RAL Seminar: Cool-season orographic snowfall extremes in the central Wasatch Mountains, Utah, USA

Abstract:
Heavy orographic snowfall can disrupt transportation and threaten life and property in mountainous regions but also provides benefits for water resources, winter sports, and tourism.  Frequently experiencing orographic snowfall extremes, Little Cottonwood Canyon (LCC) in the central Wasatch mountains of northern Utah is one of the snowiest locations in the interior western United States.  Due to a high density of avalanche paths, heavy snowfall, and extensive vehicle traffic, state route 210 (SR-210) in LCC has the highest uncontrolled avalanche hazard index of any major road in the world, with frequent closures and threats to public safety.  Using manual 12-h snow and liquid precipitation equivalent (LPE) observations, ERA5 reanalyses, and operational radar data, this presentation examines the characteristics of cool-season snowfall extremes in upper LCC.  Nearly all cool-season precipitation in upper LCC falls as snow, and 12-h extremes, defined based on new snow or LPE, occur for a wide range of crest-level flow directions, with the former most frequent during west-northwesterly flow and the latter most frequent during south-southwesterly flow.  Both are produced by several synoptic patterns, including high integrated vapor transport (𝐼𝑉𝑇), frontal systems, post-cold-frontal northwesterly flow, and closed low pressure systems.  Post-cold-frontal northwesterly flow events efficiently convert 𝐼𝑉𝑇 to precipitation and can be highly localized, with radar echoes confined to the central Wasatch.  Depending on the atmospheric environment, high 𝐼𝑉𝑇 can either produce no LPE or heavy LPE.  These results illustrate the remarkable diversity of mechanisms for orographic snowfall extremes in LCC