contained in Table 5-1. 

Table 5-1. Control measures implemented in Yuma nonattainment area 1994-2001. 

Implementing Agency Reasonably Available Control Measure 

City of Yuma Paving unpaved roads 

Closing unpaved roads 

Chemically stabilizing unpaved roads 

Paving or stabilizing parking lots 

Re-routing traffic or rapid cleanup of temporary sources of dust and spills 

Covering haul trucks 

Dust control plans for land clearing, construction projects 

Stabilizing soil; controlling dust on open lands 

Amending building codes 

Town of Somerton Re-routing traffic or rapid cleanup of temporary sources of dust and spills 

Covering haul trucks 

Dust control plans for land clearing, construction projects 

Stabilizing soil 

Yuma County Paving unpaved roads 

Stabilizing unpaved roads 

Re-routing traffic or rapid cleanup of temporary sources of dust and spills 

Covering haul trucks 

Open Burn Permit Program (Rural Metro) 

Irrigation Districts Reducing traffic on unpaved roads 

AZ Dept. of Transportation Requiring contractors to adhere to local dust control plans 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Not Reasonably Controllable or Preventable

2 

RACM for 2000 through 2004 can be found in Table 6.3 of the 2006 Yuma PM10

Maintenance Plan and is reproduced in part in Table 5-2. Chapter 7 of the Maintenance Plan 

also contains a list of contingency measures that could be implemented promptly should any 

violation of the National Ambient Air Quality Standard (NAAQS) for PM10 occur. 

Table 5-2. Yuma-area-implemented control measures, 2000-2004. 

Page 1 of 2 

Implementing Agency Reasonably Available Control Measure 

City of Yuma Pave unpaved roads 

Pave unpaved alleys 

Paving unpaved vacant land 

Chemically stabilize unpaved roads 

Watering shoulders 

Street sweeping paved roads 

Install curbs and sidewalks 

Landscaping median 

Magnesium chloride on alleys 

Magnesium chloride on City property 

Town of Somerton Water unpaved roads 

Water unpaved shoulders 

Pave unpaved roads 

Weekly cleanup of paved roads, mud, trackout, spills 

Pave unpaved lots 

Landscape shoulders 

Install curbs 

Pave/stabilize unpaved roads 

Chip/seal 

Magnesium chloride on unpaved roads 

Street sweeping 

Yuma County Pave unpaved roads 

Developers add new paved roads 

Chip/seal unpaved roads 

Magnesium chloride unpaved roads 

Street sweeping 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Not Reasonably Controllable or Preventable

3 

Table 5-2. Yuma-area-implemented control measures, 2000-2004. 

Page 2 of 2 

Implementing Agency Reasonably Available Control Measure 

Immigration & Naturalization Water drag roads 

Pipelined 

Maintain 350 “No Trespassing” signs and 50 barricades 

Patrol and water unpaved canal roads 

3 mi posted/barricaded 

Paved 2.5 mi 

2.5 mi fenced off 

Abandoned 3/8 mi 

Lined 8 mi of canal 

N. Gila Irrigation District 20 miles posted 

Unit B Irrigation District 3 mi posted/barricaded 

Bureau of Reclamation Water 960 miles of canal banks 

Marine Corps Air Station Remove 26 gas vehicles 

Remove 25 gas scooters 

Pave 240,329 ft roadway 

Pave 102,112 ft parking 

Sweeping 717,221 yd runway 

Sweeping 388,952 yd taxiway 

Sweeping 401,090 yd aprons and 121,380 yd other 

Stabilize desert 

In 2010, the Yuma Metropolitan Planning Organization (YMPO) updated the 

Transportation Improvement Plan (TIP) as required to comply with the requirements for 

transportation conformity under Section 176(c)(2) of the Clean Air Act. The update required a 

review of control measures included in the 2006 Yuma PM10 Maintenance Plan to assure that 

emissions were within the limits found in both plans for the current review years through the 

2016 projected maintenance period. Yuma’s plans related to transportation improvements can 

be found under “Plans and Reports” at http://www.ympo.org/. 

5.1.2 Additional Measures 

On August 18, 2002, Yuma recorded a 24-hr average PM10 concentration of 170 μg/m3, 

which is in exceedance of the NAAQS. A Natural Events Action Plan (NEAP) was created to 

address any measures that could be implemented to prevent future violations of the NAAQS. 

The option to develop a NEAP is no longer available; however, Yuma reviewed existing and 

developed additional measures that were later incorporated into the 2006 PM10 Maintenance 

Plan. These included: (1) a public notification and education program still in place today and 

augmented recently by a pilot flag program for public schools and facilities based on the Yuma 

Dust Control Action Forecast [Appendices D, E and F]; (2) an analysis of best available control 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Not Reasonably Controllable or Preventable

4 

measures (BACM) normally reserved for serious nonattainment areas; and (3) a review of 

existing control measures for construction sources, street sweepers, paved roads, covered 

trucks, off-highway vehicles, stationary source opacity limits, other stationary source control 

measures, and agricultural best management practices [Appendix H]. In 2002, ADEQ met with 

Yuma stakeholders and began work on the development of a Yuma Agricultural Best 

Management Practices (AgBMP) rule. The rule became effective July 18, 2005, as R18-2-613 

of the Arizona Administrative Code, and was submitted to EPA on August 16, 2006. 

5.1.3 Review of Source-Permitted Inspections and Public Complaints 

ADEQ’s Arizona Unified Repository for Information Tracking of the Environment 

(AZURITE) database was queried to compile a list of inspections for the permitted sources in 

the Yuma area around the time of the July 3, 2011, PM10 exceedance. Using a date range that 

includes the date of the exceedance, three days prior, and three days following the event, it was 

determined that no inspections occurred and no public complaints were submitted during this 

time period. 

5.2 Forecasts and Warnings 

Dust forecasts were released prior to the event by both ADEQ and the National Weather 

Service (NWS) office in Phoenix (Appendix B). The ADEQ Yuma and Vicinity Dust Control 

Action Forecast issued on Friday, July 1, 2011, stated that conditions would be “possibly gusty 

late due to thunderstorm outflow winds” and included a “Moderate” windblown dust risk. The 

NWS issued a Special Weather Statement on Saturday, July 2, 2011, at 1:08 p.m. MST, 

warning of the potential for strong gusty outflow winds from thunderstorms with gusts over 40 

miles per hour, expected to cause windblown dust and possibly a dust storm in open desert 

areas late that afternoon or evening across southwestern Arizona, including the Yuma area. 

At 12:17 a.m. MST on July 3, 2011, the NWS in Phoenix issued a Dust Storm Warning 

for Yuma and other communities due to large areas of blowing dust and sand, causing 

“substantially reduced visibilities... to 1/3 mile or less.” The time this dust storm warning was 

issued coincides with the peak PM10 concentration of 3,089 μg/m3

 recorded at the Yuma 

Supersite monitor. 

5.3 Wind Observations 

Wind data during the event were available at five Yuma-area sites (Figure 3-3 and 

Appendix A). Sustained wind speeds of over 20 mph were reported at the Yuma Supersite 

monitor during the event. The Yuma South, North Gila, and Yuma MCAS sites reported 

sustained wind speeds of over 25 mph. A peak wind gust of 48 mph was reported at the Yuma 

MCAS site. Sustained wind speeds of over 25 mph are normally sufficient to overcome most 

PM10 control measures. 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Not Reasonably Controllable or Preventable

5 

5.4 Summary 

The weather and air quality forecasts and warnings outlined in this section demonstrate 

that unpreventable outflow winds from thunderstorms resulted in strong wind gusts that caused 

the dust storm that significantly impacted Yuma. The surface wind measurements in the Yuma 

area during the event were high enough (at or above 25 mph) that most reasonable PM10

control measures would have been overwhelmed. 

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 But-For Analysis

1 

6. But-For Analysis 

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

demonstration satisfies 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 PM10

exceedance at the Yuma Supersite monitor on July 3, 2011: 

 Was not reasonably controllable or preventable, and 

 That there was a clear causal relationship between PM10 transported from 

thunderstorm outflow originating in desert areas outside the Yuma PM10

nonattainment area and the measured PM10 exceedance in Yuma. 

The weight of evidence in these sections demonstrates that, but for the existence of dust 

emissions generated by thunderstorm outflow and the associated transport of PM10, there would 

have been no exceedance of the NAAQS for 24-hr average PM10. 

As shown in Section 3, radar velocity data and time-series plots of PM10 and wind 

speeds establish a clear causal relationship between the arrival of dust-laden thunderstorm 

outflow and elevated PM10 concentrations at the Yuma Supersite monitor. Multiple independent 

measurements of wind speed, wind direction, and visibility all point to the presence of 

thunderstorm outflow as the mechanism for transport of PM10 into the Yuma nonattainment 

area. High PM10 concentrations and gusty winds were also reported in other parts of Arizona 

and southeastern California, illustrating the widespread, regional nature of this event. In 

addition, PM10 concentrations were well below the NAAQS on days immediately before and 

after the windblown dust event. The source regions for the thunderstorm outflow and PM10 are 

clearly identified as desert areas southeast of the Yuma PM10 nonattainment area. The weight 

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

July 3, 2011, to any causal source except PM10 transported by thunderstorm outflow, confirming 

that there would have been no exceedance but for the presence of these uncontrollable natural 

events. 

As detailed in Section 5, all reasonable control measures were in place and/or 

implemented on a continual basis. Air quality-related inspection and compliance data revealed 

no violations or complaints within three days before and after the time of the event. Local 

regulatory agencies, industry, and the general public were alerted to the possibility of dust 

storms due to thunderstorm activity via daily forecasts and media reports. The weight of 

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

July 3, 2011, to any causal source except PM10 transported by thunderstorm outflow, confirming 

that there would have been no exceedance but for the presence of these uncontrollable natural 

events. 

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Conclusions

1 

7. Conclusions 

The PM10 exceedance that occurred on July 3, 2011, satisfies the criteria of 40 CFR 

50.1(j) and meets the definition of an exceptional event. These criteria are: 

 The event affects air quality. 

 The event is not reasonably controllable or preventable. 

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

7.1 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. 

7.2 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 Yuma County, 

high winds overwhelmed all reasonably available controls. The PM10 exceedance discussed in 

this report was caused by naturally-occurring thunderstorm outflow that transported dust into 

Yuma County from areas largely outside the Yuma PM10 nonattainment area. These facts 

provide strong evidence that the PM10 exceedance on July 3, 2011, was not reasonably 

controllable or preventable. 

7.3 Natural Event 

As discussed above, the PM10 exceedance in Yuma on July 3, 2011, was shown to be 

caused by transport of PM10 into Yuma from thunderstorm outflow. The event therefore 

qualifies as a natural event. 

7.4 Summary 

The exceedance of the federal 24-hr PM10 standard on July 3, 2011, at the Yuma 

Supersite monitor would not have occurred but for the thunderstorm-driven high winds and 

transport of dust from areas largely outside the Yuma PM10 nonattainment area, based on the 

following weight of evidence: 

 Time-series of PM10 concentrations show that the timing of high PM10 at the Yuma 

Supersite was consistent with gusty winds and low visibilities at Yuma-area 

meteorological stations (Section 3). 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Conclusions 

2 

 High PM10 concentrations and gusty winds were reported in Yuma County, Arizona, and 

Imperial County, California, illustrating the widespread, regional, and uncontrollable 

nature of this event (Section 3). 

 PM10 concentrations were well below the NAAQS on days immediately before and after 

the windblown dust event. 

 Dry conditions preceding the event resulted in soils that were particularly susceptible to 

particulate suspension by high winds (Section 3). 

 The 24-hr average and daily 1-hr maximum PM10 values measured at the Yuma 

Supersite monitor were historically unusual compared to a multi-year data set 

(Section 4). 

 PM10 control and prevention measures were in place in the Yuma PM10 nonattainment 

area at the time of the event. Measured wind speeds and wind gusts were of sufficient 

strength to overcome reasonable control measures (Section 5). 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Appendix A 

Appendix A: Historical Fluctuation Graphs for Yuma County 

This section contains time-series of wind data for Yuma-area meteorological monitors on 

July 2 and 3, 2011. The data show a region-wide increase in wind speeds and wind gusts 

coincident with the arrival of dust and high PM10 concentrations in Yuma. 

0

5

10

15

20

25

30

35

40

45

50

0

500

1000

1500

2000

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3500

Wind Speed (mph)

Concentration (μg/m3)

July 2, 2011                          Time of Day (MST)                        July 3, 2011

Yuma North Gila Wind Speed

Yuma North Gila Wind Gust

Yuma Supersite PM10

Figure A-1. Hourly PM10 concentrations at the Yuma Supersite monitor and wind 

speeds at the Yuma North Gila monitor on July 2 and 3, 2011. PM10 concentrations and 

wind speeds sharply increased at midnight on July 3, 2011, indicating the arrival of 

windblown dust. 

A-1
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Appendix A 

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25

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35

40

45

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500

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Wind Speed (mph)

Concentration (μg/m3)

July 2, 2011                          Time of Day (MST)                        July 3, 2011

Yuma South Wind Speed

Yuma South Wind Gust

Yuma Supersite PM10

Figure A-2. Hourly PM10 concentrations at the Yuma Supersite monitor and wind 

speeds at the Yuma South monitor on July 2 and 3, 2011. PM10 concentrations and wind 

speeds sharply increased at midnight on July 3, 2011, indicating the arrival of windblown 

dust. 

A-2
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Appendix A 

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500

1000

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3000

3500 50

Wind Speed (mph)

Concentration (μg/m3)

July 2, 2011                          Time of Day (MST)                        July 3, 2011

Yuma Valley Wind Speed

Yuma Valley Wind Gust

Yuma Supersite PM10

Figure A-3. Hourly PM10 concentrations at the Yuma Supersite monitor and wind 

speeds at the Yuma Valley monitor on July 2 and 3, 2011. PM10 concentrations and wind 

speeds sharply increased at midnight on July 3, 2011, indicating the arrival of windblown 

dust. 

A-3
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Appendix A 

A-4

0

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500

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2500

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Wind Speed (mph)

Concentration (μg/m3)

July 2, 2011                          Time of Day (MST)                        July 3, 2011

Yuma MCAS Wind Speed

Yuma MCAS Wind Gust

Yuma Supersite PM10

Figure A-4. Hourly PM10 concentrations at the Yuma Supersite monitor and wind 

speeds at the Yuma MCAS monitor on July 2 and 3, 2011. PM10 concentrations and 

wind speeds sharply increased at midnight on July 3, 2011, indicating the arrival of 

windblown dust. 

DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Appendix B 

Appendix B: ADEQ and NWS Forecast Products 

YUMA AND VICINITY

DUST CONTROL ACTION FORECAST

ISSUED FRIDAY, JULY 1, 2011

Three-day weather outlook: 

High pressure will be near the Four Corners later Friday afternoon through early next week. This 

will increase moisture levels across Arizona. Thunderstorm activity will ramp up considerably across the 

mountains as well as the deserts Saturday through Monday. Outflow winds may generate desert dust 

storms. Thus, the risk of wind-blown dust in Yuma will be MODERATE as a result. 

 WINDS WIND BLOWN DUST 

RISK

Day 1: Sat 07/02/2011 No significant winds 

are expected early, 

possibly gusty late due to 

thunderstorm outflow 

winds. 

MODERATE 

Day 2: Sun 07/03/2011 No significant winds 

are expected early, 

possibly gusty late due to 

thunderstorm outflow 

winds. 

MODERATE 

Day 3: Mon 07/04/2011 No significant winds 

are expected early, 

possibly gusty late due to 

thunderstorm outflow 

winds. 

MODERATE 

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 (brown cloud). Particles with diameters between 2.5 and 10 micrometers are 

referred to as “coarse”. 

B-1
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Appendix B 

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, minimize travel on dirt roads, utilize tarps on haul trucks, 

limit use of leaf-blowers, and on high-wind days reduce outdoor activities. 

B-2
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Appendix B 

B-3

NWS Special Weather Statement 

WWUS85 KPSR 022008 

SPSPSR 

SPECIAL WEATHER STATEMENT 

NATIONAL WEATHER SERVICE PHOENIX AZ 

108 PM MST SAT JUL 2 2011 

AZZ021>023-026>028-030100- 

WEST CENTRAL DESERTS-NORTHWEST MARICOPA COUNTY-GREATER PHOENIX 

AREA-SOUTHWEST DESERTS-SOUTHWEST MARICOPA COUNTY-NORTHWEST AND 

NORTH CENTRAL PINAL COUNTYINCLUDING THE CITIES OF...BOUSE...QUARTZSITE...SALOME...BUCKEYE...LAKE 

PLEASANT...MORRISTOWN...NEW RIVER...TONOPAH...WICKENBURG... 

CAREFREE...CAVE CREEK...CHANDLER...FOUNTAIN HILLS...GILBERT... 

GLENDALE...MESA...PEORIA...PHOENIX...SCOTTSDALE...SUN CITY...TEMPE... 

DATELAND...TACNA...WELLTON...GILA BEND...APACHE JUNCTION...CASA 

GRANDE...COOLIDGE...AND FLORENCE 

108 PM MST SAT JUL 2 2011 

...STRONG GUSTY WINDS LATE THIS AFTERNOON OR THIS EVENING WITH 

BLOWING DUST POSSIBLE IN THE DESERT AREAS... 

SCATTERED THUNDERSTORMS OVER THE HIGHER TERRAIN TO THE EAST OF THE 

GREATER PHOENIX AREA ARE EXPECTED TO MOVE WEST ACROSS SOUTHERN GILA 

COUNTY AND THE TONTO NATIONAL FOREST FOOTHILLS THIS AFTERNOON. MOST 

OF THESE THUNDERSTORMS ARE EXPECTED TO DISSIPATE AS THEY MOVE 

INTO THE DESERT AREAS. HOWEVER...STRONG GUSTY OUTFLOW WINDS FROM THE 

THUNDERSTORMS ARE EXPECTED TO SPREAD FROM EAST TO WEST ACROSS THE 

DESERT INCLUDING THE GREATER PHOENIX AREA LATE THIS AFTERNOON OR THIS 

EVENING. SOME OF THE WINDS COULD GUST TO OVER 40 MPH. THESE WINDS ARE 

EXPECTED TO RESULT IN SOME BLOWING DUST ESPECIALLY IN THE OPEN DESERT 

AREAS WHERE A DUST STORM IS POSSIBLE LATE THIS AFTERNOON OR THIS 

EVENING. THIS INCLUDES MARICOPA COUNTY...NORTHWEST AND NORTH-CENTRAL 

PINAL COUNTY...AND POSSIBLY EVEN LA PAZ AND YUMA COUNTIES. 

IF YOU SEE A DUST STORM APPROACHING OR GET CAUGHT IN ONE...PULL OFF TO 

THE SIDE OF THE ROAD AS FAR AS SAFELY POSSIBLE...PUT YOUR VEHICLE IN 

PARK...TAKE YOUR FOOT OFF THE BRAKE PEDAL...AND TURN YOUR LIGHTS ALL THE 

WAY OFF. 

IT WOULD BE WISE TO SECURE LOOSE OBJECTS TO PROTECT THEM FROM THE 

WIND. THE WIND COULD ALSO MAKE DRIVING DIFFICULT ESPECIALLY FOR HIGH 

PROFILE VEHICLES. STAY UP-TO-DATE ON THE LATEST FORECASTS BY LISTENING 

TO WEATHER RADIO ON THE PUBLIC SERVICE BAND. MORE DETAILED INFORMATION 
DRAFT REPORT Exceptional Event, Yuma County, July 3, 2011 Appendix B 

B-4

IS AVAILABLE FROM THE NATIONAL WEATHER SERVICE IN PHOENIX ON THE 

INTERNET AT WEATHER.GOV/PHOENIX 

NWS Dust Storm Warning

WWUS85 KPSR 030717 

SPSPSR 

SPECIAL WEATHER STATEMENT 

NATIONAL WEATHER SERVICE PHOENIX AZ 

1217 AM MST SUN JUL 3 2011 

AZZ020-025-026-CAZ031-033-030815- 

LA PAZ AZ-YUMA AZ-IMPERIAL CA1217 AM MST SUN JUL 3 2011 

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A 

LOCAL DUST STORM WARNING FOR... 

EASTERN IMPERIAL COUNTY IN SOUTHEAST CALIFORNIA 

WEST CENTRAL YUMA COUNTY IN SOUTHWEST ARIZONA 

SOUTHWESTERN LA PAZ COUNTY IN WEST CENTRAL ARIZONA 

* COMMUNITIES IMPACTED: GORDON'S WELL...HOLTVILLE...WINTERHAVEN... 

CIBOLA...GADSDEN...MARTINEZ LAKE...SOMERTON...YUMA...ANDRADE...KINTER 

AND ARABY 

* UNTIL 115 AM MST 

* MAJOR HIGHWAYS IMPACTED INCLUDE: 

I-8... 

US 95... 

* A LOCAL DUST STORM WARNING IS ISSUED WHEN WINDS HAVE GENERATED 

LARGE AREAS OF BLOWING DUST OR BLOWING SAND THAT HAVE SUBSTANTIALLY 

REDUCED VISIBILITIES...TO 1/4 MILE OR LESS...RESULTING IN VERY HAZARDOUS 

DRIVING CONDITIONS IN SOME AREAS. BE READY FOR A SUDDEN DROP IN VISIBILITY 

TO NEAR ZERO. 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