Monday, 12 January 2004
Flash flooding during a severe drought situation: a case study of the 2002 Ogallala, NE event
case study was conducted to investigate a flash flood event which took place during severe drought conditions. During the summer of 2002, a severe drought was taking in over western Nebraska when convention developed in the early morning hours of 5 July producing 25.0-30.0 cm of precipitation and flash flooding. By the time the storm ended the town of Ogalalla, NE was flooded, the interstate was washed out and one person was dead. The presentation will investigate the atmospheric conditions before and during the event. What conditions were present to produce the devastating floods and how did these conditions fit into the normal forecasting guidelines for flash flood situations. Analysis was also conducted to determine the frequency of heavy precipitation events for the region, especially during drought conditions. The warm tropical air mass over the area had plenty of moisture (precipitable water values were between 3.7-5.0 cm), a weak cap, good instability, and upper air support. Computer simulations were run for the event using the NCAR mesoscale model (MM5). The model was run with different parameterization schemes to determine which handled the flash flood event and precipitation amounts the best. In addition, the model was run using different (wetter) surface conditions to determine what role the extreme drought conditions played in this event. As the region is also along the southern edge of the Nebraska Sand Hills, we investigated the impact of the Sand Hills on the development of this devastating flood.