Derecho: Behind Washington, D.C.’s destructive thunderstorm outbreak, June 29, 2012


Radar sequence of derecho thunderstorm complex. Storm traveled about 600 miles in 10 hours at an average speed of 60 mph. (Storm Prediction Center)

Blue marks indicate reports of damaging wind. Black squares indicate winds of over 75 mph. (National Weather Service)

This kind of fast-moving, long-lived, large, and violent thunderstorm complex is known as a derecho.

Incredible radar loop of derecho from start to finish uploaded to YouTube by akrherz

Derechos are most common in the Midwest and Great Lakes between May and July. The National Weather Service indicates they occur about once every four years in the D.C. area.

Link: Facts about derechos

They often form along the northern boundary of a hot air mass, right along or just south of the jet stream - where upper level winds zip along at high speeds.


Weather map showing fast upper level winds in blue - the location of jet stream, with a front along its southern edge. The derecho formed just south of the front as hot air surged northward.

On Friday, a historic, record-setting heat wave covered a sprawling region from the Midwest to the Southeast. All-time high temperatures records of 109 were established in Nashville and Columbia, South, Carolina and tied in Raleigh and Charlotte which hit 105 and 104. Here in Washington, D.C., the mercury climbed to an astonishing 104 degrees (breaking the previous record set in 1874 and 2011 by two degrees), our hottest June day in 142 years of records.


High temperatures on Friday, June 29, 2012 ( Stu Ostro, Weather.com )

Three-dimensional schematic of a mature derecho showing inflow and outflow channels (National Weather Service)

Winds measured by Doppler radar at 10:05 p.m. Pink shades indicate gusts of over 64 knots or 74 mph which, at the time, were raking Prince George’s county. (University Center for Atmospheric Research)

71 mph near Dulles Airport
70 mph in Damascus, Md.
79 mph in Reston, Va.
65 mph in Rockville, Md.
70 mph at Reagan National Airport
76 mph in Seat Pleasant, Md. (Prince George’s co.)
77 mph in Swan Point, Md. (Charles co.)
70 mph in Ashburn, Va.
69 mph in Leesburg, Va.

In addition, an 80 mph gust was clocked in Fredericksburg. To the north and west, 91 mph and 72 mph gusts were measured in Ft. Wayne, Indiana and Columbus, Ohio.

This derecho event is likely to go down as not only one of the worst on record in Washington, D.C. but also along its entire path stretching back to northern Indiana.

As the intensity of the heat wave, without reservation, was a key factor in the destructiveness of this derecho event - it raises the question about the possible role of manmade climate warming (from elevated greenhouse concentrations). It’s a complicated, controversial question, but one that scientists will surely grapple with in case studies of this rare, extraordinary event.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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