Widespread Declines in Snowfall Fraction over Major US Mountain Ranges

Our understanding of present-day total snowfall and snowfall fraction, as well as their trends, is complicated by a lack of precipitation phase observations and the wide variety of modeling methods used to partition rain and snow from land surface meteorological quantities. In this work we applied a selection of the best-performing precipitation phase partitioning methods to 35 years (1984-2018) of daily PRISM climate data at a 4-km resolution across the contiguous United States. We evaluated ensembles of total snowfall and snowfall fraction, weighted by method success rate, as well as the methods individually. Trend magnitudes were estimated using Sen's slope method, while trend significance was assigned using the non-parametric Mann-Kendall test. Unsurprisingly, baseline total snowfall and snowfall fraction increased with elevation and latitude, with the highest observed values in the upper elevations of the Cascades, Rockies, and California's Sierra Nevada. Variability in total snowfall—quantified as the standard deviation across all methods—was greatest in the mid-elevations of the Cascades, Olympics, Sierras, and the Coast Range of Northern California, areas characterized by winter air temperatures near freezing and heavy, seasonal precipitation. Conversely, the greatest baseline variability in snowfall fraction was found in the semi-arid Great Basin area of Nevada, California, and Utah. Significant trends in total snowfall were predominantly negative, with middle-elevation areas seeing the greatest declines. In some parts of the Washington Cascades, ensemble trends showed coherent, significant losses of snowfall approaching 100 mm per decade over the study period. Significant snowfall fraction trends were almost always negative with every major US mountain range impacted, from the Appalachians and Adirondacks on the East C­­oast to the previously mentioned western ranges. Concerningly, areas experiencing the greatest snowfall fraction declines, nearing -0.1 per decade, included the basins that comprise the major reservoirs of the Cascades and Sierra Nevada. This finding suggests that snow-to-rain shifts are having an outsized impact on the western US water storage system relative to other areas.