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In the western United States, climate warming poses a unique threat to water and snow hydrology because much of the snowpack accumulates at temperatures near 0°C. As the climate continues to warm, much of the region's precipitation is expected to switch from snow to rain, causing flashier hydrographs, earlier inlow to reservoirs, and reduced spring and summer snowpack. This study investigates historical variability in snow to precipitation proportion (Sf) and maps areas in the western United States that have demonstrated higher Sf sensitivity to warming in the past. Projected changes in Sf under 1.1, 1.8, and 3.0 °C future warming scenarios are presented in relation to historical variability and sensitivity. Our findings suggest that Sf in this region has primarily varied based on winter temperature rather than precipitation. The difference in Sf between cold and warm winters at low- and mid-elevations during 1916-2003 ranged from 31% in the Pacific Northwest to 40% in the California Sierra Nevada. In contrast, the difference in Sf between wet and dry winters was statistically not significant. Overall, in the northern Sierra, Klamath, and western slopes of the Cascade Mountains Ranges, Sf was most sensitive to temperature where winter temperature ranged between -5 to 5 °C. Results from our trend analysis show a regional shift in both Sf and signal-to-noise ratios during 1960-2003 as compared with 1916-2003. Our findings indicate that natural variability in Sf over 1916-2003 across all regions except for the Great Basin most closely resembles the projected 2040-warming scenario (+1.8 °C). [ABSTRACT FROM AUTHOR]