Synoptic and mesoscale analyses of the meteorological conditions that produced disastrous flash flooding in the Johnstown, PA., region on July 19-20, 1977, are presented. Detailed rainfall analyses based on surface rain gauges, National Weather Service (NWS) radar, and satellite rainfall estimation techniques are also shown. Dynamic processes associated with a weak short-wave trough triggered and maintained widespread thunderstorm activity over Pennsylvania and New York during the afternoon and evening of July 19. Two major squall lines moved across Pennsylvania. The western extension of the outflow boundary produced by the second squall line became quasi-stationary in western Pennsylvania, and appeared to be the primary feature that focused the heavy rainfall over a relatively small region. Warm, moist air was lifted over the rain-cooled air lying north and east of the boundary, triggering new storms in the same general region. The storms repeatedly moved southeastward over the Conemaugh River Basin, producing 8- to 9-h rainfalls of up to 12 in.