Abstract
The widespread decrease in mountain snowpack across the Western United States is a hallmark indicator of regional climate change. Observed decreases in snowpack across lowerβelevation watersheds are broadly consistent with model predictions of anthropogenic climate change; however, the magnitude of the decreases across much of the Cascades, Northern Rockies and Sierra Nevada has exceeded predictions based solely on late 20th century anthropogenic warming. To resolve the disparity between observations and predictions, the influence of intramonthly variability associated with the PacificβNorth American (PNA) pattern on both the percent of precipitation falling as snow and a proxy for snowmelt during late winter is examined. The PNA is shown to have a significant influence on the elevation of the freezing level across the Western United States, with positive values of the PNA generally associated with anomalously high freezing levels. The positive tendency of the PNA during late winter over the last halfβcentury has contributed to regional increases in the elevation of the freezing level, decreases in the percent of precipitation falling as snow, and increases in snowmelt across montane and subβmontane regions. This change in atmospheric circulation accelerated the decline in mountain snowpack predicted by anthropogenic forcing. Copyright Β© 2010 Royal Meteorological Society