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<br />..-:-<;".-'~:.r,..:, ,,;,:,~, =c<r,___.-.' <br /> <br />that make up a GCS is less thau 24 hours and the total stonn area is on thc order of a few <br />thousand square miles. <br /> <br />HMS believes tbat this GCS should be evaluated further. A preponderance of <br />radar, satellite, and conventional data are available wb.ich can be combined with <br />paleohydrologic data to better quantify this event. This GCS and the associated <br />Frijole Creek Basin l1ash 1100d conld become a landmark storm for consideration in <br />fntnre PMP studies. <br /> <br />3.7 Cheyenne, Wyoming; August 1, 1985 <br /> <br />A heavy thunderstonn system became nearly stationary over the City of Cheyenne for <br />about 2 hours during the early evening of August I, 1985 (Chappell and Rodgers, 1987). <br />This event resulted in a flash flood and hailstorm leading to the loss of 12 lives and to <br />property damage in excess of$65 million. About 8. inches of rain officially was <br />measured at the Weather Ser/ice Forecast Office with about 4 inches falling in a ] -hour <br />period from 0200 to 0300 UTC on August 2 (2000 to 2100 MDT on August I). This <br />stonn set new 24-hour and I-hour rainfall records for the State of Wyoming. As is often <br />the case with thunderstonns that produce very intense rainfall, severe weather in the form <br />ofIarge hail to 2 inches in diameter and fUllilel clouds accompanied the stom!. Drif\s of <br />hail 3 to 6 feet deep occuned and did not completely melt for 2 days. <br /> <br />Flooding began soon after the rainfall staried and eventually affected most ofthe City. <br />Particularly hard hit were Dry Creek in the northem parts of the City and Crow Creek in <br />the southem pari of the City. Figure 10 shows the topography of Cheyenne and vicinity. <br />As can be infened from Figure 10, topography likely influenced the fonnation and <br />subsequent history ofthis intense stonn. In particular, the City lies irt the flat valley of <br />Crow Creek which flows east-southeastward across southem sections of Cheyenrte. Dry <br />Creek, a tributary of Crow Creek, flows through northern sections of Cheyclme before <br />joining Crow Creek a few miles east ofthe City. <br /> <br />The Cheyenne Ridge rises 200 to 600 feet above the City about 6 miles to the south. It is <br />oriented west-northwest to east-southeast with tenain lowering 950 feet as one moves 30 <br />miles away from the Ridge toward the southeast. Moist low level air flowing out of the <br />south-southeast will be lifted by the Cheyenne Ridge causing convective cloud <br />development along the Colorado-Wyoming state line southwest of Cheyenne. <br /> <br />The Cheyenne thunderstorm system was part of a GCS which developed in a <br />conditionally unstable air mass as a low-level jet (LU) with speeds in excess of 55 mph <br />streamed out of the south-southeast (see Figure 10). A similar strong low-level flow was <br />present during the June 1965 Colorado floods (see Section 3.3). This LU interacted with <br />a low-level circulation in the lee of the Cheyenne Ridge. Such a circulation fomls <br />routinely when strong southeasterly low-level winds are present, much like the scenario <br />accompanying the fonnation of the so-called Denver Cyclone. A thunderstorm fOlmed <br />over higher tenain west of the Cheyenne Ridge. This storm eventually detached itself <br />from the terrain and moved over the City of Cheyenne. Once over the City, it <br /> <br />21 <br />