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<br />encountered a mesocyclone and intensified into a supercell in the southwest quadrant of <br />the meso-low. Air to the north ofa stationary fTont nOlth of Cheyenne was too cool to <br />sustain strong convective updrafts and the superceU remamed almost stationary over the <br />City for nearly 2 hours. Eventually, the storm was kickec. away to the east southeast of <br />the City as a short-wave trough arrived. <br /> <br />Chappell and Rodgers (ibid) offer the opinion that this type of extreme thunderstonn <br />event can pose a heavy rainfall and flash flood threat for the eastern foothills of the <br />Rocky Mountains from Montana southward through Wyoming, Colorado, and New <br />Mexico. When such moist low-level encounters particularly abrupt increases in <br />elevation, events such as the Pemose and Big Thompson Canyon flash floods occur when <br />storms become "locked in" and remain nearly stationary for a few hours UndtT weak mid- <br />level steering currents. And, as shown by this Cheyenne stOlID, even rather modest <br />telTain elevation changes can also channel the low-level flow into prefelTed areas. <br /> <br />Applying the HMS CSM to the Denver evening sounding for 0000 UTC on August 2 <br />with a surface temperature of 82 degrees F and a surface clew point of 59 degrees F shows <br />a convective warm layer of2.4 kilometers. According to the CSM, such a deep wam1 <br />layer is conducive to enhanced precipitation through coalescence processes. The CSM <br />estimates the hourly rainfall rate at 3.9 inches or about 7.8 inches for a storm nearly <br />stationary for 2 hours. This amount compares well with observed values. <br /> <br />3.8 Opal, Wyoming; August 16, 1990 <br /> <br />This event is chronicled in Storm Data.. A National Lightning Detection Network <br />(NLDN) Lightning Report shows heavy activity (569 flashes) within 25 miles of Opal <br />between 1600 and 1900 LDT on August 16, 1990 (see Figure II). <br /> <br />Opal is situated on a broad plateau in extreme southwestem Wyoming at an elevation of <br />6,912 feet msl (see Figure 12). Higher mountainous ten'ain is located 30 to 50 miles to <br />the south through west of the town. <br /> <br />Heaviest rainfall associated with the Opal storm rainfall was reported to accumulate to <br />7 inches between 1610 and 1745 LDT. If this rainfall is true, then it represents one of <br />the heaviest local rains repOlted west of the Continental Divide. A general description of <br />the storm and its associated meteorology can be found in Corrigan and Vogel (1993). <br />However, this paper leaves many questions about the stonn unanswered. The reader is <br />referred to Appendix C for a copy of this paper. <br /> <br />The Opal storm is one of the most significant Ilocal storm events cited in recent local <br />Site-Specific PMP studies for dam rehabilitation and F'MP calculation. HMS used <br />the following data sources to investigate the stonn's OCCUlTence and attempt to define its <br />rainfall aerial coverage and time distribution: <br /> <br />I. Conventional surface and upper air observations taken by the National Weather <br />Service (NWS) for Wyoming, Colorado and Utah, and <br /> <br />23 <br />