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<br />5.1 HMS site-specific local storm PMP <br /> <br />The calculation of the HMS site-specific local storm PMP utilizes HMS <br />Convective Storm Methodology (CSM) wit.h components of the historical Extreme <br />Precipitation event Atmosphere (EPA) and climatic extremes. In effect the EPA <br />emulates the HMR's historical storm review and includes consideration of the <br />atmospheric structure through the depth of each storm. The EPA was described <br />earlier in Section 2.2 of this report. The EPA is used to identify the "worst <br />case" individual components of the atmospheric structure of the historical <br />extreme precipitation events. The Extreme Thunderstorm Atmosphere (ETA) <br />consists of "piecing together" the components from t.he local storms and complex <br />convective general storms which contributed to peak 6-hour or less local storm <br />type rainfall. Again, this is analogous to the discussion of PMP contributions <br />from "among storms" found in Section 10.1.1 of HMI~ 55A <br /> <br />ETA components and storm characteristics used by HMS in the present study <br />are listed below: <br /> <br />1. Surface conditions: temperature 80 degrees F (ETA average) and <br />surface (6,000 feet msl) dew point 615 degrees F (taken from the <br />Colorado Springs climatological maxima), <br /> <br />2. Vertical temperature, moisture and wind! structure of the Big Thc)mpson <br />Canyon event above 700 mb and of the Plum Creek event below 700 mb <br />(climatic extremes for both atmospheric layers), <br /> <br />3. Precipitable Water Index (PWI)::: 1.63 inches (obtained aSSiumin!~ a <br />saturated atmosphere from the surface (6000 feet msl) to 118,000 feet msl <br />using a 66 degree F surface dew point), and <br /> <br />4. Storm characteristics: train-echo effect which allows a second <br />thunderstorm with 70 per cent of thE! first thunderstorm peak 1 hour <br />rainfall to cross the basin during a 6 hour period. (modeled after the <br />Plum Creek and Big Thompson Canyon historical events). <br /> <br />Surface conditions are based on the ETA average surface temperature for the <br />extreme historical events and the climatological extreme surface dew point. The <br />vertical structure of the atmosphere is a composite of the worst case <br />atmospheric temperature, moisture and wind field observed in the historical <br />storms for two layers of the atmosphere: the lowest 150 mb and the layer <br />extending from the top of this lower layer to 18,000 feet msl. The lowest 150 mb <br />layer has been identified by numerous authors as a key portions of the <br />atmosphere which contributes to moisture flux into thunderstorms and other rain <br />producing cloud fields. In the case of the Colorado Front Range, the 700 mb <br />level which is about 150 mb above the surface was chosen as the top of this <br />lower layer for the present study. Table 3 shows the characteristics used to <br /> <br />35 <br />