Laserfiche WebLink
<br />thunderstorms were reported with this stolTn event but rainfall rates and amounts <br />over the 72-hour duration of the storm were very hi~lh. It is possible that <br />unreported embedded thunderstorm rainfall occurred during the first 24 hours of <br />the storm with the storm focused over portions of the Front Range with no <br />observing stations, <br /> <br />Figure 5 shows the Bureau of Reclamation "rain bucket survey" of the Big Elk <br />Meadow storm melted total precipitation, The heaviest reported precipitation <br />totals at altitudes above 11,000 feet were 6 to 7 inches in 72 hours in portions of <br />western Jefferson and Boulder Counties, All but the first 12 hOUlrs of <br />precipitation of the storm would have 1'allel1 as snow at Mason ReslmfOir's <br />elevation of 11,600 feet. The relevance of the precipitation pattern and the cold <br />structure of the storm are at odds in deciding how to utilize this storm in <br />developing a site-specific general storm scenario for Mason Reservoir. Wal'ming <br />the vertical temperature profile of the atmosphere to 15.000 feet by 4 degrees <br />Celsius would have allowed all precipitation to fall as rain at this elevation. <br />However, such a warming of the atmosphere mi!~ht be accompanied by <br />thunderstorms which in turn would affect the general storm precipitation <br />pattern, This adjustment is comparable to moving the storm 15 days into the <br />warm season, which is a common site-specific PMP practice that assumes such <br />warming does not have important consequences for storm dynamics and <br />precipitation efficiency. <br /> <br />HMS includes the occurrence of embedded thunderstorms on two <br />consecutive days over the same basin as part 01' its general storm PMP <br />scenario, The Plum Creek storm (see Figure 3 and 4) in eastern Colorado <br />featured excessive thunderstorm rainfall on both the 16th and 17tt1 of June 1965 <br />supporting this decision. Additionally, HMS incorporates the rainfall spatial <br />coverage, duration, intensity and amounts observed in the Big Elk Meadow 1969 <br />storm, The merger of these two Colorado events to generate guidance for a <br />general storm PMP results in a conservative scenano which is supported as <br />plausible by the historic extreme precipitation events. This methodology is <br />analogous to the "among storms" PMP discussion found in Section 10,1,1 of <br />HMR 55A, <br /> <br />16 <br />