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<br />thunderstorms were observed to the southwest in the direction of Masonville on <br />both the 9th and the 10th of September. These observations incr,ease the <br />likelihood that the Masonville Storm rainfall fell over a period of 26 hours <br />rather than 60 minutes as reported in HMR 55A. <br /> <br />HMS transposed the Masonville storm as the controlling local storm but is <br />concerned that the values obtained overstate ttle actual threat to the Mason <br />Reservoir drainage basin. HMS has recommended to the Colorado Climate <br />Center that the Masonville storm be examined in detail for rainfall adjustment <br />based on the results of the study presented in this site-specific PMP study. <br /> <br />4.2 HMR 55A site-specific general storm PMP <br /> <br />The site-specific general storm computation is based on the maximization and <br />transposition of the historic general storms identified in Table 2 with application <br />of elevation adjustments. Table 6 identifies the storrns and their HMR 5~jA <br />classification. Note that most of the general storms are dominated by convective <br />or thunderstorm-generated rainfall rather than a steady general rain associated <br />with a large cyclonic storm system or frontal zone. <br /> <br />Table 6: HMR 55A classification of historic !~el1eral storms. <br /> <br /> <br />Bi <br /> <br />Bi <br /> <br />* Che Creek, CO 5-30131-'35 <br />Plum Creek;, CO 6-15117;;SS <br />* Note that these storms are outside <br /> <br />R 55A classification <br />lid Front-Orog@Elhic <br />-- <br />Pressure-Orographic <br />Pressu re-Orog raph ic <br />x Convective-Oro..9..':'!2hic <br />x Convective-Orogr'!2hic <br />.x Convective-Oro..9..':~phic <br />:onvective-Least On~raohic <br />'onvective-Least Oroaraohic <br />?sition envelope <br /> <br />HM <br />Cc <br /> <br />Low <br />Low <br /> <br />___fgmple <br />C~mple <br />Comple <br /> <br />Complex C <br />~plex C <br />the transp <br /> <br />The historic general storms are dominated by complex convective storm <br />systems with both orographic and least-orographic qualifiers. HMS applies <br />the HMR 55A elevation reduction to all the complex convective historic storm <br />systems after standard maximization is applied. Note that the Cherry Creek <br />storm and the Gibson Dam events are included for comparison purposes since <br />both are on the edge of the transposition envelope for the Mason F~eservoir <br />drainage basin. <br /> <br />30 <br />