Previous studies using observed snow course and SNOTEL station records have shown dramatic downward trends in snowpack in the Pacific Northwest (PNW) in recent decades. These trends are influenced both by interannual and decadal scale climate variability (particularly over the last 50 years), and also by temperature trends at longer time scales that are generally consistent with projections of global warming for the region. The use of observed data in the analysis is hampered by relatively short periods of record (with temperature and precipitation trends strongly influenced by the well-documented cool to warm phase PDO transition from 1947-present), limited sample size and spatial coverage, and a limited ability to rigorously separate the influence of temperature and precipitation on an event basis. Using long retrospective simulations of snowpack produced by a hydrologic model can help to remove all of these confounding elements from the analysis. In this study we examine snowpack over the PNW as simulated by the VIC hydrologic model implemented at 1/8 degree resolution. A new temporally and topographically corrected meteorological data set from 1915-2000 is used to drive the model. The long simulations of snowpack from 1915-2000 are used as surrogates for observations, and in particular to analyze apparent trends in snowpack over a large spatial domain. By isolating the trends in temperature and precipitation in separate simulations, the independent influence of temperature and precipitation trends on snowpack trends is also analyzed at monthly time scales.