UPDATE: The computer forecast models are not handling this event well -- anything is possible.
The SPC is forecasting a possible derecho event (what is a derecho?) tonight from Ohio into Virginia. Not every model is seeing the strong squall line yet, but below is this morning's HRW-A forecast radar around 6 p.m. The 45 percent wind damage area from SPC falls short of the 250-mile derecho requirement, but the Severe Thunderstorm Watch just issued qualifies (there would also have to be winds over 57 mph over "most" of the area).*
The last major derecho to affect this area was the Super Derecho of June 2012, which I, for some reason, neglected to blog about. However, I did list some links here, including this AccuWeather.com article. Here are a couple images showing the area that was struck. Over 5 million customers lost power and Washington, D.C. was especially hit hard.
*In 2016, a proposed rewrite of the #Derecho definition would expand length requirement to 400 miles, add width requirement of 60 miles, and require certain meteorological processes. [PDF] To my knowledge this has not been accepted.