Mike Stinneford, a meteorologist at WJLA-TV in Washington, D.C., said the derecho -- Spanish for straight -- was a straight-line windstorm associated with a fast-moving band of severe thunderstorms that stretched 700 miles from the Chicago area, through Indiana, Ohio and West Virginia, but did most of the damage in the Washington area, WTOP-AM reported.
"Outside a hurricane, a derecho is one of the worst storms that can run through the area," Stinneford said. "It's very unusual to have a derecho come this far south, also to come in the evening, also to come over the Blue Ridge mountains."
The technical definition of a derecho is a thunderstorm complex producing a damaging wind swath of at least 240 miles and sustained wind of 58 miles or more -- almost the wind strength of hurricanes that can cause massive damage because of downed trees and power lines.
Contrary to its historical pattern, Friday's derecho moved from west to east at a rapid speed, traveling up to 80 miles per hour at some points and hitting the nation's capital after dark.
"This was a very large derecho," Stinneford said. "This is probably one of the biggest ones we've ever seen ... probably one of the worst storms ever in Washington D.C., outside of a hurricane."
Once a very rare storm that occurred once or twice a decade in the United States, there have been more than 20 derechos since 2000.