A powerful thunderstorm that rolled through the region with a hurricane-force gust knocked out power to at least 280,000, some SEPTA service, and, evidently, the heat wave. The storm barreled along at a brisk 55 m.p.h. between 3 and 4 p.m., and cleared the region in time for the heart of the evening commute. Damage and debris will linger much longer.