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<br />construct the PMP atmospheric sounding. This inclusion of all the historical <br />storm atmospheres which produced extreme thunderstorm rainfall in 6 hours or <br />less assures that the HMS local storm PMP reflects the "worst case scenario" for <br />local storm rains. Unlike the methodology in HMR 55A, this HMS methodology <br />permits the occurrence of complex convective storms over the Mason Reservoir <br />drainage basin. <br /> <br />The next step in the HMS PMP methodology attempts to "maximize" the <br />precipitation production in place and then transpose this maximized production to <br />the high elevation Mason Reservoir site. The components of the ETA are <br />combined to produce an atmospheric sounclin~J which becomes thl, Local Storm <br />PMP Atmosphere. The Local Storm PMP Atmosphme is used to Galculate the <br />local storm PMP based on Equation 1 below where 1:'WI refers to the <br />Precipitable Water Index: After applying the standard HMR PWI mduction of 9 <br />per cent per 1,000 feet (beginning at the 6,000 feet rnsllevel), the PWI becomes <br />1.28 inches for the Mason Reservoir draina~Je basin at 11,600 feet msl. <br /> <br />(1) Peak 60-minute rain = PWI times D1egttl of updraft warm I,ayer times 2 <br />1.5 km <br /> <br />The depth of the updraft warm layer required in Equation 1 must be modified to <br />take into account the high elevation of the Mason Reservoir drainage basin. The <br />total depth of this warm layer is 2.9 km (after Henz and Kelly, 1985I). However, <br />as seen in Figure 9, the Mason Reservoir drainage basin is situated <br />approximately 0.6 km into this warm layer reducing its effective depth to :2.:3 km. <br />This reduction is consistent with HMR 4~levation <Idjustment te'chniqUl~s. In <br />fact, HMR 57 references the findings of Henz and Kelly (1989) as consistent <br />with its elevation-based moisture reduc:tion rationale. <br /> <br />36 <br />