Mountain snowpack in western North America is a key component of the hydrologic cycle, storing water from winter precipitation and releasing it in spring and summer. Snow is the largest water storage component in many river basins, making the region vulnerable to climate changes affecting spring snowpack. Winter and spring temperatures have increased in the region, leading to changes in hydrology and plant phenology. Snowmelt timing has shifted earlier, and snow extent and depth have generally decreased, though these observations reflect valleys and plains where snow melts earlier. Observations of winter and spring snowpack are used to predict summer streamflow, but long-term trends were not studied until Mote (2003a) analyzed snow data for the Pacific Northwest, showing substantial declines in 1 April snowpack. These declines are consistent with warming trends, and statistical regression on climate data suggests temperature plays a significant role. Similar results were found in the Swiss Alps. The study extends previous research by expanding the spatial analysis to the entire West, using a combination of historical and recent snow observations, and corroborating findings with a hydrological model. The model, VIC, simulates snowpack and shows trends consistent with observations. Spatial patterns show widespread negative trends in snowpack, with the largest losses in western Washington, Oregon, and northern California, and increases in the southern Sierra Nevada, New Mexico, and parts of the Southwest. The model and observations agree on the majority of trends, with 75% of observed and 73% of model trends being negative. Temperature and precipitation trends are key factors in snowpack changes. Temperature trends are overwhelmingly positive, with warming rates exceeding those explained by Pacific climate variability. Precipitation trends are more variable, with increases in most regions except western parts of British Columbia, Washington, and Oregon. The study concludes that widespread declines in spring snowpack have occurred in much of the North American West since midcentury, with temperature trends playing a dominant role. Future warming is likely to continue and accelerate snowpack losses, particularly in milder climates like the Cascades. The study highlights the importance of understanding these trends for water resource management in the region.Mountain snowpack in western North America is a key component of the hydrologic cycle, storing water from winter precipitation and releasing it in spring and summer. Snow is the largest water storage component in many river basins, making the region vulnerable to climate changes affecting spring snowpack. Winter and spring temperatures have increased in the region, leading to changes in hydrology and plant phenology. Snowmelt timing has shifted earlier, and snow extent and depth have generally decreased, though these observations reflect valleys and plains where snow melts earlier. Observations of winter and spring snowpack are used to predict summer streamflow, but long-term trends were not studied until Mote (2003a) analyzed snow data for the Pacific Northwest, showing substantial declines in 1 April snowpack. These declines are consistent with warming trends, and statistical regression on climate data suggests temperature plays a significant role. Similar results were found in the Swiss Alps. The study extends previous research by expanding the spatial analysis to the entire West, using a combination of historical and recent snow observations, and corroborating findings with a hydrological model. The model, VIC, simulates snowpack and shows trends consistent with observations. Spatial patterns show widespread negative trends in snowpack, with the largest losses in western Washington, Oregon, and northern California, and increases in the southern Sierra Nevada, New Mexico, and parts of the Southwest. The model and observations agree on the majority of trends, with 75% of observed and 73% of model trends being negative. Temperature and precipitation trends are key factors in snowpack changes. Temperature trends are overwhelmingly positive, with warming rates exceeding those explained by Pacific climate variability. Precipitation trends are more variable, with increases in most regions except western parts of British Columbia, Washington, and Oregon. The study concludes that widespread declines in spring snowpack have occurred in much of the North American West since midcentury, with temperature trends playing a dominant role. Future warming is likely to continue and accelerate snowpack losses, particularly in milder climates like the Cascades. The study highlights the importance of understanding these trends for water resource management in the region.