Application of an updraft warm layer depth QPF technique to June 2000 flash floods in the NWS Central and Southern Regions

During the first three weeks of June 2000 serious flash floods occurred in Fargo, ND, Kansas City, MO, Oklahoma, Arkansas and Texas. Additionally minor flash flooding events occurred in Florida, Georgia and New Mexico. Hydrologists have a challenging task in preparing flood forecasts for large and small basins during the summer flash flood season. The problem embodies how much rain can be produced from a storm system, where will it fall and when. This paper will deal with the first of these three issues.

Determining how much rain and at what intensity a thunderstorm system can produce rain can be a taunting task in itself. A proven QPF technique which is based on the depth of the updraft which is warmer than 0C can assist in proving operational answers to these questions.

The equation used to produce the thunderstorm QPF is:

PWI x UDWL(km)/RUF x 2( if UDWL/RUF=/> 1)

PWI=Surface to 500mb Precipitable Water Index UDWL=Depth of updraft warmer than 0C in km RUF=1.5km if >3,000ft elevation , 2.5km if < 3,000ft , 3.5km if dew pt=/> 70F

The equation yields the peak hourly rainfall rate in a thunderstorm or convective cloud associated with that portion of the storm with base reflectivity greater than 50dBz. Rain rates for lower reflectivities between 50dBz and 25 dBZ can be calculated by a simple step down equation.

The technique was applied the night of June 20, 2000 to flash flooding situations developing in Oklahoma, Arkansas, Georgia, Florida, New Mexico and to two floods which occurred earlier in the prior 7 days in Fargo and Kansas City. Radar data, radar rainfall estimates, surface and upper air observations, NWS flash flood warnings and flood observations were used to verify the technique. In all 12 cases the technique was able to be used to estimate thunderstorms' rainfall within 15 % several hours before the event. The technique was used to produce a forecast for the afternoon flash flooding inn Dallas and Denton Counties on June 21 during the author's paper.

In response to the other two questions concerning where and if thunderstorms would form, the author directed hydrologists to rely on both NWS WSR-88D radars to indicate areas of thunderstorm existence and intensity for the 0-3 hour periods. These predictions in the 3-6 hour period prior to the event still constitute a diffucult forecast challenge. However, use of model output for basins, surface mesonets, predicted and satellite observed soundings and improved mesocale short range modeling linked to hydrologic models will provide better forecasts as they can be used.