Abstract:

Climate models agree that Western U.S. snowpack and soil moisture will decline, but they disagree on how much. We use CMIP6 GCMs alongside the Western United States Dynamically Downscaled Dataset (WUS-D3; Rahimi et al., 2024), 14 GCMs downscaled to 9 km with WRF, to develop two independent emergent constraints that narrow this uncertainty. For snowpack, spring positive degree-day trends constrain projected April 1 SWE decline to approximately −23% by mid-century. For soil moisture, historical seasonal amplitude projects future drying across both ensembles. These are the first emergent constraints applied to a dynamically downscaled regional ensemble, and they not only tighten projections but also reveal insights into where climate model strengths and limitations.