Meteorological and hydrological precursors to widespread flash flooding across southeast Ohio on 26–28 June 1998

Widespread flash flooding occurred across parts of southeast Ohio during a 48-h period between 1200 UTC 26 June 1998 and 1200 UTC 28 June 1998. The flash floods resulted in 8 fatalities, $130 million in damages, and five severely flooded counties. Important aspects of this case include; 1) widespread flash flooding in addition to record river flooding, 2) multiple periods of intense rainfall over a very large geographic area, 3) large scale conditions that caused numerous intense thunderstorms to move at speeds near 25 m sec-1 along a northwest to southeast oriented quasi-stationary boundary, and 4) severe weather with damaging wind events coincident with the flash flooding.

This case illustrates the inherent difficulty of detecting flash flood potential when cell speeds are high. For example, high cell speeds are associated with highly sheared environments, which also support severe and possibly tornadic thunderstorms. If forecasters are preoccupied with severe thunderstorms, they may not respond adequately to a growing flash flood threat. The environment that produced the widespread flash flooding across southeast Ohio also supported severe thunderstorms with damaging winds. To further complicate matters, some forecasters still believe that flash floods only occur when convective cells and storms move slowly. Consequently, flash flooding may not be considered a threat when cell speeds are high. This case study shows that significant and widespread flash flooding is possible with high cell speeds when numerous cells move along a similar path.

A primary goal of this paper is to present flash floods as the product of both meteorological and hydrological circumstances. The meteorological ingredients that determined the scale, duration, and intensity of rainfall will be examined. Important aspects of the basin hydrology will also be addressed, including the role of flash flood detection software such as the Areal Mean Basin Estimated Rainfall (AMBER). AMBER allowed forecasters at NWSFO Pittsburgh to consider both the rainfall potential and the stream basin hydrology during this event.