P8.2 An investigation of record heavy lee side orographic snowfall during the great western storm of March 16-20, 2003

Monday, 21 June 2004
Gregory S. Poulos, NCAR, Boulder, CO; and J. Snook, D. A. Wesley, M. P. Meyers, and E. J. Szoke

This winter storm, as it progressed across the southern and central Rocky Mountains during 16-20 March 2003, was characterized by tremendous forcing both synoptically and orographically. The official Denver snowfall occurs ~ once per 50 years statistics for total snowfall (with foothill locations ~ 2 m on the upwind side of the barrier). In addition to these superlatives, the system was marked by unusually deep snowfall in the lee of the barrier also approaching 2 m, the mechanisms for which are explored herein.

Overall, this system was forced on the large scale by the development of a very strong cutoff low over the southwestern US which progressed slowly eastward over 4 days. Such a configuration will produce strong Front Range upslope flow over Colorado and Wyoming. A critical aspect of the synoptic, deep easterly flow was its fetch nearly directly out of the Gulf of Mexico region, leading to large-scale transport of copious amounts of moisture into an upslope region. Typical so-called upslope snowstorms generate snowfall that is quite heavy on the upwind side of the barrier due to orographic intensification of uplift, but only 5%-25% of the upwind-side maximum falls downwind as orographic forcing of uplift wanes and downward vertical motion warms and dries the air. In Front Range upslope storms there is typically an even greater disparity in snow water equivalent as less dense snow falls in the lee. We utilize intercomparisons of different operations numerical weather prediction models and high-resolution MM5 numerical studies (at 1.5 km horizontal grid spacing) to investigate this feature. On the basis of those analyses, we hypothesize that the stable atmospheric structure that persisted to over 300 mb during this storm enabled microphysical processes that form snow to continue well across the barrier. The Continental Divide, and thus the barrier, averages 3500 - 4100 m in this region. We also investigate the mechanisms that cause a rapid fall-off in snowfall further west and also north and south of the affected region.

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