The Agnes Rainstorm of June 1972: Surface Feature Evolution Culminating in Inland Storm Redevelopment

Abstract

The passage of Tropical Storm Agnes through the eastern United States on 21–23 June 1972 was marked by a swath of exceptionally heavy rainfall amounting to 15–35 cm. The heaviest rains fell in a narrow band from eastern Virginia northward through the Susquehanna Valley of Pennsylvania along and to the west of a slow-moving frontal boundary. The frontal boundary formed in situ to the east of the Appalachian Mountains on 21 June as warm, moist air ahead of Agnes from the Atlantic was drawn westward against somewhat cooler air along the eastern slopes of the mountains. Air was forced to flow down the pressure gradient east of the Appalachians, analogous to cold air damming situations, with comparatively low pressure along the coast. The north-south frontal zone marked the transition region between the two regimes. An analysis of all available surface observations disclosed that this boundary marked a locus of frontogenesis, moisture convergence, and cyclonic vorticity generation. The frontal boundary strengthened during 22 June in response to differential diabatic heating associated with the diurnal heating cycle and thermal advection resulting from the larger-scale circulation. After Tropical Storm Agnes reintensified while moving just offshore, new cyclogenesis began over northeastern Pennsylvania along the inland frontal boundary by 0000 UTC 23 June. Subsequently, Agnes dissipated after crossing western Long Island, while the system to the west became the dominant cyclonic circulation. The westward cyclonic redevelopment and ensuing counterclockwise looping across Pennsylvania reflected an interaction between synoptic and mesoscale circulations.