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I I I I I I I I I I I I I I I I I I '1 I\_.~ ~ REGIONAL QUANTITATIVE CONVECTIVE PRECIPITATION FORECASTING A NEW FORECAST TECHNIQUE John F. Henz Henz Meteorological Services Denver, Colorado 80211 1.0 Introduction A significant problem facing operational meteorologists on the local level is the refinement.of regional quantitative precipitation forecasts (QPF) into a local product. This problem is especially troublesome when thunderstorms are the source of the precipitation over typically small areas of less than 2,500 square miles. The problem can be compounded if the basins in the area are of rapid response nature such as small urban streams and streets found in heavily populated areas. In these areas a premium is placed on the ability to identify a flash flood situation and monitor it as soon as possible. Considerable research effort has been directed at identifying pattern recognition techniques conducive to the formation of heavy convective rainfalls. These techniques have proven to be very useful at improving local flash flood prediction through pattern recognition. Another step in improving the quantification of the QPF can corne from the analysis of the thermal updraft characteristics of the predicted and observed pre-storm environment. The Flash Flood Prediction Program (F2P2) of the Denver Urban Drainage & Flood Control District (UDFCD) has afforded the opportunity to study this problem since 1979. In 1982 analysis of predicted thunderstorm updraft characteristics revealed that the depth of the predicted updraft which was warmer than OC appeared to be directly related to both the amount of the observed rainfall and the observed surface to 500MB precipitable water index (PWI). This relationship was not verifiable on a routine basis until the F2P2 established a flood detection network of over 136 ALERT rain gauges between 1988 and 1990. The remairtder of this paper will describe the QPF forecast technique which is used daily in the F2P2 and describe its use on a number of significant flash flood in the past 15 years. Additionally an example of its use on a recent flash flood in the Phoenix Metro area during 1993 will be presented and discussed. 2.0 The Quantitative Convective Precipitation Potential (QCP2) The physical basis for the QCP2 technique was presented in Henz, 1993 but will be discussed briefly at this time. The QCP2 is directed at the kinematic analysis of the daily National Weather Service radiosonde or a locally calculated surrogate. As an example 1