Anomalously large snowfall variations across small spatial scales in the Rocky Mountain Front Range: the roles of blocking, surface winds, and the associated thermodynamic structure

During a vast winter storm that progressed across the southern and central Rocky Mountains during 16-20 March 2003 unusually large variations in local snow amounts occurred in the Front Range region near Denver, Colorado. The official Denver snowfall fell into the 1-in-50 year statistics for total snowfall (with foothill locations exceeding 2 m) but some areas at the same elevation received under 0.10 m. For obvious reasons, public impacts varied greatly between those areas where snowfall was of near-record depth and those where it was hardly notable. We investigate the meteorological and physiographic causes of these snow minima which are present on the meso-gamma-scale and are not able to be captured at typical operational numerical weather prediction model resolution.

We show that in these snow minima, locally 1-2 C warmer temperatures delayed the changeover to snow and, even when that changeover occurred, it was very wet snow that seldom accumulated (and quickly melted). As a result of this local warming, surrounding areas within 10 km that received roughly similar amounts of liquid precipitation but were colder, received snowfall of over 24”. Elevation alone does not differentiate these areas of snow minima from surrounding terrain. Because of the prominent role played by the barrier jet in advecting cold air southward along the Front Range, we hypothesize that the locally more east-west oriented ridges north of the snow minima were responsible for physically and dynamically (through downslope warming) preventing the rapid cooling experience by other locales. Combined with the local concavity of the foothills in this region it is plausible that temperatures could remain locally elevated by this mechanism. We also speculate that, in the easterly flow, downslope warming induced by local terrain or the deviated position of the cold air dome of the barrier jet may also have contributed. However, the detailed shape of the snow minima shown in satellite image cannot be explained by these concepts. Using high resolution (1.5 km horizontal grid spacing) MM5 and (2 km) Workstation Eta modeling studies underway we clarify the mechanisms responsible for these extreme snowfall variations.