A brief but powerful thunderstorm pounded the Washington area, downing power lines and trees and leaving thousands of people without electricity. The storm whipped up wind gusts of 60 m.p.h. and dumped almost 2 inches of rain in less that 15 minutes, according to the National Weather Service. About 75,000 homes and businesses lost power in the capital and surrounding areas. On Capitol Hill, the grounds were littered with branches and several uprooted trees.