Water is a valuable commodity in the arid West, where mountain snowpack provides much of the storage of water for summer use. A recent study examined snow water equivalent (SWE) at mountain sites in the Pacific Northwest and noted sharp declines in April 1 SWE since 1950, and suggested on the basis of regression analysis and elevational dependence of trends that rising temperatures were primarily responsible. The present study extends that work to the entire mountainous west, and further examines the competing influences of temperature and precipitation trends. Total accumulation is largely determined by winter precipitation, while the rate and timing of spring melt is largely determined by late winter and early spring temperature. Declines in snowpack are most acute where late-winter temperatures are near freezing and winter precipitation has been decreasing, for instance in the Oregon Cascades.