Derecho: Behind Washington, D.C.’s destructive thunderstorm outbreak, June 29, 2012
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.
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.
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.
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.
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.
Peak wind gusts in the D.C. region include the following:
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.