Some Perspective on DC’s June 2012 Derecho

June 29, 2012 Derecho on Radar (Source: NOAA)

This month marks the seventh anniversary of one of DC’s most destructive severe weather events.  It’s safe to say that prior to the evening of June 29, 2012, most Washingtonians had never heard the meteorological term “derecho.”  That date will forever live in weather infamy in the Nation’s Capital because a “derecho” knocked out power to more than a million people in the DC Metro Area. 

June 29, 2012 was DC’s hottest June day on record with a high temperature of 104°.  In addition to the record heat, there was also a high degree of atmospheric instability which helped create the ideal environment for thunderstorms.  Consequently, area meteorologists grew increasingly apprehensive as the day wore on and a cluster of severe thunderstorms developed over eastern Iowa and northern Illinois on the morning on June 29.  Thunderstorms within the cluster quickly reached severe limits and tracked eastward during the day.

Derechos are a type of squall line.  A “squall line” is a line of thunderstorms that can become severe, with the primary threat often being wind gusts of at least 58 mph.  “Derechos” are longer lasting squall lines that travel at least 240 miles, according to NOAA’s Storm Prediction Center.  Wind gusts in a derecho are typically higher than 58 mph but less than 100 mph.  Such was the case when the June 2012 derecho passed through the Mid-Atlantic Region.  The highest wind gusts in the DC Area were 71 mph at Dulles Airport, 70 mph at National Airport, and 66 mph at BWI Airport. 

Overall, the highest wind gust in the 2012 derecho was 91 mph in Fort Wayne, Indiana.  That is equivalent to the wind of an EF-1 tornado.  Unlike other types of thunderstorms, the primary type of damage that occur in derechos are straight-line winds.  That’s generally easy for experts to determine as the majority of damage occurs in linear fashion. 

The June 29, 2012 derecho came through the DC Metro Area between 9:00 – 11:30 P.M.  It knocked down countless trees and power lines, which created one of the largest power outages on record.  Many roads were impassable and treacherous.  Normally, it takes me 15-20 minutes to drive home after work, but it took more than an hour to navigate the roads that night with all the down trees and power lines. 

Not until the next morning, a Saturday, did Washingtonians fully appreciate how damaging the derecho was.  The extensive tree damage even caused Saturday’s round of the AT&T National golf tournament at Congressional Country Club in Bethesda, Maryland, to be closed to the public.