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)Between 9:30 and 11 p.m. Friday night, one of the most destructive complexes of thunderstorms in memory swept through the entire D.C. area. Packing wind gusts of 60-80 mph, the storm produced extensive damage, downing hundreds of trees, and leaving more than 1 million area-residents without power.
Blue marks indicate reports of damaging wind. Black squares indicate winds of over 75 mph. (National Weather Service)Racing along at speeds over 60 mph, the bowing line of thunderstorms formed west of Chicago around 11 a.m. and by midnight approached the Atlantic ocean. It left a massive trail of destruction spanning from northern Illinois to the Delmarva Peninsula. The National Weather Service has logged well over 800 reports of damaging winds.
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.During summer, the jet stream atop a sprawling heat dome is sometimes called a “ring of fire” due to the tendency for explosive thunderstorms to form along this weather front separating hot, humid air to the south and cooler, drier air to the north.
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 )As this stifling air bubbled northward, clashing with the weather front draped from near Chicago to just north of D.C., thunderstorms erupted. They grew in coverage and intensity as they raced southeast, powered by the roaring upper level winds and fueled by the record-setting heat and oppressive humidity in their path.
Three-dimensional schematic of a mature derecho showing inflow and outflow channels (National Weather Service)The coverage and availability of this heat energy was vast, sustaining the storms on their 600 mile northwest to southeast traverse. The storms continually ingested the hot, humid air and expelled it in violent downdrafts - crashing into the ground at high speeds and spreading out, sometimes accelerating further.
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)Peak wind gusts in the D.C. region include the following:
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.
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.
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