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The storm total rainfall was determined based on the observed storm duration and observed surface observations which wem used to calculate the next 60 to 120 minutes of rainfall. The CSM storm total rainfall compares favorably to the observed storm total rainfalls for all but the Masonville Storm, HilliS has used this equation for predicting and reconstituting rainfall in the Urban Drainage & Flood Control District's Flash Flood Prediction Program since 198:2 with a high degree of success. Recently the equation was used in the Meteorological Services Pilot Program's QPF and Flash Flood Prediction in Maricopa County, Arizona with outstanding results, HMS recommends that the mE!thod(llogy implied in the CSM equation be compared to the HMR 55A me,thodcll(lgy to determine appropriate elevation adjustments to both local stClrm PMP and to the embedded thunderstorm portion (If !lenenal storm PMP rainfall. These historic thunderstorm flooding events represent a combination of HMR partitioned local storm and general storm events. However, HMS synthesized the individual components which acted to maximize the observed thunderstorm precipitation into an Extreme Thunderstorm Atmosphere (ETA), The ETA components and associated contributing 1100d cases are described below: · Surface conditions and sub-cloud layer: Surlface temperature 80 degrees F and surface (5,000 feet msl) dew point 65 degrel~s F' (taken from Plum Creek event conditions), · Cloud layer vertical temperaturt!, moisture and wind structure (taken from Big Thompson Canyon event conditions), · Low level jet forcing of 40 knots in the sub-cloud layer (Big Thompson Canyon and Plum Creek events conditions), . Precipitable Water Index (PWI) :::: 1.44 inches (Composite of Plum Creek event moisture from the surface to 700 mb and Big Thompson Canyon event moisture from 700 mb to 500 mb), and · Train-echo effect allowing two strong thunderstorms (the second at 70 per cent of the peak ~ -hour rain of the first storm) to cross the SclmE~ basin within a 3 hour period (taKen fmm the Plum Creek, Big Thompson Canyon and Frijole Creek events and following Bertie (1982)1 j These components provide a framework for an extreme thunderstorm f1ood- producing atmospheric structure, A comparison of these components to climatological maxima can be made for dew point and PWI. The historical surface dew point maximum for Colorado Springs is 66 degrees F and the surface to 500 mb Precipitable Water Index maxima for the state is 1,54 inches, It is interesting to note that no strong storms or floods were reported to. have occurred with either of these maxima. Dr, Thomas McKee (1996, personal communication) recently observed that one of the results of the CSU Extreme Precipitation Event study was that the strcmgest thunderstorm flllodin!l events did not occur in the wettest atmospheres but rather occurred in ones with strong dynamic forcing features ;and moderate moisture. 12