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<br />4.1 HMR 55A site-specific local storm PMP <br /> <br />The site-specific local storm PMP was produced by transposing the historic local <br />storms in Table 1 to the 11,600 feet Mason Reservoir drainage basin after <br />maximizing the storm precipitation and then applying elevation adjustments, <br />Each of the storms transposed, except for the Opal, Wyoming event, was a <br />strong isolated supercell thunderstorm which was "locked" into local topography <br />by low level inflow winds, An upper level disturbance was the triggering <br />mechanism along with local heating which allowed the storms to maximize over <br />the basins affected. The small size of the Mason Reservoir basin allowed <br />relatively easy transposition of the storms. <br /> <br />In-place moisture adjustments to the historical storms rainfall followed <br />Equation 8-1 in HMR 55A. Precipitable water was calculated usin(j the storm 12- <br />hour 1000 mb persisting and historical maximum 12-hour 1000 mb persisting <br />dew points from Table 5.1 in HMR 55A and from Figures 4.5 to 4,16 (for the <br />historical maximums) in HMR 55A. For recent storms, measured maximum <br />storm dew points were taken pseudo-adiabatically to 1000 mb. Computation of <br />the precipitable water followed the moist adiabatic assumption, An in-place <br />moisture adjustment was then applied by multiplying the observed storm rainfall <br />by the in-place moisture adjustment factor to obtain the maximized storm rainfall. <br />An elevation adjustment factor was thEm calculated in the form of a reduction <br />adjustment using Figure 14.3 in HMR 55A and the maximum 12-hour 1000 mb <br />persisting dew points. This technique corresponds to reducing the storm rainfall <br />by 9 per cent for every 1,000 feet of ascent above 6,000 feet msl The elevation <br />adjustment was multiplied by the in-place storm maximized rainfall to get the <br />transposed storm PMP rainfall. <br /> <br />Table 4 shows the results of the in-place maximization and transposition of each <br />historical local storm to the Mason Reservoir basin. The dominating storm <br />appears to be the 1938 Masonville Storm with an estimated 6 inches of rainfall in <br />1 hour. However, the rainfall values oi:ltained have to be transformed into <br />incremental rainfall values for a 6-hour period to determine the dominating local <br />storm for PMP, <br /> <br />Table 5 shows the depth-duration or incremental rainfall for each of the storms <br />based on a 6-hour to 1-hour storm ratio of '1,35 using Table 12.4 (HMR 55A), <br />The extension of the storms to 6 hours results in an 8.13 inch per Ei hour local <br />storm PMP, It is curious that the rainfall potential for the Masonville storm IS 46 <br />per cent greater than any of the other local storms. Considering only the storm's <br />atmospheric structure. which is not as volatile compared to the other storms in <br />Table 3, this result was not anticipated. <br /> <br />26 <br />