The Sierra Nevada snowpack, which supplies sixty percent of California’s water for consumptive use, is under threat due to anthropogenic climate change. While previous studies have assessed the impacts of climate change on mountain snowpack using global or regional climate models, we leverage a newly generated climatological simulation using the variable resolution Community Earth System Model 2 which provides high spatiotemporal resolution estimates (14 km horizontal grid spacing, daily-to-hourly outputs) of historical and future projections of California’s hydroclimate. Our study offers a comprehensive analysis of the water cycle, such as how precipitation regimes and snowpack might change in the three major basins encompassing the western slopes of the Sierra Nevada and the Central Valley of California under +1.5°C, +2.0°C, and +3.0°C warming scenarios, and the corresponding effects on the water balance. In the +3°C warming scenario, the fraction of precipitation that falls as snow decreases significantly, dropping from 51% to 24% in the Northern Sierra Nevada and from approximately 64% to 40% in both the Central and Southern Sierra Nevada. Consequently, our projections indicate a decline and earlier peak in snow water equivalent, along with an earlier onset of snowmelt, leading to water becoming available earlier in the water year and substantial declines in runoff during the hot summer months. These trends are expected to persist and possibly intensify into the mid-to-end of the 21st century under climate change.