2002 Annual

Wednesday, 16 January 2002
Assessing the Flash Flood Potential of a Complex Environment: The Redbank Creek Flash Flood of 19 July 1996
Robert Stonefield, NOAA/NWS, Pittsburgh, PA; and J. Korotky
Widespread flash flooding developed across Venango, Clarion, and Jefferson counties in western Pennsylvania during the early morning hours (0600-1000 UTC) of 19 July 1996. The flash floods caused $225 million in property damage and one fatality; excessive rainfall also resulted in record river flooding. A large fraction of the flood damage ($100 million) was centered in Jefferson county, where rain gages measured 4-7 inches of rain during a four hour period. Flash flooding during the early morning hours of 19 July was followed by severe weather during the day, resulting in two F2 tornadoes, and several instances of wind damage.

Flash floods are usually localized, occurring typically in small watersheds. Consequently, the majority of flash floods are associated with slow-moving (or back-building) convection, usually developing in weak atmospheric flow and insignificant vertical wind shear. Storms that contributed to the Redbank Creek flash flood developed in fast atmospheric flow and significant vertical wind shear, prompting a number of organized convective storm clusters to move rapidly (at speeds of 40-50 mph) in sequence over common watersheds. The rapid storm movement created a large ground footprint of intense rainfall, resulting in the inundation of large areas. Significant vertical wind shear also contributed to the development of supercell storms, which further complicated the warning process.

This poster will illustrate the flash flood potential of the environment preceding the Redbank Creek flash flood, including the meteorological conditions that promoted a procession of convective storms to move rapidly over large common areas. Proximity soundings will be used to demonstrate an “ingredients-based” approach to assessing the potential for locally excessive rainfall, emphasizing the physical processes that contributed to vigorous rainfall production and intense rainfall rates on 19 July 1996.

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