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 January 18 and 19, 1996. This flooding event resulted in as many as 30 fatalities, numerous injuries and damage estimated at 1.5 billion dollars. 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 pre-conditioning 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 to 15 cm of snow water equivalent. A strong synoptic-scale disturbance which moved across the United States from January 17 through 20, 1996, initiated meteorological conditions that rapidly ablated the snowpack and dropped more than 7.5 cm of liquid precipitation in a 24-hour 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 dew point temperatures and high wind speeds were at least as important as high temperatures and precipitation to the severity of the flooding.