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Comparing Washington's recent derechos

Monday - 9/3/2012, 12:21am  ET

derechodildine.jpg
This graphic shows the National Weather Service Weather Radar of the June 29, 2012 derecho compared with the June 4, 2008 derecho. The 2012 complex of storms and its swath of wind damage was much larger than the 2008 event. (WTOP/Dave Dildine)
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Dave Dildine, wtop.com

WASHINGTON - The severe windstorm that roared southeast from the Great Lakes into the Mid-Atlantic in June was an extreme weather event for the Washington, D.C. metro area. The storm system left extensive tree damage in its wake and more than one million people were without power.

The storm complex was a "derecho." Although the intensity of the June storms was exceptional, it's happened before.

A derecho of similar speed and intensity struck the region the afternoon of June 4, 2008. Like June 2012's windstorm, the 08 storms tracked west to east causing widespread wind damage and power outages.

Derechos, by definition, are long-lived complexes of severe thunderstorms that track more than 250 miles and produce wind gusts of 60 mph or more. The 2008 derecho developed over central Indiana and tracked roughly 600 miles due east toward the Mid-Atlantic coast. The 2012 derecho had roots in northern Illinois and raced east-southeast toward the Atlantic coastline, a track length of more than 800 miles.

The swath of wind damage from the 2008 event was smaller in width than the 2012 derecho. The north-south axis of the 2008 storms caused wind damage from Baltimore to central Virginia. The 2012 derecho's fierce winds stretched from the Mason- Dixon Line into southern Virginia.

Steven Zubrick at the National Weather Service attributes the intensity and broader coverage of the 2012 derecho to a hotter, more unstable airmass over Washington.

"The airmass on June 29, 2012 resulted in much more significant and widespread thunderstorms than the airmass supported in June 2008," Zubrick says.

The 2008 storms moved through the metro area during the beginning of a Wednesday rush hour, toppling trees and snarling traffic. Numerous downed trees on the above-ground portion Metro's orange line between East Falls Church and West Falls Church forced it to close for most of the afternoon. The 2012 storms arrived late on a Friday night, with slightly less impact on transit due to lower travel volumes.

The 2012 derecho was less predictable than the 2008 storms.

"National Weather Service's Storm Prediction Center in Norman, Ok. was more confident of severe weather occurring with storms in our area on June 4, 2008 than on June 29, 2012," Zubrick says.

Official forecasts from the Storm Prediction Center placed the area under a moderate risk of severe weather five hours before the 2008 storms rolled through. There was more uncertainly involved with forecasting the 2012 derecho's passage over the Appalachian Mountains.

Peak wind gusts around the Washington area during the 2008 derecho include a 59 mph gust at Reagan National Airport, 59 mph at Dulles Airport, 63 mph at the USGS in Reston, Va. and 66 mph at Andrews Air Force Base. An automatic weather- reporting station in Frederick, Md. reported a 76 mph gust at the storm's height.

As the 2012 derecho stormed through Washington, winds gusts reached or exceeded those in 2008. In Seat Pleasant, Md. winds gusted to 76 mph. National Airport recorded an instantaneous wind speed of 70 mph and Dulles Airport recorded a wind gust of 71 mph. The USGS weather station recorded a 79 mph gust.

The 2008 derecho spawned at least five brief tornadoes that produced concentrated areas of significant wind damage. A tornado was reported in White Oak, Md. along with strong straight-line winds. The National Weather Service determined an EF-0 tornado touched down near Chesapeake Beach in Calvert County, Md. In Virginia, a tornado tracked 12 miles from Fauquier County toward Aldie, Va. producing an estimated $400,000 in damage alone. The 2012 derecho didn't produce any confirmed tornadoes.

In all, the Storm Prediction Center catalogued 172 reports of wind damage between the Ohio Valley and Atlantic coast on June 4, 2008. The large, long-track 2012 derecho resulted in 636 reports of wind damage and severe wind gusts east of the Mississippi River.

According to the Storm Prediction Center in Norman, Ok. the Washington area typically experiences a derecho about once every four years.

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