WeatherWhys for July 18,2014 from accuweather.com

Johnstown, Pennsylvania, has faced several devastating flood events in its history, including the "Great Johnstown Flood of May 1889." This weekend marks the anniversary when another such event occurred in 1977. A train of heavy thunderstorms moved over the area, dropping up to 12 inches of rain on the hills surrounding Johnstown. Runoff from the rain swelled Little Conemaugh River and smaller creeks and caused the Laurel Run Dam and Sandy Run dams to collapse. That sent a 10-foot-high wall of water into Johnstown, sweeping away residents in their homes and cars.