С»ÆÊé

The western United States has emerged as a global hotspot for snow drought, posing a growing threat to water availability in the Upper Colorado River Basin (UCRB), where high-elevation snowpack generates the majority of Colorado River streamflow. Record-low snow water equivalent (SWE) during the anomalously warm, dry 2025-26 winter highlights the urgency of this risk, yet key uncertainties remain in the timing and magnitude of future snowpack deficits. Using WRF-CESM2, a high-resolution (3-km) ensemble of dynamically downscaled climate simulations (1980-2100), we define an impact-based threshold grounded in 2026 snowpack conditions (~55% of historical median peak SWE) and evaluate changes in the frequency, persistence, and clustering of threshold-exceeding years. We show that low-snow years comparable to or more extreme than 2026 shift from rare, isolated events in the historical period to frequent, multi-year sequences by late century. This transition marks a fundamental shift in snowpack reliability in runoff-generating headwaters, with direct implications for water supply planning in the Colorado River Basin.