Forecast tools and considerations for four recent flash floods

Rainfall intensity and distribution along with watershed size and surface characteristics play a vital role in flash flood situations. Four flash flood events in 2003 and 2004 demonstrate a variety of rainfall amounts and watershed characteristics that led to significant flash floods. All four have been used extensively in training at UCAR COMET. These events are 1) the Las Vegas, Nevada flash flood on 19 August 2003, 2) the Kansas Turnpike flash flood on 30 August 2003, 3) the Hurricane Gaston flash flood in Richmond, Virginia on 30 August 2004, and 4) the Hurricane Ivan floods of 16-18 September 2004.

Two of these cases (Las Vegas and Richmond) demonstrate the importance of understanding the impacts of urban development on runoff from intense rainfall in both humid and arid locations. Studies of urban watersheds suggest that streams need to carry significantly more volume due to greater runoff ratios and the shorter travel time from headwater to outlet. Streams would evolve to become wider and deeper if permitted. Most are not permitted to enlarge and thus flash floods occur at lower rainfall thresholds. The Kansas Turnpike example illustrates the need to have (1) information about small watersheds and (2) the ability to recognize the radar and satellite signature of effective warm-rain processes. The Hurricane Ivan case reminds us how important it is to understand the impacts of antecedent moisture, terrain, and frontogenetic forcing on the distribution of excessive rainfall and the subsequent runoff. Copious precipitation, including heavy rains from the remnants of Hurricane Frances 7-10 days earlier, created saturated or near-saturated soil conditions across many sections of the Appalachians and upper Ohio Valley. As Ivan moved inland from the Gulf Coast, strong southeasterlies focused intense rainfall production along the eastern slopes of the lower Appalachians triggering flash floods and debris flows. Later intense bands of rainfall affected Maryland, Pennsylvania, Ohio, and New York as tropical moisture ahead of Ivan was lifted over a polar front.