A Flash Flood from a Lake-Enhanced Rainband

Abstract

An analysis of a flash flood caused by a lake-enhanced rainband is presented. The flood took place near Erie, Pennsylvania, on 17 September 1996. It was found that the flood resulted from a complex interplay of several scales of forcing that converged over the Erie region. In particular, the flood occurred during a period when 1) a lake-enhanced convective rainband pivoted over the city of Erie with the pivot point remaining quasi-stationary for about 5 h; 2) a deep, surface-based no-shear layer, favorable for the development of strong lake-induced precipitation bands, passed over the eastern portion of Lake Erie; 3) the direction of flow in the no-shear layer shifted from shore parallel to onshore at an angle that maximized frictional convergence; 4) an upper-level short-wave trough contributed to low-level convergence, lifting, and regional destabilization; and 5) a strong land?lake diurnal temperature difference produced a lake-scale disturbance that locally enhanced the low-level convergence. Analysis of the Weather Surveillance Radar-1988 Doppler radar data from Buffalo, New York, and Cleveland, Ohio, revealed that most of the radar-derived precipitation estimates for the region were overdone except for the region affected by the quasi-stationary rainband, which was underestimated. Reconstruction of the conditions in the vicinity of the band indicate that cloud bases were considerably lower and equivalent potential temperatures higher than for the areas of precipitation farther east over northwestern Pennsylvania and southwestern New York State. It is postulated that, due to the long distance from the radar sites to the Erie area, the radar was unable to observe large amounts of cloud condensate produced by warm-rain processes below 4 km. Estimates of precipitation rates from a simple cloud model support this interpretation.