The rapid ablation of a snowpack and intense rainfall combined to cause catastrophic flooding across much of the mid-Atlantic region of the United States on 18 and 19 January 1996. This flooding event resulted in as many as 30 fatalities, numerous injuries, and damage estimated at $1.5 billion. The Loyalsock and Lycoming Creek watersheds in northern Pennsylvania were arguably the hardest hit areas in the region, at least from the perspective of fatalities and injuries. This flooding event, which was certainly devastating from a human and economic perspective, also represents an interesting scientific situation in which climatological preconditioning and the occurrence of an unusual synoptic-scale weather event combined to create disastrous results.

Before the event, snow fell at record levels across northern Pennsylvania during the late autumn and early winter seasons. By the middle of January 1996, more snow had fallen across the combined watershed region than is expected during an average snowfall season. The heavy snowfall, in conjunction with anomalously low temperatures, combined to create an exceptionally deep snowpack containing 7.5–15 cm of snow water equivalent. A strong synoptic-scale disturbance that moved across the United States from 17 through 20 January 1996 initiated meteorological conditions that rapidly ablated the snowpack and dropped more than 7.5 cm of liquid precipitation in a 24-h period in the combined watershed region.

A one-dimensional mass and energy balance snowpack model (SNTHERM) was used to evaluate the magnitude of the surface energy fluxes that were associated with the movement of the synoptic-scale system through the area. Further, the model was used in a sensitivity analysis mode to determine the meteorological variables most important to the ablation of the snowpack. Results indicate that high dewpoint temperatures and high wind speeds were at least as important as high temperatures and precipitation to the severity of the flooding. It was shown that a thorough understanding of the hazardous potential of an event of this type can best be achieved when assessments from diverse disciplines within the natural sciences are employed.

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*Center for Climatic Research, Delaware State Climatologist, University of Delaware, Newark, Delaware.

+National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland.

#National Weather Service, Middle Atlantic River Forecast Center, State College, Pennsylvania.