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Monsoons and Haboobs in the American Desert

Phoenicians watch the dust storm approach | Creative Commons photo by Christopher Marks

The nation's eyes were briefly on Phoenix, AZ this week, as that sprawling city found itself in the path of a violent, mile-high dust storm -- a haboob, from the Arabic word for "phenomenon." Bearing down on the city with little warning, the storm cut visibility in Phoenix to near-zero on Tuesday evening, forcing the closure of Sky Harbor airport and slowing ground traffic to a halt.

Though Phoenix got all the attention the storm was actually remarkably widespread, reaching as far as central Riverside County in California before petering out at about 1:00 am Wednesday just east of the Coachella Valley. John Slama of desertweather.com grabbed this Doppler screen shot from his phone as he tried to outrun the storm on I-10 in the Chuckwalla Valley west of Blythe.

The embedded time-lapse video gives a better sense of the scale of the storm as seen from the ground:

The haboob started out as a typical monsoon-season thunderstorm, the same kind of storm as those that have dumped several inches of rain on Joshua Tree National Park this week. As desert temperatures surpass triple digits, the superheated air rises. That region-wide updraft creates a low-pressure area which draws moist air in from the nearby Sea of Cortez, with some additonal moisture from the more distant Gulf of Mexico. When oceanic moist air reaches the hot desert, the result is burst after burst of intense thunderstorms.

Tuesday evening's storm started near Tucson and headed northward. As it crossed the hot desert plains south of Phoenix -- home to sprawling suburban development and fallow fields -- the intense front began to pick up soil and debris. Winds reaching up to 70 miles an hour lifted soil more than a mile into the sky, and transported it for hundreds of miles. At its peak the storm was estimated at about a hundred miles across. The eastern edge of the dust cloud mantled Phoenix: the western edge rolled up toward the Colorado River and into California.

It's been an excruciatingly dry few years in the eastern parts of the American desert, so more and perhaps bigger haboobs are not at all out of the question. Even if the soil mostly stays on the ground from here on in, though, monsoon thunderstorms can be formidable, and are not to be trifled with. A seemingly small storm system can dump inches of rain on a watershed in just a few minutes, causing flash floods that can remain dangerous for many miles downstream.

You might be hiking in a wash or a canyon under blue skies with only a few clouds on the horizon, and find yourself faced with a powerful flash flood that came out of those distant clouds. People drown in the desert in the summer almost every year as a result. Lightning strikes take a few more -- unsurprising in a land where people can be taller than the trees -- and a few more come to harm when they drive into flooded dips in the road where washes cross the pavement.

By all means enjoy the desert summer, including the beauty to be found in its dramatic storms. But do keep an eye on the horizon.

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