Drought less predictable under declining future snowpack

Climate warming causes less mountain precipitation to fall as snow. Hydrologic simulations predict that in a high-end emissions scenario, this decreases the predictability of seasonal water resources across the western United States, with low-elevation coastal areas impacted most strongly. Mountain snowpack serves as an immense natural water reservoir, and knowledge of snow conditions helps predict seasonal water availability and offers critical early warning of hydrologic drought. This paradigm faces an impending challenge given consensus that a smaller fraction of future precipitation will fall as snow. Here, we apply downscaled hydrologic simulations from 28 climate model projections to show that by mid-century (2036-2065), 69% of historically snowmelt-dominated areas of the western United States see a decline in the ability of snow to predict seasonal drought, increasing to 83% by late century (2070-2099). Reduced predictability arises when peak snowpack approaches zero or because of decreased warm-season runoff efficiency. Changes in drought prediction skill show significant (P < 0.01) elevation dependence, with lower-elevation coastal areas most impacted by warming. Ancillary predictive information can only partially mitigate snow-based predictability losses to 65% of areas, underscoring the importance of declining future snowpack.