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<br />I: <br /> <br />.' . -. <br />. .o.~~" ....,-',;'i::.U~t~~~,_.. <br /> <br />The surface southwesterly winds in Utah were attempting to advect air with dew points in <br />the 55 to 62 degree F range into southwestern Wyoming ahead of the surface trough. <br />Topography rises from about 4,500 feet msl near Salt Lake City to about 8,500 feet msl <br />30 to 50 miles west to southwest of Opal. It then descends to about 6,900 feet msl near <br />Opal. If the air mass over Salt Lake City (T=75 degrees F and Td = 63 degrees F) is <br />lifted over this topographic balTier, thunderstoml formation occurs with cloud bases at <br />about 9,200 feet MSL. Given thc generally light south-southwesterly winds of 10 knots <br />in the cloud steering level of 700 mb to 500 mb, the developing storms would have <br />moved generally to the north well west of Opal. The NWS reported strong stonn activity <br />that produced highway flooding about IS miles west of Kemmerer, Wyoming with strong <br />radar echoes about IS miles south of Kemmerer at ] 552 LDT or 2152 UTC. --I!le~_ <br />storms were not associated with the Opal flash flood. However, moist outflowJrom <br />these storms could have fed moist surface air with tem{Jeratures in the low 70's and <br />dew {Joints of 53-56F from the west into the OMI area about the time the.JU!lstJront <br />arrived from the south. <br /> <br />The August 16, 1990 1200 UTC 850 mb, 700 mb, 500 mb and 300 mb charts show a <br />well-defined thernlal short wave and associated surface trough approaching southwestern <br />Wyoming from Utah and Idaho. The short wave produced about 2 degrees C cooling in <br />the 700 mb-500 mb layer with passage and advected in copious amounts of middle-level <br />moisture. The August 17, 1990 0000 UTe (1800 LDT on August 16) soundings for <br />Lander (LND), Salt Lake City (SLC) and Grand Junction (GJT) can be seen in <br />Figure 13. Each of these soundings is unstable and very moist. The surface to 500 mb <br />precipitable water index is estimated at 1.04" for Opal. Key factors favoring the <br />fonnation of thunderstorms with locally heavy rain are the approach of the themlal short <br />wave with its favorable destabilizing effects and the fetch of moist air from the southwest <br />to feed stOlm development. These soundings were loaded into the NWS RAOB program <br />and an objective Opal sounding was created. <br /> <br />Analysis of this upper air sounding for Opal to simulate pre-storm conditions is <br />accomplished by lifting a surface parcel with a temperature of 75 degrees F and dew point <br />of 54 degrees F. The resulting thunderstonn forms with cloud bases near 9,000 feet msl <br />and storm tops near 45,000 feet msl. The resulting cloud depth is 10.2 km and warm <br />layer in the cloud updraft is about 2.2 km which is substantially greater than the <br />minimum 1.5 km warm layer depth needed for enhanced wann coalescence rainfall. The <br />average temperature excess of the updraft is approximately 7 degrees C. This <br />temperature excess is greater than the 6 degrees C excess associated with severe <br />thunderstOlms in the western United States. <br /> <br />As earlier noted, the vertical wind shear is light with an average cloud layer wind speed <br />about IS knots or 7 mps. Sub-cloud layer winds are about 20 to 30 kuots giving the <br />atmosphere an inverse wind shear which is conducive to the formation of slow <br />moving thunderstorms with locally heavy rainfall. Such inverse wind shear was also <br />present in the Rapid City Flash Flood of 1972, the Big Thompson Flash Flood of 1976 <br />and the Frijole Creek Flood of 1981. Each of these storms displayed a multi-cell <br /> <br />27 <br />