84th AMS Annual Meeting

Thursday, 15 January 2004: 11:45 AM
Utah's 100 Inch Snowstorm: Structure, orographic precipitation processes, and implications for the NWS IFPS era
Room 6A
W. James Steenburgh, NOAA/CIRP and University of Utah, Salt Lake City, UT
This presentation examines the synoptic, orographic, and lake-effect precipitation processes during the largest winter-storm cycle observed over the Wasatch Mountains of northern Utah in the past 10 years. The storm cycle, which occured from 22-27 Nov 2001, included two distinct storm systems that produced 108 inches (274 cm) of snow at Alta Ski Area, including 100 inches (254 cm) during a 100 h period. Hence, the storm cycle has been coined the "100 inch storm." Each storm system featured an intrusion of low equivalent potential temperature air aloft, well in advance of a surface-based cold front. Prefrontal orographic precipitation became increasingly convective as the low equavalent potential temperature air aloft moved over northern Utah, while cold-frontal passage was accompanied by a convective line and stratiform precipitation region. Postfrontal destabilization led to orographic and lake-effect snowshowers that produced two-thirds of the observed snow-water equivalent at Alta.

Storm stages were defined based on passage of the above features and their accompanying changes in stability and precipitation processes. Contrasts between mountain and lowland precipitation varied dramatically from stage-to-stage and storm-to-storm, and frequently deviated from that expected from climatology, which predicts a four-fould increase in precipitation between the lowlands and the Wasatch Crest. Based on the two storms, as well as other studies, a conceptual model has been developed that summarizes the evolution of Intermountain snowstorms featuring an intrusion of low equivalent potential temperature air aloft ahead of a surface cold front. This conceptual model will be compared with prior conceptual models of orographic precipitation by Marwitz, Hobbs, and others, and implications for short-range quantitative precipitation forecasting, precipitation downscaling, and the generation of high-resolution gridded precipitation forecasts by the National Weather Service IFPS system, will be discussed.

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