State of Arizona 

Exceptional Event Documentation 

for the Event of August 18, 2011, 

for the Phoenix PM10 Nonattainment Area 

Final Report Prepared for 

Arizona Department of Environmental Quality 

Maricopa County Air Quality Department 

Maricopa Association of Governments 

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

Exceptional Event Documentation 

for the Event of August 18, 2011, 

for the Phoenix PM10 Nonattainment Area 

Final Report 

STI-912090-5545-DR

Prepared by 

Daniel Alrick 

Angela Ekstrand 

Briana Gordon 

Hilary Hafner 

Clinton MacDonald 

Hilary Minor 

Ashley Russell 

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

January 23, 2013 

Cover image illustrates windblown dust due to thunderstorm outflow moving into Phoenix, AZ at 1810 LST on August 

18, 2011. This photo was taken by an ADEQ visibility camera from North Mountain looking south toward downtown 

Phoenix.

Exceptional Event, Maricopa County, August 18, 2011 Acknowledgments 

Acknowledgments 

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

for helpful discussion and feedback during the preparation of this document. 

iii

Exceptional Event, Maricopa County, August 18, 2011 Table of Contents 

Table of Contents 

Section Page

List of Figures ..............................................................................................................................vii

List of Tables................................................................................................................................ ix

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

1.1 Report Contents .........................................................................................................1

1.2 Exceptional Event Rule Requirements.......................................................................1

1.2.1 Public Notification That the Event Was Occurring (40 CFR 

50.14(c)(1)(i)) .................................................................................................2

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

1.2.3 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))................................................................................................2

1.2.4 Document That the Public Comment Process Was Followed for Event 

Documentation (40 CFR 50.14(c)(3)(iv))........................................................2

1.2.5 Submit Demonstration Supporting Exceptional Event Flag (40 CFR 

50.14(a)(1-2)).................................................................................................3

1.2.6 Documentation Requirements (40 CFR 50.14(c)(3)(iii)) ................................3

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

2.1 Geographic Setting and Monitor Locations ................................................................1

2.2 Climate, Monsoon, and Thunderstorms .....................................................................5

2.3 Event Day Summary ..................................................................................................6

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

3.1 Discussion..................................................................................................................1

3.2 Summary....................................................................Error! Bookmark not defined.

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

4.1 Analysis......................................................................................................................1

4.2 Summary....................................................................................................................1

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

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

5.1.1 Control Measures...........................................................................................1

5.1.2 Additional Measures.......................................Error! Bookmark not defined.

5.1.3 PM10 Rule Effectiveness ................................................................................3

5.1.4 Compliance and Enforcement Activities.........................................................4

5.1.5 Review of Source-Permitted Inspections and Public Complaints ..................5

5.2 Forecasts and Warnings ............................................................................................6

5.3 Wind Observations.....................................................................................................6

5.4 Summary....................................................................................................................6

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

6.1 Discussion..................................................................................................................1

6.2 Summary....................................................................................................................1

v
Exceptional Event, Maricopa County, August 18, 2011 Table of Contents 

vi

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

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

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

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

7.4 Clear Causal Relationship..........................................................................................2

7.5 Historical Norm...........................................................................................................2

7.6 Not Reasonably Preventable .....................................Error! Bookmark not defined.

7.7 But For .......................................................................................................................2

Appendix A: Air Quality and Meteorological Data...............................................................1

Appendix B: Media Coverage, Videos, and Images ...........................................................6

Video Links ..........................................................................................................................6

Media Article and Image Links ............................................................................................7

Appendix C: Historical Fluctuation Time-Series Graphs...................................................20

Appendix D: ADEQ and NWS Forecast Products ............................................................22

Appendix E: Affidavit of Public Notice ................................................................................1
Exceptional Event, Maricopa County, August 18, 2011 List of Figures 

List of Figures 

Figure Page

Figure 2-1. Location of air quality monitors with exceedances of the 24-hr PM NAAQS 

and representative NWS meteorological monitors............................................................2

10

Figure 2-2. Location of air quality sites monitoring PM10 statewide on August 18, 2011............3

Figure 2-3. Drainage System of Phoenix, Arizona. .....................................................................4

Figure 2-4. Average monthly temperatures and precipitation for Phoenix, 1981–2010. .............5

Figure 2-5. Cross-section of a thunderstorm creating an outflow boundary and haboob. 

(Source: Desert Meteorology, Thomas T. Warner, 2004.) ...............................................6

Figure 2-6. Thunderstorms (bright white areas) over central and eastern Arizona 

produced outflow boundaries that propagated westward and transported dust to 

the Phoenix area on August 18, 2011...............................................................................7

Figure 3-1. Visible satellite image from 16:30 LST on August 18, 2011 (GOES-West). 

Bright white areas indicate tall, convective (thunderstorm) clouds. Thunderstorms 

over central and eastern Arizona generated outflow boundaries that carried dust 

westward across the Phoenix area. ..................................................................................1

Figure 3-2. Phoenix-area air quality and meteorological monitors and general wind 

direction. Wind and PM data at the monitors are shown in Table 3-1. Gusty 

winds associated with thunderstorm outflow transported dust across the Phoenix 

area during the late afternoon hours of August 18, 2011..................................................2

10

Figure 3-3. Radar base velocity data at 18:06 LST on August 18, 2011, from NWS 

Phoenix Doppler radar. Green indicates flow toward the radar; dark orange and 

red indicate flow toward the radar, and yellow indicates little motion toward the 

radar. Two well-defined outflow boundaries (dashed liens) approached the 

Phoenix area from the southeast (A) and east (B). These boundaries carried dust 

northward and westward across the Phoenix metropolitan area.Error! Bookmark not defined.

Figure 3-4. Hourly PM concentrations and wind speeds at the West Chandler monitor 

on August 18 and 19, 2011. PM concentrations and wind speeds sharply 

increased at 16:00 LST on August 18, 2011, indicating the arrival of windblown 

dust. Sustained winds of 20 mph were measured with gusts over 35 mph......................5

10

10

Figure 3-5. Hourly PM concentrations and wind speeds at the Buckeye monitor on 

August 18 and 19, 2011. PM concentrations sharply increased with wind gusts 

over 35 mph at 20:00 LST on August 18, 2011, indicating the arrival of windblown 

dust. ..................................................................................................................................5

10

10

Figure 3-6. Hourly PM concentrations at the Phoenix area PM monitors and visibility 

at KPHX. Visibility was significantly reduced at KPHX between 18:00 and 19:00 

LST, coincident with the sharp increase in PM concentrations at area PM

monitors, indicating the arrival of windblown dust.............................................................6

10 10

10 10

vii
Exceptional Event, Maricopa County, August 18, 2011 List of Figures 

viii

Figure 4-1. 24-hr average PM concentrations at the Central Phoenix monitor 

(2007-2011). The 24-hr average PM concentration on August 18, 2011, is 

highlighted in red...............................................................................................................1

10

10

Figure 5-1. Timeline of Maricopa County fugitive dust rules and ordinances..............................4

Figure C-0-1. 24-hr average PM concentrations at the Buckeye monitor (2007–2011). 

The 24-hr average PM concentration on August 18, 2011, is highlighted in red..........20

10

10

Figure C-0-2. 24-hr average PM concentrations at the West Chandler monitor (2007–

2011). The 24-hr average PM concentration on August 18, 2011, is highlighted 

in red. ..............................................................................................................................21

10

10

Figure C-0-3. 24-hr average PM concentrations at the South Phoenix monitor (2007–

2011). The 24-hr average PM concentration on August 18, 2011, is highlighted 

in red. ..............................................................................................................................21

10

10
Exceptional Event, Maricopa County, August 18, 2011 List of Tables 

List of Tables 

Table Page

Table 2-1. Statewide monitor information and PM10 measurements on August 18, 2011. 7

Table 3-1. Observed wind speeds and wind gusts at Maricopa County monitors and KPHX on 

August 18, 2011. All monitors reported high PM concentrations from 18:00 to 20:00 LST on 

August 18, 2011, coincident with high wind speeds and wind gusts. 3

10

Table 5-1. Rules and ordinances regulating particulate matter emissions in Maricopa County. 2

Table 5-2. Pinal County rules regulating existing and new non-point sources in Pinal County. 3

Table 5-3. Pinal County rules regulating fugitive dust in Pinal County portion of Phoenix PM

Nonattainment Area. 3

10

ix
Exceptional Event, Phoenix, August 18, 2011 Introduction

1

1. Introduction 

On August 18, 2011, four air quality monitors in the Phoenix PM10 nonattainment area 

recorded 24-hr average PM10 concentrations in excess of the National Ambient Air Quality 

Standard (NAAQS) of 150 μg/m3

. The purpose of this report is to demonstrate that these 

exceedances were due to naturally occurring windblown dust, were historically unusual, were 

not reasonably controllable or preventable, and would not have occurred “but-for” the windblown 

dust and, therefore, are Exceptional Events as defined by the U.S. Environmental Protection 

Agency’s (EPA) Exceptional Events Rule (EER). 

1.1 Report Contents 

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

that transpired on August 18, 2011, and provides a background narrative of the exceptional 

event and an overall explanation of how 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 August 18, 2011, and the exceedances of the 24-hr PM10 standard at the four 

monitoring stations. 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 illustrates that the event of August 18, 2011, produced 

PM10 concentrations in excess of normal historical 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 August 

18, 2011, was not reasonably controllable or preventable. 

Section 6 of this assessment builds upon the demonstrations made in the previous 

sections, showing a clear causal connection between the natural event and the exceedances, 

and concludes that the exceedances of the 24-hr PM10 standard on August 18, 2011, would not 

have occurred but for the event. 

Appendix A contains time-series graphs and data tables to supplement Section 3. 

Appendix B contains links to videos, images, and media reports to supplement Section 3. 

Appendix C contains time-series graphs to supplement Section 4. Appendix D contains air 

quality forecasts issued by the Arizona Department of Environmental Quality (ADEQ) and 

weather statements and warnings issued by the National Weather Service. Appendix E

contains a copy of the affidavit of public notice concerning this assessment report. 

1.2 Exceptional Event Rule Requirements 

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

requirements must also be met in order for the EPA to concur with the flagged air quality 
Exceptional Event, Phoenix, August 18, 2011 Introduction

2

monitoring data. This section of the report contains the requirements of the EER and 

associated guidance, and discusses how ADEQ addressed those requirements. 

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

ADEQ issued Dust Control Action Forecasts and Air Quality Forecasts for Maricopa 

County, advising citizens of the potential for high-wind dust events on August 18, 2011. More 

information on ADEQ’s forecasting program can be found in Section 5 of this report. The 

forecast products that were issued pertaining to this event are included in Appendix D. 

1.2.2 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 the EPA’s Air 

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

AQS includes particulate matter (PM) data from both filter-based and continuous monitors 

operated in Arizona. 

When ADEQ (or another agency operating monitors in Arizona) suspects that data may 

be influenced by an exceptional event, ADEQ (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 (or the other 

operating agency) also submits data from continuous monitors into AQS after quality assurance 

is complete. 

If ADEQ (or other operating air quality agencies) have determined that the potential 

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

flag is submitted for those measurement(s) in AQS. The data are not official until they undergo 

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 flags on the August 18, 2011, data can be confirmed in AQS. 

1.2.3 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 EER. The PM10 exceedances that occurred at 

four monitors on August 18, 2011, in the Phoenix PM10 nonattainment area were included on 

this list. This assessment report serves as demonstration supporting the flagging of these data. 

1.2.4 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 hard copy 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 
Exceptional Event, Phoenix, August 18, 2011 Introduction 

3

with any comments received, will be submitted to EPA, consistent with the requirements of 40 

CFR 50.14(c)(3)(iv). See Appendix E for a copy of the affidavit of public notice. 

1.2.5 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 9 headquarters 

in San Francisco, California. The deadline for the submittal of this package is September 30, 

2014. 

1.2.6 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. 

Exceptional Event, Phoenix, August 18, 2011 Conceptual Model

1 

2. Conceptual Model 

This section provides a narrative background and summarizes the meteorological and 

air quality conditions in place on August 18, 2011, in the Phoenix area. Elements described in 

this section include 

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

monitors. 

 A description of Phoenix’s climate. 

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

2.1 Geographic Setting and Monitor Locations 

Phoenix is located in the Salt River Valley in south-central Arizona. It lies at an 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. 

A fairly dense network of air quality and meteorological monitors exists throughout the 

Phoenix area, with a much less dense network of monitors throughout the rest of Arizona. 

Figure 2-1 shows the general geographic setting of Phoenix, as well as the locations of 

meteorological monitors and the PM10 monitors that recorded exceedances on August 18, 2011. 

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

run days). AQS monitors typically measure both air quality and meteorological data. Some of 

the AQS monitors in the Phoenix area are run by the Maricopa County Air Quality Department 

(MCAQD) while others are run by ADEQ. The PM10 exceedances on August 18, 2011, were 

recorded at the West Chandler, South Phoenix, Central Phoenix, and Buckeye monitors. The 

National Weather Service (NWS) monitors measure meteorological data only. Phoenix Sky 

Harbor International Airport (KPHX), Chandler Municipal Airport (KCHD), and Phoenix 

Goodyear Airport (KGYR) were the primary NWS sites used in this demonstration package 

because of those sites’ high data quality, data completeness, proximity to AQS sites, and 

representativeness of meteorological conditions in the Phoenix area. Figure 2-2 shows 

monitors statewide measuring PM10 on August 18, 2011. 

Figure 2-3 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 in the air and add to regional PM10

loadings during high wind events. Much of this alluvial matter and fine soil is deposited in the 
Exceptional Event, Phoenix, August 18, 2011 Conceptual Model

2 

low-lying areas of central and southern Arizona, with larger depositional areas focused in and 

around the confluences of dry river channels. 

Figure 2-1. Locations of air quality monitors that recorded exceedances of the 24-hr 

PM10 NAAQS and NWS monitors in the Phoenix area. 
Exceptional Event, Phoenix, August 18, 2011 Conceptual Model

3 

Figure 2-2. Location of sites monitoring PM10 in Arizona on August 18, 2011. 
Exceptional Event, Phoenix, August 18, 2011 Conceptual Model

4 

Figure 2-3. Drainage system of Phoenix, Arizona. 
Exceptional Event, Phoenix, August 18, 2011 Conceptual Model

5 

2.2 Climate, Monsoon, and Thunderstorms 

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

average summer high temperatures are among the hottest of any populated area in the United 

States (Figure 2-4). Temperatures reach or exceed 100°F an average of 110 days annually, 

and reach or exceed 110°F an average of 18 days annually. Phoenix receives an average of 

7.66 inches of rain per year. 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 mid- to late-summertime period, monsoonal 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 2-4. Average monthly temperatures and precipitation for Phoenix, 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. 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 the outflow boundary (also called gust 

fronts), as shown in Figure 2-5. 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. 
Exceptional Event, Phoenix, August 18, 2011 Conceptual Model

6 

Cross‐section schematic of a haboob caused by the cool outflow from a

thunderstorm, with the leading edge that is propagating ahead of the storm

called an outflow boundary.  The strong, gusty winds that prevail at the

boundary are defined as a gust front.  The leading edge of the cool air is called

the nose, and the upward‐protruding part of the features is referred to as the

head.  Behind the roll in the windfield at the leading edge is a turbulent wake.  

The rapidly moving cool air and the gustiness at the gust front raise dust

(shaded) high into the atmosphere.

Figure 2-5. 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, the extremely hot temperatures can 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 afternoon and evening of August 18, 2011, strong winds generated by 

thunderstorms over central and eastern Arizona transported dust westward into the Phoenix 

area (Figure 2-6). The windblown dust resulted in 24-hr average PM10 concentrations in 

exceedance of the NAAQS at four air quality monitors (Table 2-1). The PM10 concentrations 

measured at these monitors were in excess of normal historical fluctuations. The dust was 

naturally occurring and likely originated over undeveloped lands east of the Phoenix area, and 

wind speeds in excess of 30 to 40 mph overwhelmed reasonable dust control measures. 

Monitors across Maricopa, Pinal, and Pima counties recorded high PM10 concentrations during 

this dust storm, illustrating the widespread, regional nature of the event. 
Exceptional Event, Phoenix, August 18, 2011 Conceptual Model

7 

Transport of

windblown dust

Figure 2-6. Thunderstorms (bright white areas) over central and eastern Arizona 

produced outflow boundaries that propagated westward and transported dust to the 

Phoenix area on August 18, 2011. 

Table 2-1. Maricopa County area monitor information and PM10 measurements on 

August 18, 2011. The four Phoenix-area monitors discussed in this report are in bold. 

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

(LST) 

AQS 

Qualifier 

Flag 

Apache County 

N/A N/A WMAT 04-001-1003-81102-1 9 29 0700 

Coconino County 

N/A N/A ADEQ 04-005-1237-81102-1 N/A N/A N/A 

Gila County 

Hayden Old Jail TEOM ADEQ 04-007-1001-81102-3 30 74 1200 

Maricopa County 

Buckeye TEOM MC 04-013-4011-81102-1 296 2946 2000 RJ 

Central Phoenix TEOM MC 04-013-3002-81102-4 232 4373 1800 RJ 

Durango Complex TEOM MC 04-013-9812-81102-1 152 1395 1800 

Dysart TEOM MC 04-013-4010-81102-1 71 723 1900 
Exceptional Event, Phoenix, August 18, 2011 Conceptual Model

8 

Fort 

McDowell/Yuma 

Frank 

TEOM FMIR 04-013-5100-8112-1 N/A N/A N/A 

Glendale TEOM MC 04-013-2001-81102-1 76 902 1900 

Greenwood TEOM MC 04-013-3010-81102-1 107 961 1800 

Higley TEOM MC 04-013-4006-81102-1 121 832 1900 

JLG Supersite BAM ADEQ 04-013-9997-81102-3 80 985 1800 

JLG Supersite TEOM ADEQ 04-013-9997-81102-4 87 1155 1800 

North Phoenix TEOM MC 04-013-1004-81102-1 N/A N/A N/A 

South Phoenix TEOM MC 04-013-4003-81102-1 179 2496 1800 RJ 

West Chandler TEOM MC 04-013-4004-81102-1 186 1564 1800 RJ 

West 43rd Avenue TEOM MC 04-013-4009-81102-1 144 1300 1900 

West Phoenix TEOM MC 04-013-0019-81102-1 76 622 1900 

Zuni Hills TEOM MC 04-013-4016-81102-1 100 1292 1800 

Navajo County 

N/A N/A WMAT 04-017-1002-81102-1 12 44 1300 

Pima County 

Ajo TEOM ADEQ 04-019-0001-81102-3 206 1354 2100 RJ 

Geronimo BAM PCDEQ 04-019-1113-81102-1 21 132 1600 

Green Valley BAM PCDEQ 04-019-1030-81102-1 N/A N/A N/A 

Orange Grove FRM PCDEQ 04-019-0011-81102-2 18 N/A N/A 

Rillito TEOM ADEQ 04-019-0020-81102-3 65 758 1500 IJ 

South Tucson FRM PCDEQ 04-019-1001-81102-1 21 N/A N/A 

Pinal County 

Apache Junction 

Fire Station TEOM PCAQCD 04-021-3002-81102-3 N/A N/A N/A 

Casa Grande 

Downtown TEOM PCAQCD 04-021-0001-81102-3 200 2883 1700 RJ 

Combs School TEOM PCAQCD 04-021-3009-81102-3 195 3323 1800 RJ 

Cowtown TEOM PCAQCD 04-021-3013-81102-3 731 6493 2000 RJ 

Maricopa TEOM PCAQCD 04-021-3010-81102-3 199 1939 2000 RJ 

Pinal County 

Housing TEOM PCAQCD 04-021-3011-81102-3 454 5600 1700 RJ 

Stanfield TEOM PCAQCD 04-021-3008-81102-3 224 2655 1700 RJ 

Santa Cruz County 

Nogales Post Office BAM ADEQ 04-023-0004-81102-3 45 97 400 

Yuma County 

Yuma Supersite TEOM ADEQ 04-027-8011-81102-3 42 262 1700 

TEOM: Tapered element oscillating microbalance monitor 

BAM: Beta attenuation monitor 

FRM: Federal reference method 

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

ADEQ: Arizona Department of Environmental Quality 

MC: Maricopa County Air Quality Department 

FMIR: Fort McDowell Indian Reservation 

PCAQCD: Pinal County Air Quality Control District 
Exceptional Event, Phoenix, August 18, 2011 Conceptual Model 

9 

PCDEQ: Pima County Department of Environmental Quality 

RJ: qualifier flag for high winds 

IJ: qualifier flag for high winds 

Exceptional Event, Phoenix, August 18, 2011 Causal Relationship

3. Causal Relationship 

3.1 Discussion 

Meteorological and air quality observations indicate that dust carried by thunderstorm 

outflow was directly responsible for the high PM10 concentrations observed in the Phoenix area 

on August 18, 2011. On the afternoon of August 18, thunderstorms developed over the higher 

terrain of central and eastern Arizona, just east of the Phoenix area (Figure 3-1). These 

thunderstorms generated dust-carrying outflow boundaries that propagated westward across 

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

monsoon season can generate strong winds that producing blowing dust across Arizona. The 

likely source regions for PM10 during the August 18, 2011, event were the deserts east and 

southeast of the Phoenix area and along the path of the outflow boundaries from those deserts 

westward into Maricopa County. This region largely consists of natural, undisturbed desert. 

The last rainfall in the Phoenix area preceding the August 18, 2011, event was minimal, with 

0.17 inches recorded at Phoenix-Sky Harbor International Airport (KPHX) on August 3, 2011. 

The combination of geography and lack of rainfall preceding the event resulted in a large source 

region of soils that were particularly vulnerable to particulate suspension. 

Figure 3-1. Visible satellite image from 1630 LST on August 18, 2011 (GOES-West). 

Bright white areas indicate tall, convective (thunderstorm) clouds. Thunderstorms over 

central and eastern Arizona generated outflow boundaries that carried dust westward 

across the Phoenix area. 

1 
Exceptional Event, Phoenix, August 18, 2011 Causal Relationship

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

boundaries and windblown dust as it moved through southwestern Arizona. Initially, one outflow 

boundary approached the Phoenix area from the southeast while a second outflow boundary 

approached from the east. The southeastern boundary first arrived in the Phoenix area around 

1700 LST on August 18, which carried windblown dust to the West Chandler monitor. 

Sustained winds of 32 mph and a peak gust of 41 mph were reported at the Chandler Municipal 

Airport (KCHD). The outflow boundary continued northwestward and carried windblown dust to 

the Central Phoenix and South Phoenix monitors during the 1800 LST hour. Phoenix Sky 

Harbor International Airport (KPHX) reported sustained winds of 28 mph with gusts to 39 mph. 

Figure 3-3 shows radar velocity data that illustrate the locations of the two outflow boundaries 

as they approached Phoenix. The two outflow boundaries merged and continued to propagate 

westward across the Phoenix metropolitan area, eventually carrying windblown dust to the 

Buckeye monitor at 2000 LST. Other PM10 monitors in the Phoenix area also recorded elevated 

PM10 concentrations as the outflow boundary passed, but the concentrations were not high 

enough to result in exceedances of the NAAQS. 

Figure 3-2. Phoenix-area air quality and meteorological monitors, and general wind 

direction. Wind and PM10 data for the monitors are shown in Table 3-1, along with the 

names of the stations. Gusty winds associated with thunderstorm outflow transported 

dust across the Phoenix area during the late afternoon hours of August 18, 2011. 

2 
Exceptional Event, Phoenix, August 18, 2011 Causal Relationship

Table 3-1. Observed wind speeds and wind gusts at Maricopa County air quality and 

meteorological monitors on August 18, 2011. All air quality monitors reported high PM10

concentrations from 1800 to 2000 LST on August 18, 2011, coincident with high wind speeds 

and wind gusts. 

Monitor 

1-hr Max 

PM10

(μg/m3) 

Time of 

Observation 

(LST) 

Wind 

Speed 

(mph) 

Wind Gust 

(mph) 

Wind 

Direction 

(degrees) 

KCHD - 1727 32 41 180 

West 

Chandler 1564 1800 18 38 132 

South 

Phoenix 2496 1800 10 35 158 

Central 

Phoenix 4373 1800 14 39 153 

KPHX - 1835 28 39 140 

KGYR - 1950 26 31 120 

Buckeye 2946 2000 16 34 106 

3 
Exceptional Event, Phoenix, August 18, 2011 Causal Relationship

Radar site

A

B

Figure 3-3. Radar base velocity data at 1806 LST on August 18, 2011, from NWS Phoenix 

Doppler radar. Green indicates flow toward the radar; dark orange and red indicate flow away 

from the radar, and yellow indicates little motion away from the radar. Two well-defined outflow 

boundaries (dashed lines) approached the Phoenix area from the southeast (A) and east (B). 

These boundaries carried dust northward and westward, respectively, across the Phoenix 

metropolitan area. 

Monitors in the Phoenix area measured sustained winds of over 20 mph and wind gusts 

in excess of 30 mph, coincident with the sharp increase in PM10 concentrations (Figure 3-4, 

Figure 3-5, and Appendix A). Visibility at KPHX also decreased significantly with the arrival of 

the dust (Figure 3-6), prompting the NWS office in Phoenix, Arizona, to issue a Dust Storm 

Warning for Maricopa County (see Appendix D for the statements issued by NWS). Similar 

reductions in visibility were reported at KCHD (near the West Chandler PM10 monitor) and at 

KGYR (just east of the Buckeye PM10 monitor). It is also noteworthy that, before the abrupt 

increase in PM10 concentrations at each monitor, winds were comparatively much lighter and 

PM10 concentrations were much lower, illustrating the correlation between the high winds and 

the dust. Visibility cameras in the Phoenix area also clearly depict the rapid arrival of dust and 

the resulting significant reduction in visibility after 1700 LST on August 18, 2011. Links to these 

videos, other media coverage, and images pertaining to this windblown dust event are shown in 

Appendix B. 

4 
Exceptional Event, Phoenix, August 18, 2011 Causal Relationship

Figure 3-4. Hourly PM10 concentrations and wind speeds at the West Chandler monitor 

on August 18 and 19, 2011. PM10 concentrations and wind speeds sharply increased at 

1600 LST on August 18, 2011, indicating the arrival of windblown dust. Sustained winds 

of 20 mph were measured with gusts over 35 mph. 

Figure 3-5. Hourly PM10 concentrations and wind speeds at the Buckeye monitor on 

August 18 and 19, 2011. PM10 concentrations sharply increased with wind gusts over 35 

mph at 2000 LST on August 18, 2011, indicating the arrival of windblown dust. 

5 
Exceptional Event, Phoenix, August 18, 2011 Causal Relationship 

Figure 3-6. Hourly PM10 concentrations at the Phoenix-area PM10 monitors and visibility 

at KPHX. Visibility was significantly reduced at KPHX between 1800 and 1900 LST, 

coincident with the sharp increase in PM10 concentrations at area PM10 monitors, 

indicating the arrival of windblown dust. 

3.2 Summary 

The information presented in this section demonstrates a clear causal relationship 

between the windblown dust and the PM10 exceedances measured at four Phoenix-area 

monitors on August 18, 2011. The radar and wind data shown in this section illustrate the 

spatial and temporal extent of the dust storm as it moved through the Phoenix area. In addition, 

the time-series plots of air quality and meteorological data found in this section and in Appendix 

A show that the sharp increase in PM10 concentrations coincided with the strong wind speeds 

and wind gusts, and that the strong winds were experienced over a large area. 

6 
Exceptional Event, Phoenix, August 18, 2011 Historical Norm 

4. Historical Norm 

4.1 Analysis 

PM10 concentrations measured at the Central Phoenix monitor on August 18, 2011, 

were unusual and in excess of normal historical fluctuations. To establish the severity of this 

event, PM10 concentrations measured on August 18, 2011, were compared to a historical 2007–

2011 five-year annual data set at each monitor (see Figure 4-1 and Appendix C). The PM10

concentrations measured at the Central Phoenix monitor (Figure 4-1) on August 18, 2011, were 

among the highest 24-hr averages measured over the five-year period. Similar time-series plots 

for the West Chandler, South Phoenix, and Buckeye monitors are shown in Appendix C. 

4.2 Summary 

Given the recorded values and using methods for assessing historical air quality similar 

to those accepted by the EPA, it is clear that the PM10 levels on August 18, 2011, were outside 

of normal historical fluctuations. This analysis provides evidence that the event affected air 

quality on an historic scale. 

0

50

100

150

200

250

300

350

400

450

500

PM10 Concentration (μg/m3)

Date

Central Phoenix 5-Year Historical Fluctuation - 24 Hour Averages

August 18, 2011

July - Sept, 2011

Figure 4-1. Twenty-four-hour average PM10 concentrations at the Central Phoenix 

monitor (2007-2011). The 24-hr average PM10 concentration on August 18, 2011, is 

highlighted in red. 
Exceptional Event, Phoenix, August 18, 2011 Not Reasonably Controllable or Preventable

1 

5. Not Reasonably Controllable or Preventable 

5.1 Background 

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. These programs include 

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

2. Maricopa County’s Inspection and Compliance Program 

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 Maricopa County Dust Control Forecast of “High Risk”, (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 of greatest risk for elevated PM 

emissions. 

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 adopted into the Arizona 

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

§49-4571

. 

5.1.1 Control Measures 

Maricopa County regulations of PM10 emissions are listed in Table 5-1. 

 

1

 Updates to the AgBMP program in December, 2011, clarified BMPs for crops 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. 
Exceptional Event, Phoenix, August 18, 2011 Not Reasonably Controllable or Preventable

2 

Table 5-1. Rules and ordinances regulating PM 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 dustgenerating 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 nontraditional 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 which are not regulated by Rule 310 and 

which 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. 

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 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 wood 

burning restriction Establishes restrictions for residential wood burning. 

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: Off-road 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. 
Exceptional Event, Phoenix, August 18, 2011 Not Reasonably Controllable or Preventable

3 

5.1.2 Additional Measures 

In addition to the rules and regulations listed in Table 5-1, other PM10-reducing control 

measures (e.g., paving 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 state implementation plan (SIP) 

through PM10 plans such as the Revised Maricopa Association of Governments’ (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 5-2). 

Additionally, the PCAQCD implements specific nonattainment rules for that part of the Phoenix 

PM10 nonattainment area that resides in Pinal County (see Table 5-3). 

Table 5-2. Pinal County rules regulating existing and new non-point sources. 

Article Number & Title Description 

Article 2: Fugitive dust Provides a mechanism to reasonably regulate operations which 

periodically may cause fugitive dust emissions into the atmosphere. 

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 5-3. Pinal County rules regulating fugitive dust in Pinal County portion of the 

Phoenix PM10 Nonattainment Area. 

Article Number & Title Description 

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. 

Article 5: Nonattainment area 

rules; stabilization for residential 

parking and drives 

Establishes rules for stabilizing residential properties. 

Article 6: Restrictions on vehicle 

parking and use on vacant lots Establishes rules for unpaved or unstable vacant lots. 

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. 

Article 8: Nonattainment area 

rules, requirement for stabilization 

of disturbed areas at vacant lots 

Establishes rules for stabilizing disturbed areas at vacant lots. 

5.1.3 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
Exceptional Event, Phoenix, August 18, 2011 Not Reasonably Controllable or Preventable

4 

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 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, 95% compliance (from 85%) for Rule 310.01 sources, and 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 sources, and 73% for Rule 316 

sources. The timeline below (Figure 5-1) illustrates the improvements in RE over the last 

several years; it also points out significant revisions to previous rules, as well as newly adopted 

rules and ordinances. 

Figure 5-1. Timeline of Maricopa County fugitive dust rules and ordinances. 

5.1.4 Compliance and Enforcement Activities 

MCAQD is prepared to proactively respond to high-wind events and protect human 

health and well-being. MCAQD’s approach consists of two primary components: routine 

proactive inspections and surveillance inspections, conducted both during and after significant 

air quality events. MCAQD routinely inspects dust control-permitted sites and increases the 

frequency of inspections for permits covering areas of 10 acres or more. Rule 316 sources are 
Exceptional Event, Phoenix, August 18, 2011 Not Reasonably Controllable or Preventable

5 

also regularly inspected multiple times every year. Maricopa County 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 

increased surveillance before, during, and after the forecast event(s). MCAQD also conducts 

event surveillance and post-event activities during 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. Post-event 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. 

During 2011, a total of 14 MCAQD air monitoring sites were upgraded with new 

equipment that allows the monitoring sites to automatically report measured readings at 5-min 

intervals. Previously, only hourly readings were available. The real-time data reporting system 

includes a mechanism to alert MCAQD field staff when PM concentrations are elevated. The 

system allows MCAQD responders to review concentrations at the monitors and to consult the 

NWS website to check for weather event activity. This capability allows the MCAQD 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 concentraitons. 

5.1.5 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 Phoenix area around the time of the August 18, 2011, PM10 exceedances. An evaluation of 

all inspection reports, air quality complaints, compliance reports, and other documentation do 

not indicate any evidence of unusual anthropogenic-based PM10 emissions. During the time 

period of August 15–21, 2011, MCAQD inspectors conducted a total of 236 inspections of 

permitted facilities, 177 of which were sources of fugitive dust. Additionally, MCAQD conducted 

35 inspections on vacant lots and unpaved parking lots during this period. During this 7-day 

period, a total of 19 violations were issued countywide for PM10 and non-PM10-related 

violations. No violations were issued for PM10 emissions within a 4-mile radius of any 

exceeding monitor. 

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

ADEQ’s AgBMP inspector also monitors the ADEQ Five-Day Dust Control Forecast and the 

MCAQD air monitoring sites that include real-time data. The ADEQ AgBMP 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 high-wind 

events. Additional outreach is conducted with facility representatives prior to forecast high-wind 
Exceptional Event, Phoenix, August 18, 2011 Not Reasonably Controllable or Preventable

6 

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

the ADEQ AgBMP inspector is dispatched to contact the owners and operators of agricultural 

fields in the area to discern if their activities are causing negative impacts. The AgBMP 

inspector is prepared to respond to most agriculture complaints within 24 hours. 

Based on a review of the inspection reports and site visit documentations, there is no 

evidence to suggest that agricultural activities produced unusual PM10 emissions. On August 

17, 2011, the ADEQ AgBMP inspector performed one site visit in the Gilbert area. 

5.2 Forecasts and Warnings 

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

NWS office in Phoenix (Appendix D). For August 18, 2011, a Maricopa County Dust Control 

Forecast was issued by ADEQ, indicating a moderate risk level for unhealthy PM10. The Dust 

Control Forecast also indicated a potential for strong and gusty winds throughout the area as 

well as dense blowing and transported dust generated by outflow from thunderstorms. 

At 1644 LST on August 18, 2011, the NWS in Phoenix issued a Dust Storm Warning for 

Maricopa County, indicating that “winds have generated large areas of blowing dust or blowing 

sand that have substantially reduced visibilities... to 1⁄4 mile or less.” Similarly, at 1654 LST, the 

NWS issued a Severe Thunderstorm Warning for Maricopa County, indicating damaging winds 

in excess of 60 mph were possible and that “dense blowing dust may accompany” the 

thunderstorms.” At 1744 LST, a statement from the NWS indicated that “the leading edge of 

gusty south to southeast winds 25 to 40 mph had reached the southeast Valley... the gust front 

and areas of dense blowing dust will push toward the north and west... and affect much of the 

central and southeast portion of the Greater Phoenix Metropolitan Area by 6 p.m.” The 

issuance of these statements by the NWS coincided with the peak PM10 concentration, of 1,564 

μg/m3

, recorded at the West Chandler monitor; of the four monitors with PM10 exceedances, this 

was the first monitor to be impacted by the windblown dust. The NWS issued several similar 

statements through 2000 LST as the windblown dust continued westward across Maricopa 

County. 

5.3 Wind Observations 

Sustained wind speeds of over 20 mph with gusts over 30 mph were reported at 

monitors across the Phoenix area as the outflow boundaries moved through (Figure 3-4 and 

Appendix A). 

5.4 Summary 

The thunderstorm outflow event of August 18, 2011 produced strong winds 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 and southern Maricopa County. The 

Phoenix area is designated as a serious nonattainment area for PM10 and is required to have 
Exceptional Event, Phoenix, August 18, 2011 Not Reasonably Controllable or Preventable 

7 

BACM for all significant sources of PM10. BACM 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, 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. Widespread thunderstorm outflows with sustained winds in 

excess of 20 mph with gusts over 30 mph were enough to overwhelm all available efforts to limit 

PM10 concentrations during the event. The fact that these were natural events involving strong 

thunderstorm outflow winds that transported PM10 emissions into and across the Phoenix area, 

with a majority of the PM10 emissions recorded by Phoenix area monitors coming from sources 

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

exceedances of August 18, 2011 recorded within the Phoenix PM10 nonattainment area were 

not reasonably controllable or preventable. 

Exceptional Event, Phoenix, August 18, 2011 But-For Analysis

1 

6. But-For Analysis 

6.1 Discussion 

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, in regard to 

the PM10 exceedances in the Phoenix PM10 nonattainment area on August 18, 2011, 

 the exceedances were not reasonably controllable or preventable, and 

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

outflow originating in desert areas outside the Phoenix metropolitan area and the 

measured PM10 exceedances. 

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 exceedances 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 monitors in the Phoenix area. Multiple 

independent measurements of wind speed, wind direction, and visibility all show that 

thunderstorm outflow was the mechanism for transport of PM10 into the Phoenix PM10

nonattainment area. 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 east and southeast of 

Phoenix. The weight of evidence presented in this submittal provides no alternative that could 

tie the exceedances of August 18, 2011, to any causal source except PM10 transported by 

thunderstorm outflow, confirming that there would have been no exceedances but for the 

presence of this uncontrollable natural event. 

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 or 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. 

6.2 Summary 

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

the exceedances of August 18, 2011, to any causal source except PM10 transported by 

thunderstorm outflow, confirming that there would have been no exceedances but for the 

presence of this uncontrollable natural event. 

Exceptional Event, Phoenix, August 18, 2011 Conclusions

1

7. Conclusions 

The PM10 exceedances that occurred on August 18, 2011, satisfy the criteria of the 

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

must be provided for the following elements: 

 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; 

 There is a clear causal relationship between the measurement(s) under consideration 

and the event; 

 The event is associated with a measured concentration(s) in excess of normal historical 

fluctuations; and 

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

7.1 Affects Air Quality 

As stated in the preamble to the EER, 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 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, high winds overwhelmed all reasonably available controls (Section 5). The PM10

exceedances discussed in this report were caused by naturally occurring thunderstorm outflow 

that transported dust into the Phoenix area from areas largely outside the Phoenix PM10

nonattainment area. These facts provide strong evidence that the PM10 exceedances on 

August 18, 2011, were not reasonably controllable or preventable. 

7.3 Natural Event 

As discussed above, the PM10 exceedances on August 18, 2011, were shown to be 

caused by PM10 transported into the Phoenix area by thunderstorm outflow. The event 

therefore qualifies as a natural event. 
Exceptional Event, Phoenix, August 18, 2011 Conclusions 

2

7.4 Clear Causal Relationship 

The following points demonstrate that the high PM10 concentrations were caused by 

windblown dust: 

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

Phoenix-area monitors was consistent with gusty winds and low visibilities (Section 3). 

 High PM10 concentrations and gusty winds were reported throughout the Phoenix 

metropolitan area, 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 (Section 3). 

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

particulate suspension by high winds (Section 3). 

 Visibility camera imageryshows that large quantities of PM10 were transported into the 

Phoenix area on August 18, 2011. The timing of the dust storm depicted in the visibility 

camera imagery is consistent with the PM10 concentration measurements, elevated 

winds, and reduced visibility. 

7.5 Historical Norm 

The 24-hr average PM10 values measured at the exceedance monitors were in excess 

of normal historical fluctuations, based on a comparison with a multi-year data set (Section 4). 

7.6 But For 

On the basis of the weight of evidence described above and in Section 6, the 

exceedances of the federal 24-hr PM10 standard on August 18, 2011, in the Phoenix PM10

nonattainment area would not have occurred but for the thunderstorm-driven high winds and 

transport of dust from areas largely outside of the Phoenix PM10 nonattainment area. 
Exceptional Event, Phoenix, August 18, 2011 Appendix A 

Appendix A: 

Air Quality and Meteorological Data 

This section contains time-series graphs of PM10 concentrations and wind data for 

Phoenix-area monitors on August 18 and 19, 2011. The data show a region-wide increase in 

wind speeds and wind gusts coincident with the arrival of dust and high PM10. 

Figure A-1. Hourly PM10 concentrations and wind speeds at the Central Phoenix 

monitor on August 18 and 19, 2011. PM10 concentrations and wind gusts sharply 

increased at 1800 LST on August 18, 2011, indicating the arrival of windblown dust. 

1
Exceptional Event, Phoenix, August 18, 2011 Appendix A 

Figure A-2. Hourly PM10 concentrations and wind speeds at the South Phoenix monitor 

on August 18 and 19, 2011. PM10 concentrations and wind gusts sharply increased at 

1800 LST on August 18, 2011, indicating the arrival of windblown dust. 

2
3 

U.S. Department of Commerce National Oceanic & Atmospheric Administration

QUALITY-CONTROLLED LOCAL CLIMATOLOGICAL DATA 

HOURLY OBSERVATIONS TABLE 

PHOENIX SKY HARBOR INTL AIRPORT (23183) 

PHOENIX, AZ (08/2011) 

Nation

Asheville

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

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-hrChg

(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

0051 0151 0251 0351 0451 0551 0651 0751 0851 0951 1051 1151 1251 1351 1451 1551 1651 1751 1800 1804 1812 1835 1840 1848 1849 1851 1918 1947 1951 2033 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 11 11 11

SCT200 BKN250 FEW150 SCT250 FEW150 SCT250 FEW150 SCT250 FEW150 SCT250 SCT150 BKN250 FEW150 SCT250 FEW150 SCT250 FEW150 SCT250 FEW100 SCT180 SCT250 FEW110 SCT180 SCT250 FEW120 SCT180 SCT250 FEW120 SCT250 FEW120 SCT250 SCT120 SCT190 BKN250 SCT120 SCT190 BKN250 FEW090 SCT120 BKN200 SCT090 SCT120 BKN200 SCT090 SCT120 BKN200 BKN006 OVC110 BKN006 BKN110 BKN250 BKN010 BKN110 BKN250 BKN010 BKN110 BKN250 BKN020 BKN110 BKN250 BKN020 BKN110 BKN250 BKN020 BKN110 BKN250 SCT110 SCT150 BKN200 SCT110CB SCT150 SCT110CB SCT150 SCT120 BKN150 BKN200 SCT120 BKN150 BKN200 FEW120 SCT150 BKN200 FEW120 SCT150 BKN250 FEW120 SCT150 BKN250

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 1.25 0.50 0.50 1.00 2.00 2.00 4.00 4.00 8.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

DS DS BLDU BLDU BLDU BLDU BLDU 

9998999796939396100102104106108109111111109107102999993939191929084858284868484

37.236.737.236.135.633.933.935.637.838.940.041.142.242.843.943.942.841.739.037.037.034.034.033.033.033.332.029.029.428.028.930.028.928.9

71717171717272737271717172727373727272747474747474747472737373727171

21.421.521.621.821.722.022.022.522.321.921.721.822.322.222.522.522.422.322.323.523.523.523.523.223.223.423.122.122.622.522.522.121.821.5

55565658586161615855535253525252535457636366666666666666676867656564

12.813.313.314.414.416.116.116.114.412.811.711.111.711.111.111.111.712.214.017.017.019.019.019.019.018.919.019.019.420.019.418.318.317.8

23 25 24 27 28 34 34 31 25 21 18 17 16 15 14 14 16 17 23 31 31 41 41 44 44 42 45 55 55 63 57 50 53 51

6 3 0 0 0 3 5 5 7 3 0 7 3 9 10 14 9 8 22 24 23 28 21 25 23 22 25 24 18 24 22 9 9 9

250 260 000 000 000 100 080 110 010 VR 000 VR VR 310 270 250 260 270 150 160 170 140 140 140 140 130 120 120 120 100 110 150 150 150

21 24 18 33 34 32 39 38 38 38 38 36 34 34 31 32 

28.64 28.64 28.64 28.64 28.65 28.66 28.68 28.69 28.69 28.69 28.69 28.66 28.63 28.60 28.57 28.54 28.54 28.57 28.58 28.58 28.59 28.62 28.62 28.63 28.63 28.64 28.66 28.70 28.71 28.72 28.72 28.73 28.73 28.71

0 3 1 8 8 5 3 0

000003016003028019050006

Figure A-3. Quality-controlled local climatological data hourly observations table for Phoenix Sky Harbor International Airport, 

(08/18/2011). Note in the Weather Type column that a DS (dust storm) and BLDU (blowing dust) were reported. Dynamically g

http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
4 

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

HOURLY OBSERVATIONS TABLE 

PHOENIX GOODYEAR AIRPORT (03186) 

GOODYEAR, AZ 

(08/2011) 

Elevation: 968 ft. above sea level Latitude: 33.416 Longitude: -112.383 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)

P

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

0547 0647 0747 0847 0947 1047 1147 1247 1347 1447 1547 1650 1747 1847 1950 2030 2050

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

FEW100 SCT180 BKN250 FEW100 SCT180 BKN250 FEW100 SCT180 BKN250 FEW120 FEW180 SCT250 FEW180 SCT250 FEW180 SCT250 FEW180 SCT250 FEW120 FEW180 SCT250 FEW120 FEW180 SCT250 FEW120 SCT250 FEW120 BKN250 FEW120 FEW180 BKN250 FEW090 SCT120 SCT200 FEW090 BKN120 OVC200 FEW090 BKN120 OVC200 SCT120 BKN200 SCT120 BKN200

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 5.00 1.00 5.00 5.00

BLDU BLDU BLDU BLDU

90909397100102106108109111111109102100908684

32.032.034.036.038.039.041.042.043.044.044.043.039.038.032.030.029.0

7272727474727173737575757675757473

22.222.222.123.223.122.421.822.522.923.623.623.824.223.723.723.122.8

6363616361575254555757596463686868

17.017.016.017.016.014.011.012.013.014.014.015.018.017.020.020.020.0

41 41 34 33 28 23 17 17 17 17 17 20 29 30 48 55 59

8 6 8 7 8 6 3 9 9 11 14 14 11 16 26 23 21

240240240250250230VR 250250280270260210210120120120

20 21 29 34 34 29

28.80 28.82 28.84 28.84 28.85 28.84 28.81 28.78 28.75 28.72 28.68 28.68 28.70 28.71 28.81 28.84 28.86

M M M M M M M M M M M M M M M M M

Figure A-4. Quality-controlled local climatological data hourly observations table for Phoenix Goodyear Airport, Goodyear, AZ 

Note in the Weather Type column that BLDU (blowing dust) was reported for several hours. Dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
5 

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

HOURLY OBSERVATIONS TABLE 

CHANDLER MUNICIPAL AIRPORT (53128) 

CHANDLER, AZ 

(08/2011) 

Elevation: 1243 ft. above sea level Latitude: 33.268 Longitude: -111.812 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)

L

Pr

(i

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

0552 0648 0749 0847 0947 1047 1147 1248 1348 1447 1547 1647 1727 1750 1810 1830 1848 1947 2050

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

SCT150 BKN200 SCT150 BKN200 FEW150 BKN200 FEW150 SCT150 FEW110 SCT250 FEW110 SCT250 SCT110 SCT200 SCT100 BKN200 SCT100 BKN250 SCT100 BKN250 SCT120 BKN250 BKN050 BKN050 BKN050 BKN050 BKN050 BKN100 BKN050 BKN150 BKN050 BKN150

40.00 40.00 40.00 40.00 40.00 35.00 35.00 30.00 20.00 30.00 20.00 20.00 0.50s 1.00 3.00 2.00 8.00 10.00 10.00

BLDU DS BLDU BLDU 

86909397100102106108109109111108100939084828181

30.032.034.036.038.039.041.042.043.043.044.042.038.034.032.029.028.027.027.0

71717273747373737373737375737273717271

21.521.622.022.623.122.822.922.522.622.622.922.523.622.622.422.821.822.321.6

63616161615957545454545463636468666866

17.016.016.016.016.015.014.012.012.012.012.012.017.017.018.020.019.020.019.0

46 38 34 30 28 24 20 17 16 16 15 17 30 37 42 59 58 65 60

3 5 7 6 7 3 7 9 9 6 13 11 32 17 23 26 30 25 29

100 VR 160 150 180 150 240 210 230 290 280 290 180 170 150 100 100 090 060

23 41 26 29 39 39 34

28.53 28.55 28.56 28.57 28.57 28.56 28.53 28.50 28.47 28.44 28.40 28.42 28.46 28.48 28.50 28.52 28.54 28.56 28.60

M M M M M M M M M M M M M M M M M M M

Figure A-5. Quality-controlled local climatological data hourly observations table for Chandler Municipal Airport, Chandler, AZ Note in the Weather Type column that a DS (dust storm) and BLDU (blowing dust) were reported. Dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

Appendix B: Media Coverage, Videos, and Images 

Video Links 

ADEQ visibility cameras in the Phoenix area: 

South Mountain: www.phoenixvis.net/videos/mpeg4/SOMT_08182011.mp4

Estrella Mountains: www.phoenixvis.net/videos/mpeg4/ESMO_08182011.mp4

Camelback Mountains: www.phoenixvis.net/videos/mpeg4/CAME_08182011.mp4

Superstition Mountains: www.phoenixvis.net/videos/mpeg4/SUPM_08182011.mp4

Local citizens often create videos during dust storms, documenting their observations. Here are 

a few links to these videos. 

http://www.youtube.com/watch?v=HJP4daOMUdM

http://www.youtube.com/watch?v=xZeUucphi-I

http://www.youtube.com/watch?v=MlyKfzaBH4g
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

Media Article and Image Links 

http://articles.latimes.com/2011/aug/18/nation/la-naw-phoenix-storm-20110819

Dust storm in Phoenix makes driving — and breathing — risky: Flights are delayed, poles 

are toppled and thousands lose power. It was the third major dust storm to hit the city 

since July. 

August 18, 2011 

Los Angeles Times Staff Writer 

PHOENIX — A giant wall of dust rolled through the Phoenix area Thursday for the third time 

since early July -- turning the sky brown, creating dangerous driving conditions and delaying 

some airline flights. 

The dust storm, also known as a haboob in Arabic and around Arizona, swept through Pinal 

County and headed northeast, reaching Phoenix at about 6 p.m. 

Some incoming and departing flights at Phoenix Sky Harbor International Airport were 

temporarily delayed because of the storm, according to airport officials who couldn't immediately 

provide exact numbers. Takeoffs and landings resumed at about 6:50 p.m. 

National Weather Service meteorologists said a powerful thunderstorm packing winds of up to 

60 mph hit Pinal County and pushed the dust storm toward Arizona's most populous county. 

There were several reports of downed poles, and Salt River Project officials said 3,500 of its 

customers were without electricity, mostly in the Queen Creek area southeast of Phoenix. 

There were no immediate reports of any weather-related auto accidents. 

It was the third major dust storm to hit the Phoenix metro area since last month. A haboob on 

July 5 brought a mile-high wall of dust that halted airline flights, knocked out power for 10,000 

people and covered everything in its path with a thick sheet of dust. Another dust storm hit July 

18, reaching heights of 3,000 to 4,000 feet, delaying flights and cutting off power for more than 

2,000 people in the Phoenix metro area. 

Weather officials say haboobs only happen in Arizona, the Sahara desert and parts of the 

Middle East because of dry conditions and large amounts of sand. 

Pollution levels skyrocket during dust storms and create even more breathing problems for 

people with asthma and other similar conditions. 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

http://www.cbsnews.com/2100-201_162-20094755.html

Another giant dust storm coats Phoenix in dirt 

August 19, 2011, 3:08 PM 

AP 

PHOENIX - A thick cloud of dust has swallowed Phoenix at sundown three times this summer, 

covering the city with grit and baffling even longtime residents who can't remember seeing so 

many dramatic "haboobs" during a monsoon season. 

A 1,000-feet-high wall of dust traveled at least 50 miles into metro Phoenix and neighboring 

Pinal County on Thursday evening before dissipating. 

It turned the skies brown, reduced visibility, created dangerous driving conditions and caused 

some airline flights to be delayed. The storm, also known by the Arabic word haboob, also 

coated anything left outside in a thin layer of fine dirt and left some people who walked outside 

for a minute or two with grit between their teeth. 

Danny Shepherd, a deejay who was driving to a gig at a coffee house during the storm, has 

lived in metropolitan Phoenix since 1989 and can't recall so many big dust storms in one year. 

"They're typical but I also think there's been a lot more this year -- big ones and small ones and 

the haboob, the grand-daddy of them all," Shepherd said. 

National Weather Service meteorologist Ken Waters said the area is experiencing a typical 

number of dust storms this year, but what sets this season apart from others is the size and 

power of three of the storms. 

"Each year, you are going to get some variety of the dust storms," Waters said. "We don't see a 

single causative factor for why they seem to be stronger this year." 

Waters said thunderstorms moving through southern Arizona supplied winds of up to 60 mph 

that stirred up fine dust in the agricultural fields and sent Thursday's storm to the nation's sixthlargest city. 

"It was a strong dust storm, but nothing on the order of the big one in July," he said. 

This season's most powerful dust storm came on July 5 when a mile-high wall of dust halted 

airline flights, knocked out power for 10,000 people and covered everything in its path with a 

thick sheet of dust. Another dust storm hit July 18 reaching heights of 3,000 to 4,000 feet, 

delaying flights and cutting off power for more than 2,000 people in the Phoenix metro area. 

Weather experts said such massive dust storms only happen in Arizona, Africa's Sahara desert 

and parts of the Middle East because of dry conditions and large amounts of sand. 

At a Honda car dealership in Phoenix, a crew of four men used a portable pressure washer to 

hose down vehicles covered in a thick coat of dust that made the car windows opaque. 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

Erik Fisher, a new car director at the dealership, estimated his crew will have to wash about 350 

cars Friday thanks to the dust storm. "They'll probably be out there half a day," Fisher said. "It's 

a lot of extra man power." 

Pollution levels skyrocket during dust storms and create even more breathing problems for 

people with asthma and other conditions. 

Enemencio Campos, right, and Pedro Lopez, both of Eloy, Ariz., pick up roofing debris Aug. 19, 

2011, from the middle of an Eloy street the day after a violent storm hit the area. Pinal County 

authorities are assessing damage in Eloy after a summer storm rolled through, leaving behind a 

path of destruction. Homes were damaged, roofs were torn off businesses and a junior-high 

school had part of its roof missing. 

Dennis Dickerson, a compliance manager at the Maricopa County Air Quality Department, said 

Thursday's dust storm dramatically raised air pollution levels. Dickerson said even people 

without health issues should stay out of severe dusts storms. 

Dickerson said the normal level of dust in the air is anywhere from 20 to 70 micrograms per 

cubic meter. Thursday's 24-hour average for Phoenix reached nearly 160 micrograms per cubic 

meter. That figure was just slightly above what federal standards deem healthy. 

Dr. Art Mollen, who operates a family medicine practice in Scottsdale, Ariz., said there were 

twice as many patients as usual in his waiting room at the start of work Friday. People came in 

with asthma flare-ups, sinus inflammation, opthalmological migraines and other problems. 

"We have been inordinately busy today because of the dust storm," Mollen said, noting also that 

since early July he has seen more patients with allergy problems, even though Arizona's allergy 

season happens in the spring and fall. 

Mollen also said he expects to see more cases of valley fever, a fungal pneumonia that thrives 

in the hot, arid Southwest in dirt found just a few feet beneath the earth's surface. It can be 

stirred up by construction, wind or other activities. 

Meteorologists also were trying to determine whether microbursts caused storm damage in 

metro Phoenix and neighboring Pinal County. 

Power lines landed on top of several cars and one elementary school bus in Pinal County, just 

south of metro Phoenix. County spokesman Elias Johnson said there were no reported injuries 

and everyone got out safely, including the four students on the bus. 

Some incoming and departing flights at Phoenix Sky Harbor International Airport were delayed 

about 40 minutes because of the storm. Take-offs and landings resumed after the storm 

subsided. 

One utility, the Salt River Project, said at the peak of the storm 3,500 of its customers were 

without electricity, mostly in the Queen Creek, Ariz., area southeast of Phoenix, but power had 

been restored to nearly everyone within about five hours of the storm's arrival. 

Authorities in Pinal County were assessing damage in the city of Eloy, Ariz., where homes were 

damaged, roofs were torn off businesses and a junior high school has part of its roof missing. 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

KTVK-TV reports that elsewhere in Eloy, trees landed on top of homes and winds sheared off 

the roof of a business in the downtown area. 

A dust storm moves to consume Camelback Mountain in Phoenix Aug. 18, 2011. / AP 

Photo/The Arizona Republic 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

http://www.reuters.com/article/2011/08/19/us-weather-phoenix-duststormidUSTRE77I0P620110819

Billowing dust storm engulfs downtown Phoenix 

Fri Aug 19, 2011 9:30am EDT 

PHOENIX 

(Reuters) - A billowing wall of dust engulfed downtown Phoenix on Thursday, cutting visibility to 

a few hundred yards and delaying flights at the international airport, authorities and news 

reports said. 

Driving rains and winds gusting at nearly 60 miles per hour also buffeted San Tan Valley, 

southeast of Phoenix in the early evening, according to the National Weather Service. 

Roaring gusts downed trees and power lines in Pinal County, trapping drivers in their cars and 

preventing commuters from reaching their homes south of the state capital. 

"This storm hit during a very busy time for traffic, bringing down power lines over several miles 

on top of cars with visibility near zero," Pinal County Sheriff's Office spokesman Elias Johnson 

said in a statement. 

Deputies "will keep residents out until we can determine their safety is no longer at risk," he 

added. 

The dust storm prevented planes from taking off or landing at Phoenix Sky Harbor International 

Airport for about 40 minutes, The Arizona Republic newspaper reported. 

A call to the airport seeking confirmation of the delays was not immediately returned on 

Thursday. 

In early July, a huge sand storm -- dubbed a haboob -- swallowed the Phoenix valley, darkening 

the streets, sending residents scrambling for cover and cutting power to thousands of homes. 

Weather experts say haboobs frequently occur during the swelteringly hot summer monsoon 

season in the southwest United States. 

(Reporting by Tim Gaynor; Editing by Cynthia Johnston) 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

Pedestrians wait for a transit bus along Indian School Road during a dust storm in Phoenix, 

Arizona August 18, 2011. Credit: REUTERS/Joshua Lott. 

A dust storm surrounds high rise buildings in Phoenix, Arizona August 18, 2011. Credit: 

REUTERS/Joshua Lott. 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

A pedestrian walks along Indian School Road during a dust storm in Phoenix, Arizona August 

18, 2011. Credit: REUTERS/Joshua Lott. 

A pedestrian crosses Central Avenue during a dust storm in Phoenix, Arizona August 18, 2011. 

Credit: REUTERS/Joshua Lott. 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

A dust storm surrounds high rise buildings in Phoenix, Arizona August 18, 2011. Credit: 

REUTERS/Joshua Lott. 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

http://www.npr.org/blogs/thetwo-way/2011/08/19/139780992/video-third-haboob-of-summercovers-phoenix-in-sand-and-dust (follow the link to see the video) 

VIDEO: Third Haboob Of Summer Covers Phoenix In Sand And Dust

by Mark Memmott 

August 19, 2011 9:15 AM 

The area around Phoenix was hit by its third "haboob" of the summer last evening, as strong 

winds sent a huge cloud of sand and dust rolling over the area. 

The Arizona Republic says that "blowing dust blanketed much of the Valley ... reducing visibility 

to less than a quarter-mile." 

As during previous dust storms, news outlets in Phoenix caught much of the action and have 

posted video. Here's one from the Republic. 

By the way, "haboob" is the word they've been using in Arizona and in meteorological circles for 

many years, the Republic reports. As Merriam-Webster says, it's Arabic in origin and means "a 

violent dust storm or sandstorm especially of Sudan." 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 

http://news.nationalgeographic.com/news/2011/08/pictures/110819-haboob-dust-storm-phoenixarizona-lightning-science/#/haboob-hits-phoenix-dust-storm-lightning_39333_600x450.jpg

Pictures: Huge Dust Storm Swallows Phoenix 

Flash in a Haboob 

Published August 19, 2011 

AP 

A flash of lightning accentuates a thick blanket of dust over Gilbert, Arizona, on Thursday—the 

third major dust storm to hit the Phoenix metropolitan area (map) since early July. 

The billowing cloud of dust was about a half mile (0.8 kilometer) high and 50 to 55 miles (80 to 

88 kilometers) wide. The storm rolled across Arizona's Pinal County last night with winds 

reaching 30 to 40 miles (48 to 64 kilometers) an hour, the Arizona Republic reported. 

Dust storms are relatively common between June and September in parts of the U.S. 

Southwest. Severe dust storms are also called haboobs—Arabic for "violent wind"—since they 

often appear in northern Africa and the Middle East. (See a picture of a haboob in western Iraq.) 

Photos below are credited to Matt York/AP. 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 
Exceptional Event, Phoenix, August 18, 2011 Appendix B 
Exceptional Event, Phoenix, August 18, 2011 Appendix C 

Appendix C: Historical Fluctuation Time-Series Graphs 

0

50

100

150

200

250

300

350

400

450

500

PM10 Concentration (μg/m3)

Date

Buckeye 5-Year Historical Fluctuation - 24 Hour Averages

August 18, 2011

Figure C-1. Twenty-four--hour average PM10 concentrations at the Buckeye monitor 

(2007–2011). The 24-hr average PM10 concentration on August 18, 2011, is shown in 

red and highlighted by the arrow. 
Exceptional Event, Phoenix, August 18, 2011 Appendix C 

0

50

100

150

200

250

300

350

400

450

500

PM10 Concentration (μg/m3)

Date

West Chandler 5-Year Historical Fluctuation - 24 Hour Averages

August 18, 2011

Figure C-2. Twenty-four-hour average PM10 concentrations at the West Chandler 

monitor (2007–2011). The 24-hr average PM10 concentration on August 18, 2011, is 

shown in red and highlighted by the arrow. 

0

50

100

150

200

250

300

350

400

450

500

PM10 Concentration (μg/m3)

Date

South Phoenix 5-Year Historical Fluctuation - 24 Hour Averages

August 18, 2011

Figure C-3. Twenty-four-hour average PM10 concentrations at the South Phoenix 

monitor (2007–2011). The 24-hr average PM10 concentration on August 18, 2011, is 

shown in red and highlighted by the arrow.
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

Appendix D: 

ADEQ and NWS Forecast Products 

MARICOPA COUNTY DUST CONTROL FORECAST

ISSUED WEDNESDAY, AUGUST 17, 2011

Five-day weather outlook:

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

POSSIBLE EVEN FROM DISTANT THUNDERSTORMS

Active summer monsoon weather will continue over or in the vicinity of the Phoenix metro area thru Friday followed by a brief lull as

dry westerly winds aloft occur. Moisture will be on the increase again by Monday. As a result, thunderstorm outflow boundaries 

from even distant thunderstorms will pose a risk for periods of dense blowing dust and unhealthy PM-10 levels thru Friday and 

again on Monday. 

R I S K F A C T O R S

WINDS STAGNATION

UNHEALTHY PM-10

RISK LEVEL

Day 1: Thu 08/18/2011

Southwest to westerly 

10-20 mph during the 

afternoon except strong 

and gusty due to 

outflow from 

thunderstorms. 

No significant 

+ stagnation expected. = MODERATE

Day 2: Fri 08/19/2011

Southwest to westerly 

10-20 mph during the 

afternoon except strong 

and gusty due to 

outflow from 

thunderstorms. 

No significant 

+ stagnation expected. = MODERATE

Day 3: Sat 08/20/2011

Southwest to westerly 

10-20 mph during the 

afternoon. 

No significant 

+ stagnation expected. = LOW

EXTENDED OUTLOOK

Day 4: Sun 08/21/2011

Southwest to westerly 

10-20 mph during the 

afternoon. 

No significant 

+ stagnation expected. = LOW

Day 5: Mon 08/22/2011

Southwest to westerly 

10-20 mph during the 

afternoon except strong 

and gusty due to 

outflow from 

thunderstorms. 

No significant 

+ stagnation expected. = MODERATE
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

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
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

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 THURSDAY, AUGUST 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

FORECAST 

DATE

NOTICES 

(*SEE BELOW 

FOR DETAILS)

AIR POLLUTANT 

YESTERDAY

TUE 08/16/2011

DUST

Highest AQI Reading/Site

(Preliminary data only)

TODAY

WED 08/17/2011

DUST

TOMORROW

THU 08/18/2011

DUST

EXTENDED

FRI 08/19/2011

DUST

O3* 58

QUEEN VALLEY

84

MODERATE

80

MODERATE

77

MODERATE

CO* 05

GREENWOOD

06

GOOD

07

GOOD

06

GOOD

PM-10* 60

SOUTH PHOENIX

75

MODERATE

75

MODERATE

75

MODERATE

PM-2.5* 33

GLENDALE

28

GOOD

34

GOOD

31

GOOD

* 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.
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

“DUST” means that short periods of high PM-10 concentrations caused by outflow from thunderstorms are possible.
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

Health message for Wednesday, August 17: Unusually sensitive people should consider 

reducing prolonged or heavy exertion outdoors.

Health message for Thursday, August 18: Unusually sensitive people should consider 

reducing prolonged or heavy exertion outdoors.

Synopsis and Discussion

OZONE: Considerable cloud cover held local ozone levels down on Tuesday but today is starting out under mostly 

sunny skies. This, along with rather subdued gradient winds, should support highest ozone concentrations reaching the 

upper moderate range of the Air Quality Index. Winds aloft are now being advertised to become westerly thru a deep 

layer by Thursday morning. Since this component aligns with the typical upslope flow this time of year, higher 

afternoon winds are forecast to resume tomorrow and continue thru the weekend. This situation usually means that 

highest ozone levels will occur over the far eastern Valley and at this time are forecast to be in the mid-moderate range 

of the AQI. 

PARTICLES: Another outflow boundary from thunderstorm activity impacted parts of the Valley Tuesday evening 

with wind gusts as strong as 36 mph and reductions in visibilities to as low as two miles in blowing dust. This 

contributed to a max hourly PM-10 concentration of 591.2ug/m3 at the Higley monitoring site at 8:00 p.m. Active 

summer monsoon weather will continue over or in the vicinity of the Phoenix metro area thru Friday followed by a 

brief lull as dry westerly winds aloft occur. Moisture will be on the increase again by Monday. As a result, 

thunderstorm outflow boundaries from even distant thunderstorms will continue to pose a risk for periods of dense 

blowing dust and unhealthy PM-10 levels the next few evenings. 

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 TUESDAY, AUGUST 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) 44 37

Apache Junction (Pinal County) 61 54

Blue Point 59 50

Buckeye 34 29

Casa Grande (Pinal County) 44 37

Cave Creek 49 42

Central Phoenix 44 37

Dysart 38 32

Falcon Field 53 45

Fountain Hills 60 51

Glendale 45 38

Humboldt Mountain 49 42

North Phoenix 51 43

Phoenix Supersite 48 41

Pinal Air Park (Pinal County) 47 40

Pinnacle Peak 55 47

Queen Valley (Pinal County) 62 58

Rio Verde 56 47

South Phoenix 43 36

South Scottsdale 52 44

Tempe 42 36

Tonto Nat’l Mon. (Gila County) 52 44

West Chandler 52 44

West Phoenix 44 37

Yuma (Yuma County) 35 30
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

CO (CARBON MONOXIDE)

SITE NAME MAX 8-HR VALUE (PPM) MAX AQI AQI COLOR CODE

Central Phoenix 0.1 01

Greenwood 0.4 05

Phoenix Supersite NOT AVBL NOT AVBL NOT AVBL

West Phoenix 0.2 02

PM-10 (PARTICLES)

SITE NAME MAX 24-HR VALUE (ug/m3) MAX AQI AQI COLOR CODE

Buckeye 39 36

Central Phoenix 38 35

Combs School(Pinal County 76 61

Durango 51 47

Dysart 32 30

Glendale 54 50

Greenwood 37 34

Higley 61 54

Maricopa (Pinal County) 62 54

Phoenix Supersite 33 31

South Phoenix 73 60

West Chandler 49 45

West Forty Third 66 56

West Phoenix 46 43

Zuni Hills 36 33

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 8.2 27

Dysart 6.4 21

Estrella Mountain Park 4.3 14

Glendale 10.1 33

Phoenix Supersite 6.8 22

South Phoenix 6.6 21

Vehicle Emissions Lab 5.5 18

West Phoenix 8.3 27
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

Statements, Advisories, and Warnings issued by the National Weather Service office in 

Phoenix, AZ, pertaining to the August 18, 2011, dust storm event: 

URGENT - WEATHER MESSAGE 

NATIONAL WEATHER SERVICE PHOENIX AZ 

444 PM MST THU AUG 18 2011 

AZZ023-027-190100- 

/O.EXA.KPSR.DS.W.0013.000000T0000Z-110819T0100Z/ 

/O.CON.KPSR.EH.W.0007.000000T0000Z-110819T0300Z/ 

GREATER PHOENIX AREA-SOUTHWEST MARICOPA COUNTYINCLUDING THE CITIES OF...BUCKEYE...MESA...PHOENIX...GILA BEND 

444 PM MST THU AUG 18 2011 

...DUST STORM WARNING IN EFFECT UNTIL 6 PM MST THIS EVENING... 

...EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 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. AN 

EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM MST THIS 

EVENING. 

* AFFECTED AREA: SOUTHWEST AND CENTRAL MARICOPA COUNTY. 

* LOCATIONS INCLUDE: GILA BEND...BUCKEYE...MESA...PHOENIX 

* VISIBILITY: LESS THAN ONE QUARTER MILE DUE TO BLOWING DUST. 

* IMPACTS: SUDDEN REDUCTION IN VISIBILITY DUE TO BLOWING DUST CAN BE 

 VERY HAZARDOUS...ESPECIALLY TO MOTORISTS. EXCESSIVELY HOT 

 TEMPERATURES CAN BE STRESSFUL TO HUMANS AND ANIMALS. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

A 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 

HAZARDOUS DRIVING CONDITIONS IN SOME AREAS. BE READY FOR A SUDDEN 

DROP IN VISIBILITY TO NEAR ZERO. USE EXTRA CAUTION AND SLOW DOWN 

WHILE DRIVING...AS OBJECTS ON AND NEAR ROADWAYS WILL BE SEEN ONLY 

AT CLOSE RANGE. 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. 

BULLETIN - EAS ACTIVATION REQUESTED 

SEVERE THUNDERSTORM WARNING 

NATIONAL WEATHER SERVICE PHOENIX AZ 

454 PM MST THU AUG 18 2011 

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A 

* SEVERE THUNDERSTORM WARNING FOR... 

 EAST CENTRAL MARICOPA COUNTY IN SOUTH CENTRAL ARIZONA... 

 CENTRAL PINAL COUNTY IN SOUTH CENTRAL ARIZONA... 

* UNTIL 545 PM MST 

* AT 446 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED 

 SEVERE THUNDERSTORMS CAPABLE OF PRODUCING DAMAGING WINDS IN 

 EXCESS OF 60 MPH. IN ADDITION...DENSE BLOWING DUST MAY ACCOMPANY 

 THESE STORMS. THESE STORMS WERE LOCATED 14 MILES EAST OF 

 MAGMA...OR 6 MILES SOUTHEAST OF FLORENCE JUNCTION...AND WERE NEAR 

 STATIONARY. 

* OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO 

 SANTAN AND SACATON 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

THIS IS A DANGEROUS STORM. IF YOU ARE IN ITS PATH...PREPARE 

IMMEDIATELY FOR DAMAGING WINDS AND DEADLY CLOUD TO GROUND LIGHTNING. 
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

MOVE TO SHELTER INSIDE A STRONG BUILDING. STAY AWAY FROM WINDOWS. 

SEVERE WEATHER STATEMENT 

NATIONAL WEATHER SERVICE PHOENIX AZ 

520 PM MST THU AUG 18 2011 

AZC013-021-190045- 

/O.CON.KPSR.SV.W.0032.000000T0000Z-110819T0045Z/ 

PINAL AZ-MARICOPA AZ520 PM MST THU AUG 18 2011 

...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 545 PM MST 

FOR EAST CENTRAL MARICOPA AND CENTRAL PINAL COUNTIES... 

AT 510 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED TWO 

SEVERE THUNDERSTORMS IN THE WARNED AREA. SEVERE THUNDERSTORMS WERE 

LOCATED SOUTHEAST OF SUN LAKES AND SOUTHEAST OF SAN TAN VALLEY. BOTH 

STORMS WERE NEAR STATIONARY AND WERE PRODUCING STRONG OUTFLOW FINDS 

IN EXCESS OF 60 MPH. OUTFLOW BOUNDARIES FROM THESE STORMS ARE ALSO 

GENERATING AREAS OF BLOWING DUST. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

THESE ARE DANGEROUS STORMS. INDIVIDUALS IN THE VICINITY SHOULD TAKE 

COVE INDOORS. 

REPORT SEVERE WEATHER TO THE NEAREST LAW ENFORCEMENT AGENCY. THEY 

WILL RELAY YOUR REPORT TO THE NATIONAL WEATHER SERVICE FORECAST 

OFFICE. 

GREATER PHOENIX AREA-SOUTHWEST MARICOPA COUNTYNORTHWEST AND NORTH CENTRAL PINAL COUNTYINCLUDING THE CITIES OF...BUCKEYE...MESA...PHOENIX...GILA BEND... 

APACHE JUNCTION...CASA GRANDE...FLORENCE 

544 PM MST THU AUG 18 2011 

...EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM MST THIS 

EVENING... 

...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. AN 

EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM MST THIS EVENING. 

* AT 540 PM...THE LEADING EDGE OF GUSTY SOUTH TO SOUTHEAST WINDS 25 

 TO 40 MPH HAD REACHED THE SOUTHEAST VALLEY. A DUST STORM...WITH 

 VISIBILITY LESS THAN ONE QUARTER MILE AT TIMES...ACCOMPANIED THE 

 GUSTY WINDS. THE GUST FRONT AND AREAS OF DENSE BLOWING DUST WILL 

 PUSH TOWARD THE NORTH AND WEST...AND AFFECT MUCH OF THE CENTRAL AND 

 SOUTHEAST PORTION OF THE GREATER PHOENIX METROPOLITAN AREA BY 6 PM. 

* AFFECTED AREA: SOUTHWEST AND CENTRAL MARICOPA COUNTY. 

* LOCATIONS INCLUDE: 

 MESA...CHANDLER...PHOENIX...GILBERT...TEMPE...QUEEN CREEK...SCOTTSDALE 

* VISIBILITY: LESS THAN ONE QUARTER MILE DUE TO BLOWING DUST. 

* IMPACTS: SUDDEN REDUCTION IN VISIBILITY DUE TO BLOWING DUST 

 CAN BE VERY HAZARDOUS...ESPECIALLY TO MOTORISTS. EXCESSIVELY 

 HOT TEMPERATURES CAN BE STRESSFUL TO HUMANS AND ANIMALS. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

A 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 

HAZARDOUS DRIVING CONDITIONS IN SOME AREAS. BE READY FOR A SUDDEN 

DROP IN VISIBILITY TO NEAR ZERO. USE EXTRA CAUTION AND SLOW DOWN 

WHILE DRIVING...AS OBJECTS ON AND NEAR ROADWAYS WILL BE SEEN ONLY 

AT CLOSE RANGE. 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. 
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

AIRPORT WEATHER WARNING FOR SKY HARBOR AIRPORT 

NATIONAL WEATHER SERVICE PHOENIX AZ 

553 PM MST THU AUG 18 2011 

...AIRPORT WEATHER WARNING FOR SKY HARBOR AIRPORT IN EFFECT UNTIL 

745 PM MST... 

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED AN AIRPORT WEATHER 

WARNING FOR SKY HARBOR AIRPORT FOR STRONG GUSTY WINDS. 

THUNDERSTORM WIND GUSTS BETWEEN 36 AND 40 KNOTS FROM THE SOUTHEAST. 

VISIBILITY 1/4 MILE IN BLOWING DUST AT TIMES. 

URGENT - WEATHER MESSAGE...UPDATED 

NATIONAL WEATHER SERVICE PHOENIX AZ 

600 PM MST THU AUG 18 2011 

AZZ027-028-190300- 

/O.EXP.KPSR.DS.W.0013.000000T0000Z-110819T0100Z/ 

/O.CON.KPSR.EH.W.0007.000000T0000Z-110819T0300Z/ 

SOUTHWEST MARICOPA COUNTYNORTHWEST AND NORTH CENTRAL PINAL COUNTYINCLUDING THE CITIES OF...GILA BEND...APACHE JUNCTION... 

CASA GRANDE...FLORENCE 

600 PM MST THU AUG 18 2011 

...EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM MST THIS 

EVENING... 

...DUST STORM WARNING WILL EXPIRE AT 6 PM MST THIS EVENING... 

THE DUST STORM WARNING WILL EXPIRE AT 6 PM MST THIS EVENING. AN 

EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM MST THIS 

EVENING. 

* GUSTY WINDS WITH BLOWING DUST WILL PERSIST ACROSS PORTIONS OF 

 SOUTHWEST MARICOPA COUNTY AS WELL AS NORTHWEST AND NORTH-CENTRAL 

 PINAL COUNTY THROUGH 7 PM. VISIBILITY WILL GENERALLY BE AT AND 

 ABOVE 1 MILE WITH THE DUST WHILE WINDS GRADUALLY DIMINISH. 

* AFFECTED AREA: SOUTHWEST AND CENTRAL MARICOPA COUNTY. 

* LOCATIONS INCLUDE: CASA GRANDE...GILA BEND 

* VISIBILITY: LESS THAN ONE QUARTER MILE DUE TO BLOWING DUST UNTIL 6 

 PM...WITH IMPROVING VISIBILITY THEREAFTER. 

* IMPACTS: SUDDEN REDUCTION IN VISIBILITY DUE TO BLOWING DUST 

 CAN BE VERY HAZARDOUS...ESPECIALLY TO MOTORISTS. EXCESSIVELY 

 HOT TEMPERATURES CAN BE STRESSFUL TO HUMANS AND ANIMALS. 

GREATER PHOENIX AREAINCLUDING THE CITIES OF...BUCKEYE...MESA...PHOENIX 

600 PM MST THU AUG 18 2011 

...DUST STORM WARNING NOW IN EFFECT UNTIL 8 PM MST THIS 

EVENING... 

...EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM MST THIS 

EVENING... 

THE DUST STORM WARNING IS NOW IN EFFECT UNTIL 8 PM MST THIS 

EVENING. AN EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM 

MST THIS EVENING. 

* AT 555PM...THE LEADING EDGE OF GUSTY SOUTH TO SOUTHEAST WINDS 25 TO 

 35 MPH HAD REACHED THE SOUTHEAST VALLEY...AND EXTENDED FROM SKY 

 HARBOR AIRPORT TO DOWNTOWN SCOTTSDALE. A DUST STORM...WITH 

 VISIBILITY LESS THAN ONE QUARTER MILE AT TIMES...ACCOMPANIED THE 

 GUSTY WINDS. THE GUST FRONT AND AREAS OF DENSE BLOWING DUST WILL 

 PUSH TOWARD THE NORTH AND WEST...AND AFFECT MUCH OF THE GREATER 

 PHOENIX METROPOLITAN AREA BETWEEN 6 PM AND 8 PM. WIND SHOULD SLOWLY 

 DECREASE WITH TIME...AND VISIBILITY WILL SLOWLY IMPROVE. 

* AFFECTED AREA: SOUTHWEST AND CENTRAL MARICOPA COUNTY. 
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

* LOCATIONS INCLUDE: MESA...CHANDLER...PHOENIX...GILBERT... 

 TEMPE...QUEEN CREEK...SCOTTSDALE...PARADISE VALLEY 

* VISIBILITY: LESS THAN ONE QUARTER MILE DUE TO BLOWING DUST. 

* IMPACTS: SUDDEN REDUCTION IN VISIBILITY DUE TO BLOWING DUST 

 CAN BE VERY HAZARDOUS...ESPECIALLY TO MOTORISTS. EXCESSIVELY 

 HOT TEMPERATURES CAN BE STRESSFUL TO HUMANS AND ANIMALS. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

A 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 

HAZARDOUS DRIVING CONDITIONS IN SOME AREAS. BE READY FOR A SUDDEN 

DROP IN VISIBILITY TO NEAR ZERO. USE EXTRA CAUTION AND SLOW DOWN 

WHILE DRIVING...AS OBJECTS ON AND NEAR ROADWAYS WILL BE SEEN ONLY 

AT CLOSE RANGE. 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. 

BULLETIN - EAS ACTIVATION REQUESTED 

SEVERE THUNDERSTORM WARNING 

NATIONAL WEATHER SERVICE PHOENIX AZ 

612 PM MST THU AUG 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...YOUNGTOWN...SURPRISE...SUN CITY 

 WEST...SUN CITY...PEORIA...EL MIRAGE... 

* UNTIL 700 PM MST 

* AT 606 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED 

 SEVERE THUNDERSTORMS CAPABLE OF PRODUCING DAMAGING WINDS IN 

 EXCESS OF 60 MPH. THESE STORMS WERE LOCATED NEAR PEORIA...AND 

 MOVING SOUTHEAST AT 5 MPH. 

* THE SEVERE THUNDERSTORM WILL OTHERWISE REMAIN OVER MAINLY RURAL 

 AREAS OF THE INDICATED COUNTY. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

THESE ARE DANGEROUS STORMS. SEEK SHELTER NOW INSIDE A STURDY 

STRUCTURE AND STAY AWAY FROM WINDOWS! 

IF YOU ENCOUNTER DENSE 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. 

SEVERE WEATHER STATEMENT 

NATIONAL WEATHER SERVICE PHOENIX AZ 

640 PM MST THU AUG 18 2011 

AZC013-190200- 

/O.CON.KPSR.SV.W.0033.000000T0000Z-110819T0200Z/ 

MARICOPA AZ640 PM MST THU AUG 18 2011 

...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 700 PM MST 

FOR CENTRAL MARICOPA COUNTY... 

AT 634 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS CONTINUED TO 

DETECT SEVERE THUNDERSTORMS CAPABLE OF PRODUCING DAMAGING WINDS IN 

EXCESS OF 60 MPH. THESE STORMS WERE LOCATED NEAR PEORIA AND WERE 

NEAR STATIONARY. 

THE SEVERE THUNDERSTORM WILL OTHERWISE REMAIN OVER MAINLY RURAL AREAS 

OF THE INDICATED COUNTY. 

THE STORMS HAVE WEAKENED...BURT WILL LIKELY REINTENSIFY AS THEY 

INTERSECT AN OUTFLOW BOUNDARY APPROACHING FROM THE EAST. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

IF YOU ARE IN THE WARNING AREA...GO INSIDE A STURDY BUILDING. 

REMEMBER...A SEVERE THUNDERSTORM WILL PRODUCE DAMAGING WINDS AND 

DEADLY LIGHTNING 

REPORT SEVERE WEATHER TO THE NEAREST LAW ENFORCEMENT AGENCY. THEY 

WILL RELAY YOUR REPORT TO THE NATIONAL WEATHER SERVICE FORECAST 

OFFICE. 

URGENT - WEATHER MESSAGE...UPDATED 

NATIONAL WEATHER SERVICE PHOENIX AZ 

719 PM MST THU AUG 18 2011 

AZZ023-190330- 

/O.CAN.KPSR.EH.W.0007.000000T0000Z-110819T0300Z/ 

/O.CON.KPSR.DS.W.0013.000000T0000Z-110819T0300Z/ 

GREATER PHOENIX AREAINCLUDING THE CITIES OF...MESA...PHOENIX...SCOTTSDALE...PEORIA 

| 

719 PM MST THU AUG 18 2011 

...DUST STORM WARNING REMAINS IN EFFECT UNTIL 8 PM MST THIS 

EVENING... 

...EXCESSIVE HEAT WARNING IS CANCELLED... 

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS CANCELLED THE 

EXCESSIVE HEAT WARNING. A DUST STORM WARNING REMAINS IN EFFECT 

UNTIL 8 PM MST THIS EVENING...MAINLY FOR THE NORTH AND WEST PORTION 

OF THE GREATER PHOENIX AREA. 

* AT 715PM...THE LEADING EDGE OF GUSTY SOUTH TO SOUTHEAST WINDS 25 TO 

 40 MPH HAD REACHED THE NORTH VALLEY. BETWEEN 630 AND 650 PM...DEER 

 VALLEY AIRPORT REPORTED A PEAK GUST OF 44 MPH...WHILE SCOTTSDALE 

 AIRPORT REPORTED A PEAK GUST OF 40 MPH. A DUST STORM..WITH 

 VISIBILITY OCCASIONALLY LESS THAN ONE QUARTER MILE...ACCOMPANIED 

 THE LEADING EDGE OF THE GUSTY WINDS. THE GUST FRONT WILL PUSH WEST 

 ACROSS MUCH OF THE WEST VALLEY...WITH LOCALIZED AREAS OF DENSE 

 BLOWING DUST POSSIBLE. VISIBILITY HAS IMPROVED OVER CENTRAL AND 

 SOUTHEAST PORTIONS OF THE GREATER PHOENIX AREA...THOUGH DUSTY 

 CONDITIONS PERSIST. 

* AFFECTED AREA: CENTRAL AND NORTH-CENTRAL MARICOPA COUNTY. 

* LOCATIONS INCLUDE: MESA...CHANDLER...PHOENIX...GILBERT... 

 TEMPE...QUEEN CREEK...SCOTTSDALE...PARADISE VALLEY 

* VISIBILITY: OCCASIONALLY LESS THAN ONE QUARTER MILE DUE TO BLOWING 

 DUST...WITH WIDESPREAD VISIBILITY LESS THAN 2 MILES. 

* IMPACTS: SUDDEN REDUCTION IN VISIBILITY DUE TO BLOWING DUST CAN BE 

 VERY HAZARDOUS...ESPECIALLY TO MOTORISTS. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

A 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 HAZARDOUS DRIVING 

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

NEAR ZERO. USE EXTRA CAUTION AND SLOW DOWN WHILE DRIVING...AS OBJECTS 

ON AND NEAR ROADWAYS WILL BE SEEN ONLY AT CLOSE RANGE. 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. 

URGENT - WEATHER MESSAGE...UPDATED 

NATIONAL WEATHER SERVICE PHOENIX AZ 

755 PM MST THU AUG 18 2011 

AZZ023-190400- 

/O.EXP.KPSR.DS.W.0013.000000T0000Z-110819T0300Z/ 

GREATER PHOENIX AREA- 
Exceptional Event, Phoenix, August 18, 2011 Appendix D 

INCLUDING THE CITIES OF...MESA...PHOENIX 

755 PM MST THU AUG 18 2011 

...DUST STORM WARNING WILL EXPIRE AT 8 PM MST THIS EVENING... 

THE DUST STORM WARNING WILL EXPIRE AT 8 PM MST THIS EVENING. WINDY 

CONDITIONS WILL PERSIST THROUGH MUCH OF THE EVENING...BUT VISIBILITY 

HAS IMPROVED NOTICEABLY AT MOST LOCATIONS. THEREFORE...THE DUST STORM 

WARNING WILL BE ALLOWED TO EXPIRE AT 8 PM. ISOLATED THUNDERSTORMS 

OVER THE NORTH VALLEY COULD GENERATE LOCALIZED AREAS OF DENSE BLOWING 

DUST THROUGH 9 PM...BUT VISIBILITY RESTRICTION DUE TO BLOWING DUST IS 

NOT EXPECTED TO LOWER TO ONE QUARTER MILE. 

* AFFECTED AREA: CENTRAL AND NORTH-CENTRAL MARICOPA COUNTY 

* LOCATIONS INCLUDE: PHOENIX...PEORIA...GLENDALE...SCOTTSDALE 

* WINDS: SOUTHEAST 20 TO 35 MPH...WITH LOCALLY STRONGER GUSTS. 

* VISIBILITY: OCCASIONALLY BELOW 4 MILES DUE TO BLOWING DUST OVER THE 

 WEST AND NORTH VALLEY THROUGH 9 PM. 

* IMPACTS: THE AIR IS STILL RATHER DUSTY...WHICH COULD POSE A 

 PROBLEM FOR PEOPLE WITH RESPIRATORY PROBLEMS.| 

$$ 

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT 

WEATHER.GOV/PHOENIX 
Exceptional Event, Phoenix, August 18, 2011 Appendix E 

Appendix E: 

Affidavit of Public Notice