An Examination of the Eastern Nebraska and Western Iowa Flash Flood Event of 6-7 August 1999

Authors

Catherine M. Zapotocny

Document Type

Article

Citation

National Weather Digest (June 2002) 26(1-2): 7-26

Comments

United States government work

Abstract

Flash flooding occurs each summer in eastern Nebraska and western Iowa. While 10-inch rains are extraordinary, it is not uncommon to have at least one excessive rain event during the annual convective season. A significant flash flood occurred in east central Nebraska and western Iowa the night of August 6, 1999 into the morning of August 7, 1999. Antecedent hydrologic conditions limited the loss of life, however considerable property damage resulted as the heavy rain fell over the urbanized areas in eastern Nebraska and western Iowa.

Several flash flood ingredients came together over the area. These meteorological factors are typical of similar events in the Plains of the United States. The frontal type flash flood pattern, as described by Maddox and others (1979), became established during the afternoon of August 6, 1999 with a west to east warm front near the Kansas and Nebraska border. Strong inflow of moist, unstable air normal to the surface front contributed to the development of convection on the cool side of the surface front. An intensifying convergent nocturnal low-level jet (LLJ) transported high fJe air northward above the surface front, which increased thermodynamic instability and enhanced the upward motion. While individual thunderstorm cells moved northeast, backward cell development relative to the mean cell motion vector persisted for a prolonged period (Chappell 1986) playing a significant role in the character of this heavy rain event. These cells moved northeast, growing to maturity and producing heavy rain. During the intensification of the low-level jet, a gradual change in propagation increased storm cell mergers. The result of these factors was a quasi-stationary convective event producing very heavy rains. The use of pattern recognition, knowledge of flash flood forecasting techniques, and utilization of real-time and forecast data, enabled the National Weather Service (NWS) operational forecasters to anticipate the flash flood potential. This case illustrates the importance of understanding the hydrometeorological processes that lead to the production of heavy rain and flash flooding, allowing the successful issuance of a flash flood watch prior to the nocturnal event. Short-term monitoring of cell movement and new cell initiation was integral to forecasting the evolution of the convective system and the location of the heaviest nocturnal rainfall. In addition, this case will be compared with another heavy rain event to identify common features utilized for the anticipation of heavy rain events.