Abstract
The heavy rains responsible for the disastrous flash floods new 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 South Dakota nearly 96 h earlier. The complex tends to become more compact and intense during the night, with a radius of ∼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 north 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. Airflow relative to the moving storm is southeasterly at the surface, veering rapidly to southwesterly in the boundary layer and then to westerly in the upper troposphere.
Substantial convective instability is present throughout the environment of the storm, least in the northern sectors and greatest in the region of low-level inflow southwest of the centroid. A temporary reduction of this instability results in a weakening of the storm over the Great Lakes on the 18th, but a renewal of the supply of low-level heat and moisture is linked to a rejuvenation as the system approaches Pennsylvania. After crossing the Atlantic coast, the system is associated with a cyclonic circulation at the surface, which develops to tropical-storm intensity.
A study of the mass budget reveals prominent storm-scale ascent within 200 km of the rainfall centroid, compensated largely by descent in the annulus between the 200 and 400 km radii. Budgetary calculations over the storm area show that water vapor is made available principally by storm-scale convergence, with surface evaporation and moist horizontal advection playing smaller but by no means negligible roles. Convection removes the accumulating vapor long before large-scale saturation is reached, in a highly efficient process whereby nearly 90% of the water vapor made available by horizontal transport and surface evaporation falls as precipitation. The vertical motion and components of the water vapor budget are strongly modulated, but not entirely dislocated, as the storm passes through the region of strong diurnal low-level wind oscillation typical of the central United States in summer.
