The National Weather Service and AccuWeather both have post-mortems on the massive dust storm that swallowed Phoenix on July 5, 2011.
AccuWeather says, “The dust storm was estimated to reach a peak height of at least 5,000 to 6,000 (about a mile) with an aerial coverage on the leading edge stretching nearly 100 miles, according to the National Weather Service. The storm traveled at least 150 miles, much farther than the average 25 to 50 miles that dust storms typically travel.” (See report and videos here. and another report and videos here.)
The National Weather Service provides a report and charts here. Some excerpts:
The Setup
Strong to severe thunderstorms developed east of Tucson, AZ during the afternoon hours of July 5, 2011. The storms intensified as they progressed west into the Tucson Metropolitan Area, producing downburst winds in excess of 70 MPH (110 KPH). Aided by gravity (Tucson is approximately 1500 ft/460 m higher than Phoenix) and additional downbursts from the parent storms, these strong outflow winds proceeded to race off to the northwest, with the leading edge moving at 30 to 40 MPH (45 to 65 KPH). By 630 PM (0130 UTC) the first calls came in to NWS Phoenix that a large wall of dust was approaching the Casa Grande/Eloy, AZ area, roughly 50 mi (75 km) southeast of Downtown Phoenix.
Lack of significant rain during the winter contributed to the dust storm.
The Delivery
At 7 PM MST (02 UTC) the leading edge of the massive dust storm hit the far southeast portions of the Phoenix area. The dust continued to push further and eventually through the entire metropolitan area during the next two hours….A few measured wind gusts even approached 70 MPH (110 KPH).
Background – How unusual (or usual) was this? What exactly is a Dust Storm?
Dust storms are a common phenomenon across the Sonoran Desert in the Southwest U.S. during the North American Monsoon. During an average year, generally one to three dust storms will move into the Phoenix area – predominantly from the southeast. Across all of Arizona, over 100 dust storms have been reported in the past 10 years according to NCDC Storm Data. While records of most widespread, most intense, largest, etc., dust storms are not kept, NWS meteorologists that have worked in Phoenix for almost 30 years have said this was one of the most significant dust storms they have experienced.
A dust storm usually arrives suddenly in the form of an advancing wall of dust and debris which may be miles long and several thousand feet high. They strike quickly, making driving conditions hazardous. Blinding, choking dust can reduce visibility to near zero in just one to two minutes, causing accidents that may involve chain collisions, creating massive pileups. Dust storms usually last only 10 to 30 minutes, though dusty conditions may remain for some time afterward.
If dense dust is observed blowing across or approaching a roadway, pull your vehicle off the pavement as far as possible, stop, turn off lights, set the emergency brake, take your foot off of the brake pedal to be sure the tail lights are not illuminated. In the past, motorists driving in dust storms have pulled off the roadway, leaving lights on. Vehicles approaching from the rear and using the advance car’s lights as a guide have inadvertently left the roadway and in some instances collided with the parked vehicle. Make sure all of your lights are off when you park off the roadway. Don’t enter the dust storm area if you can avoid it. If you can’t pull off the roadway, proceed at a speed suitable for visibility, turn on lights and sound horn occasionally. Use the painted center line to help guide you. Look for a safe place to pull off the roadway. Never stop on the traveled portion of the roadway.
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