The Johnstown flood of July 1977: a long-lived convective system ( Pennsylvania).

The heavy rains responsible for the disastrous flash floods near Johnstown, Pennsylvania, on the night of 19-20 July 1977 are shown to be part of a large quasi-circular mesoscale convective complex. This complex can be traced back to an origin in S Dakota nearly 96 h earlier. The complex tends to become more compact and intense during the night, with a radius of approx 150 km, and to expand in response to new and peripheral convection during the day. On the small synoptic scale, the system is characterized by a pool of cool air at the surface and throughout the lower troposphere, just N of the centroid of the rainstorm, by a cyclonic circulation of substantial intensity in the lower and middle troposphere, and by anticyclonic circulation in the upper troposphere.-from Authors