On September 12, 2011, the Phoenix region experienced typical late summertime weather conditions with a 

maximum temperature of 100°F, relative humidity of 20-40%, variable to southeasterly winds at 5-12 miles 

per hour (mph) through the afternoon, and visibility at 10 miles all day, as reported at the Phoenix Sky Harbor

Airport. Despite the spikes in PM10 concentrations at most of the PM10 monitoring sites in the region on 

September 11th and a few hours of elevated PM10 concentrations in the very early morning hours, PM10 

concentrations on September 12th were generally low during the day (<50 μg/m3). Between 6 p.m. MST and 8 

p.m. MST on the 12th, thunderstorms developed over portions of Arizona to the south of the Phoenix region (as 

indicated in Figure 5-12) and by 7 p.m., outflows from the thunderstorms entrained dust from desert areas of 

Pinal County and progressed northwestward into Maricopa County by 8 p.m., transporting large amounts of 

PM10 into the south-central portion of the Phoenix nonattainment area. As shown in Figure 5-13, the arrival 

of the dust front had the following impacts in the Phoenix area: south-southeasterly winds increased to 20-25 

mph, with gusts up to 32 mph; visibility dropped from 10 miles to 6 miles; and hourly PM10 concentrations 

increased to nearly 3000 μg/m3 at the Durango Complex monitoring site and exceeded 1000 μg/m3 at three 

additional sites. However, once the winds subsided by the 10 p.m. hour, visibility returned to 10 miles and 

PM10 concentrations rapidly decreased, resulting in exceedances of the 24-hour PM10 standard at only three 

monitoring sites on September 12. As a result, it is very clear that without the existence of the emissions 

generated by the thunderstorm outflows, there would not have been any exceedances of the 24-hour PM10 

standard in the Phoenix PM10 nonattainment area. 

12:00 p.m. 2:00 p.m. 4:00 p.m. 6:00 p.m. 

1900z Sep 12 2100z Sep 12 2300z Sep 12 0100z Sep 13 

 

8:00 p.m. 10:00 p.m. 12:00 a.m. 3:00 a.m. 

0300z Sep 13 0500z Sep 13 0700z Sep 13 1000z Sep 13 

    
     

Satellite imagery showing thunderstorms developing much of Arizona through the afternoon and 

evening and dissipating as they moved northeastward. Strong storms over southern/central Arizona are 

believed to have been responsible for the thunderstorm outflow boundary that caused PM10 exceedances in 

Maricopa and Pinal counties on September 12. 

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Figure 5-13 depicts the dust event that occurred in Maricopa County impacting the Phoenix area on September 

12, 2011. Hourly averaged PM10 concentrations at monitors throughout Maricopa County are shown along 

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

Measurements at Sky Harbor were used due to its close proximity to the monitors that experienced 

exceedances on September 12. As can be seen in the graph, visibility was reduced to 6 miles at the Sky Harbor 

Airport during the windblown dust event, and this visibility reduction coincided with rapidly increasing PM10

concentrations and sustained wind speeds up to 25 mph in the Phoenix area during the 8 p.m. hour. Maps of 

the PM10 concentrations, sustained wind speeds, and wind gusts in Figures 5-15 to 5-27 also show the rapid 

increases in PM10 concentrations and wind speeds as the primary outflow boundary reached the Phoenix area 

during that hour. 

As shown in the graph in Figure 5-14 and on the map in Figure 5-15, several monitoring sites in Pinal County 

measured increased 1-hour average PM10 concentrations during the 3 p.m. hour on September 12th due to 

sustained thunderstorm outflow winds over 25 mph. However, the winds were primarily out of the westsouthwest, limiting dust impacts to the Pinal County monitoring sites. During the next few hours, PM10 

concentrations were generally low, but there were missing PM10 data at a few of the monitoring sites for 

multiple hours. By 7 p.m., a second spike in PM10 concentrations due to a thunderstorm outflow boundary 

with sustained southerly winds between 20-25 mph and gusts above 30 mph, can be seen. The peak 

monitoring site—Cowtown—had a maximum 1-hour average PM10 concentration of 613 μg/m3

, and the 

Maricopa and Stanfield monitoring sites also measured increased PM10 concentrations of 243 μg/m3 and 346 

μg/m3

, respectively. These spikes preceded the PM10 concentration spikes at the monitoring sites in Maricopa 

by 1-1.5 hours, which is consistent with wind speeds and directions observed at the Phoenix Sky Harbor 

Airport in Maricopa County and the Casa Grande Airport in Pinal County for the propagation of the 

thunderstorm outflow boundary across the region. The increase in PM10 concentrations between the Pinal 

County sites and the Maricopa County sites indicates that the winds were strong enough to entrain additional 

dust into the atmosphere as the winds traveled over the same northern desert areas of Pinal County as they did 

on the previous day. Only monitoring sites in Maricopa and Pinal counties exceeded the NAAQS due to 

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

concentrations reflect the magnitude of the dust storm. Additionally, the NWS airport meteorological data 

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

outflow boundary were out of the south. These winds, combined with the high PM10 concentrations in Pinal 

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

nonattainment area and then transported into the Phoenix region by the strong winds. Additional evidence of 

the transport of windblown dust into the region is presented in the GIS time series maps in Figure 5-15 through 

Figure 5-27 below. 

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

animation here: 

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

www.phoenixvis.net/videos/mpeg4/SOMT_09122011.mp4

This video from September 12th is from the same camera view as the video from the 11th and shows multiple 

moderate dust plumes blowing into the Phoenix area in the afternoon and then the main dust front moving into 

the region from the south around 8 p.m., exactly when the hourly average PM10 data showed sharp spikes in 

the PM10 concentrations. 

Videos showing other views across the Valley on this date are also available (see the Visibility Cameras 

section).
    
     

Phoenix nonattainment area PM10 monitor readings and Sky Harbor wind speed and 

visibility for September 12, 2011.

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Pinal County PM10 monitor readings and Casa Grande visibility and wind speed for 

September 12, 2011. 

            ! " # 

Figures 5–15 through 5–27 provide a time series GIS-based visualization of the meteorology and PM10 

concentrations associated with the thunderstorm outflows observed on September 12, 2012. The data 

displayed in the following maps were gathered from the same five data sources listed in Table 5-1. All 

available meteorological and air quality data was used in order to present the most complete story of the 

event. 

! " # $  %   # &  ' ( 

A description of each time series map is provided to highlight important data in each map and explain the 

progression of the meteorology and PM10 concentrations through time. Taken as a whole, the maps and 

associated explanatory text describe the clear causal connection between the windblown dust generated 

and transported by the thunderstorm outflow winds and the PM10 exceedances at the nonattainment area 

monitors. 

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September 12, 2011, 3:00 PM – 4:30 PM

Figures 5-15 through 5-17 show light to moderate southeasterly winds and low PM10 concentrations 

throughout the Phoenix region and the strong westerly winds previously mentioned in Pinal County that 

generated high PM10 concentrations during the 3 p.m. hour. By 4:30 p.m., the strong winds had subsided 

and PM10 concentrations decreased back to low levels again. 

September 12, 2011, 5:30 PM – 7:00 PM

Figures 5-18 through 5-20 show gusty, east-southeasterly winds that developed across southeastern 

Maricopa County and central Pinal County due to outflows from thunderstorms that formed over the 

Coolidge area between 5 and 6 p.m. During the 6-7 p.m. hour, dust entrained into the atmosphere began 

to impact PM10 monitors on the southern edge of the Phoenix PM10 nonattainment area, which had 

hourly concentrations shown in the graph in Figure 5-13 reaching 400 μg/m3

 during the 6-7 p.m. hour. 

The plumes of dust transported into the Phoenix area can also be seen in the visibility camera video linked 

above. 

 

September 12, 2011, 7:00 PM – 8:00 PM

Figures 5-21 through 5-22 show lingering elevated PM10 concentrations in the central Phoenix area and 

the development of strong southerly winds at the Pinal County monitoring sites and airports due to 

thunderstorms to the south. PM10 concentrations also spiked at the Pinal County monitors closest to the 

strong winds during this hour. 

September 12, 2011, 8:00 PM – 9:30 PM

Figures 5-23 through 5-25 show the arrival of the winds that impacted the Pinal County sites in the 

previous hour and very large, abrupt increases in PM10 concentrations at nearly every monitoring site in 

the central Phoenix area, which coincides with the dust front that is clearly evident in the visibility camera 

videos and is shown on the cover of this report. 

September 12, 2011, 9:30 PM – 10:30 PM

Figures 5-26 and 5-27 show subsiding winds across the region and rapidly decreasing PM10 

concentrations behind the outflow boundary and as deposition of windblown dust occurred. 
GIS Time Series Map for September 12, 2011, 3:00 PM – 3:30 PM.

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GIS Time Series Map for September 12, 2011, 3:30 PM – 4:00 PM.

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GIS Time Series Map for September 12, 2011, 4:00 PM – 4:30 PM.

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GIS Time Series Map for September 12, 2011, 4:00 PM – 4:30 PM.

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GIS Time Series Map for September 12, 2011, 5:30 PM – 6:00 PM.

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