Meteorological analysis of the Cheyenne, Wyoming, flash flood and hail storm of 1 August 1985
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Meteorological analysis of the Cheyenne, Wyoming, flash flood and hail storm of 1 August 1985

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    Analysis of a devastating storm that struck Cheyenne, Wyoming, on 1 August 1985 shows that the storm began as an east-west multicellular system just south of the city near the summit of the Cheyenne Ridge. This system developed in a conditionally unstable air mass that formed over southeast Wyoming as a southeasterly flow of very moist air at low levels became juxtaposed with an area of steepening lapse rates to the west. Early cells drifted slowly northward in agreement with the pressure-weighted vector mean wind of the environment. New convective growth on the southwest flank of this multicellular system eventually produced a wave-shaped convective system, which rapidly developed supercell structure. As the supercell began to rotate, the storm became stationary over the city for nearly 2 hours. This lack of motion is believed to have been due to helicity, which promoted the transverse propagation of the supercell's updraft at a rate that counteracted the effects of the vector mean wind of the environment. The storm began to move southeastward with the arrival of a short-wave trough and soon dissipated as it encountered increasingly stable conditions. The results of the study suggest that the eastern foothills of the Rocky Mountains and the adjacent high plains may be particularly vulnerable to this type of storm. Deep convection frequently occurs over this area when moist air arrives from the Great Plains, driven by a low-level easterly jet. The combination of strong low-level easterly flow topped by weak middle-level southerly flow can apparently produce sufficient wind shear for supercell formation, while producing a vector mean wind for the environment that gives little or no eastward motion relative to the ground.
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