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5. The occurrence of the "train-echo etfect" twice in the first 15 hours of the general storm. This amounts to allowing two groups of tw,o thunderstorms each to occur over the draina!~e basin: Following Bertie (1982) and reflecting the sequence of events 4Jbserved during th4~ 1965 Plum Creek event, the rainfall from the second thunderstorm in E!ach group is set equal to 10 per cent of the peak 1-hour rainfall from the first thunderstorm of each group, 6. The second 6 hours of day 1 assumE~S a rain rate of 0.5 inches pE~r hour (taken from the Big Elk Meadow evellt) Clind the final 12 hOlJlrsassumes a rate of 0.4 inches per hour (also takEtn from the Big Elk Meadow I~vent) assuming all precipitation falls in th4~ form of rain, and 7. Rainfall on day 2 and 3 ofthe general storm emulate the Big Elk Meadow rain rates assuming all prec:ipit,ation falls in the form of rain. The HMS PMP general storm total event is obtained by combining or compositing one or more historical events. The combination of thE' double train- echo thunderstorm rainfall occurring in cornbination with the heavy high elevation rain rates of the May 1969 Big Elk Meadow stOl'm is very severe. Additionally the continuing moderate rainfalls over 48 hours is also severe but consistent with the duration of the May 1969 storm rainfall. The same additional assumptions about 1000 mb dew point, PWI reductions and elevation adjustments were applied to the general storm cases. The application of the extreme precipitation event atmospheric data and the Convective Storm Methodology have resulted in significant reductions in the site- specific local storm and general storm PM!" compared to the HMR PMP values. Despite these reductions, HMS believes that its calculated values are still conservative. The primary difference between the HMR and HMS PMP values is due to the use of different PWI values used in the two methodologies. One of the reasons for this difference is that the HIVIR values were obtained assuming moist adiabatic conditions to 50,000 feet ITlsl while the HMS values were obtained taking this assumption to 18,000 feet msl. The HMS treatment of atmospheric moisture is supported by findings of both the historical storm search reported in the presents study and by the results of the CSU Extreme Precipitation Study. Both studies found that extreme rain events do not occur with totally saturated atmospheres but rather with atmospheres with very moist conditions iinto the middle levels overlain by drier air aloft. Additionally, the use of the updraft warm layer depth used in the CSM is based on a reliable empirical relationship for estimating the maximum rainfall potential for a particular atmosphel"ic: structure. The use of enhanced data sets and improved methodology when lIsed in concert with the HMR standard methodology provides a valid framework for estimating 40