Changes to Western U.S. Snow Accumulation throughout the Twenty-First Century: Predictions from Dynamical Downscaling

In the western United States, snow accumulation and resulting snowmelt is a fundamental component of springtime streamflow. Over the course of the twenty-first century, snow accumulation will be impacted by climate change, both through rising temperatures and changing precipitation patterns. Here we aim to understand how snow accumulation is likely to change, particularly in mountain regions, which act as natural reservoirs for regions downstream through the storage of seasonal snow. We downscale three bias-corrected global climate models (GCMs) for historical, mid-century, and end-century decades with the Weather Research and Forecasting (WRF) regional climate model at high spatial resolution (9 km). For future decades, we consider an RCP 8.5 scenario. By comparing to historical simulations, we estimate not only how snow water equivalent (SWE) may change by end-century, but also how the spatial patterns of SWE may change, particularly during the snowmelt season. We also address whether the three GCMs agree over individual mountain ranges, including the Sierra Nevada of California, the Cascades of the Pacific Northwest, and the Colorado Front Range of the Rocky Mountains. Disagreement between WRF/GCM simulations suggests uncertainty for how snow accumulation and ablation may change throughout the twenty-first century. We conclude with how changes to SWE will impact the timing and magnitude of spring streamflow for major watersheds. Preliminary results suggest that snowmelt-dominated watersheds, including the Columbia and the Upper Colorado, will experience decreased springtime maximum runoff and lower baseflow during the summer. The impact of climate change on mountain SWE will have implications beyond the mountainous regions of the western United States.