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In western North America (WNA), mountain snowpack supplies much of the water used for irrigation, municipal, and industrial uses. Thus, snow droughts (a lack of snow accumulation in winter) can have drastic ecological and socioeconomic impacts. In this study, the historical (1951-2013) frequency, severity, and risk (frequency x severity) of dry, warm, and warm and dry snow droughts are quantified at the grid-cell and ecoregion scale for snow-dominated regions in the western United States and southwestern Canada (sWNA). Based on multiple linear regression analysis, relationships between mean winter temperature, snow drought risk, and snow water equivalent sensitivity are explored. Piecewise linear regression is used to identify temperature thresholds for mapping temperature-related snow drought susceptibility. Results highlight spatial differences in snow drought regimes across sWNA and reveal that temperature thresholds exist at -3.1 degrees C (+/- 0.3 degrees C) and 1.4 degrees C (+/- 0.3 degrees C), above which the warm snow drought risk increases more rapidly. Approximately 3% of the nonglaciated snow storage in this region has high susceptibility to temperature-related snow drought, representing 11 km(3) of water, or approximately one third the capacity of Lake Mead. Under a +2 degrees C climate scenario, an additional 8% (28 km(3)) of this snow storage volume will transition to high susceptibility.

Plain Language Summary In western North America, mountain snowpack fills reservoirs for agricultural, municipal, and industrial uses and sustains streamflow in summer when ecosystem needs are high. Thus, snow droughts (a lack of snow accumulation in winter) can have large social, economic, and environmental impacts. An analysis of the frequency and severity of past snow droughts shows that warm and dry winter conditions occurring together produce the most severe snow droughts, while warm winter conditions alone produce the least severe snow droughts. The severity and frequency of warm snow droughts, however, is dependent on mean winter temperature, and the risk of warm snow droughts is substantially higher for locations with mean winter temperatures above -3.1 degrees C (+/- 0.3 degrees C). Approximately 3% of the volume of the western United States' and southwestern Canada's nonglaciated snowpack is highly susceptible to warm snow droughts, and an additional 24% exhibits medium susceptibility.