ACINN Graduate Seminar - SS 2023

2023-06-28 at 12:00 (on-line and on-site)

Avalanches, Cool-Season Orographic Precipitation Extremes, and Deep-Powder Skiing in Little Cottonwood Canyon, Utah

Jim Steenburgh and Michael Wasserstein

Department of Atmospheric Sciences, University of Utah, USA


The Wasatch Range is a narrow, meridionally oriented mountain range in the North American interior that rises 2000 m locally to 3500 m MSL. The central Wasatch Range broadens southeast of Salt Lake City, Utah and contains a series of zonally oriented alpine ridges, creating a region of high topography exposed to flow from many directions. Snowfall and liquid precipitation equivalent (LPE) maximize in the high terrain near Little Cottonwood Canyon (LCC) where mean annual snowfall exceeds 1250 cm and a major highway ascends >1000 m in 11 km to Alta Ski Area, traversing 50 avalanche paths. Due to heavy traffic and a lack of snowsheds and other avalanche protection structures, the highway has the highest uncontrolled avalanche hazard index of any major road in the world. Using a 23-year record of 12-hour manual snow and LPE observations from Alta, ERA5 reanalyses, and operational radar data from a US National Weather Service radar, we analyze the synoptic and mesoscale characteristics of cool-season (October–April) precipitation extremes in upper LCC, where most of the cool-season precipitation falls as snow. Major precipitation events occur over a wide range of 700-hPa (near-crest-level) flow directions. Major snowfall events are most frequent, however, in northwesterly flow, whereas major LPE events are most frequent in southwesterly flow. Precipitation efficiency, defined as the fraction of vertically integrated water-vapor flux converted to precipitation during storm periods, is highest during northwesterly flow, with high snow-to-liquid ratios yielding low-density snow and large snowfall accumulations. Southwesterly flow contains larger vertically integrated water-vapor fluxes and LPE, but snowfall is limited by smaller snow-to-liquid ratios. Regionally, there are two pathways for water vapor penetration to the central Wasatch. One is across the relatively low northern Sierra Nevada of northern California and southern Cascades of southern Oregon, the other through the lower Colorado River Basin. Both avoid the high terrain of the Sierra Nevada in southern California. During southwesterly flow, precipitation systems have broad coverage with a high frequency of radar echoes in the central Wasatch and surrounding mountains and valleys. In contrast, during periods of northwesterly flow, precipitation enhancement in the central Wasatch and LCC can be highly localized due to orographic forcing or lake-effect processes. These results illustrate the large variety of storm types that produce heavy precipitation in the central Wasatch and LCC, including local forcing mechanisms and key vapor-transport pathways enabling heavy precipitation over an interior, continental mountain range.




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