A derecho is a widespread, long-lived, straight-line windstorm that is associated with a fast-moving band of severe thunderstorms. http://en.wikipedia.org/wiki/Derecho
ORIGINAL CAPTION: A shelf cloud in a severe thunderstorm develops around 11:45am in Matteson, IL south of Chicago. In this 700-mile long "super derecho" storm cells grew in Illinois and caused a devastating path of destruction across the eastern United States all the way to Washington DC and north through New Jersey. This photo was taken not long after the storm formed in western Illinois near I-57 and US 30 (Lincoln Highway). (courtesy of JoeyBLS Photography)
As many as 183,000 customers in the Chicago were left without power this morning after a small derecho blasted through the area. According to ComEd, the hardest hit areas were Aurora and Rockford where 66,000 customers were left without power. In the Chicago area, about 76,000 customers were left without power including 56,000 in the south metro area.
Originally just a thunderstorm complex (MCS) that had developed along a northwest to southeast frontal boundary across portions of Minnesota and South Dakota late last night, the storms moved into a more favorable atmosphere for strengthening and began to produce wind damage across southwestern Wisconsin earlier this morning. After travelling approximately 280 miles (from southwestern Wisconsin to northeast Illinois to northern Indiana) with winds between 60 and 80 mph, this thunderstorm complex was safely classified as a small derecho.
READ MORE: http://www.examiner.com/article/derecho-hits-the-chicago-area-downing-trees-and-power-lines-183-000-lose-power