june 29th, 2012 mid atlantic derecho

What is a derecho? Many people across or region had never heard the term before. That was up until June 29th, 2012. A derecho is a powerful line of severe thunderstorms that produces widespread and significant wind damage. To meet the criteria of a derecho, this line of storms must travel at least 240 miles, and produce widespread wind gusts of 60mph, and at least widely scattered wind gusts over 75mph. That's exactly what happened that night. 

The cluster of storms first developed near Chicago, and then raced ESE across the midwest and into the mid atlantic. The atmosphere was incredibly unstable with record high temps. As the storms reached the mountains, they weakened slightly. But as soon as they hit the extremely unstable air to the east of the mountains they rapidly intensified as it plowed across the state. The lead time on this derecho (like most derechos) was not very good. This likely caught many people off guard, and may have been the reason behind some of the fatalities that resulted from this storm. That said, NWS did a very good job at issuing timely severe thunderstorm warnings as the storms approached. In fact, every county in the NWS Sterling forecast area was under a severe thunderstorm warning at some point.

The amount of wind damage that this derecho produced was historic. Wind gusts of 70-90mph were measured all across the path with widespread trees and powerline damage. During the peak, over a quarter million people in maryland alone lost power. Locally, winds gusted to 71mph at Dulles, 70mph at Regan National, and 66mph at BWI. Several weather stations across central/southern MD recorded wind gusts of 75-85mph. Hundreds if not thousands of trees were blown down.

Forecaster Jack

Image 1 credit: NWS Sterling
Image 2 credit: Storm Prediction Center