It knocked out power to millions, and in some areas for weeks.

Three years ago tonight I was introducing the viewers of my evening weather broadcast to a new weather term: The Derecho. Here on the Eastern Shore of Maryland, I first mentioned it on air around 5 PM. Through the early evening, I watched it get closer and closer, and by mid evening it was likely that we would be affected. When it arrived at our studios in Salisbury, the winds were so strong we could not open the door. It reached the Eastern Shore and Delaware about midnight, but it moved through the DC and Baltimore area around 11:15 PM, and the damage was horrific to trees and power lines. A friend had no power for two weeks in Silver Spring, Md.

The NWS report on this event makes good reading.