Observational and synoptic analyses of winter precipitation regime change over Utah

Surface observational analyses from prior studies have revealed an increase in surface air temperatures over the U.S. Intermountain West accompanied by widespread declines in snowpack as well as in the fraction of precipitation that falls as snow. This paper focuses on the state of Utah, using three distinct lines of investigation. First, by means of observation-based, gridded daily temperature and precipitation data and, utilizing a climatological rain-snow threshold (RST) temperature method, the breakdown of the hydroclimate regime indicates that the proportion of precipitation falling as snow has decreased by 7% during the last half century. Meanwhile observed snow depth has decreased by 35% across Utah - an observation supported by consistent decreases in snow cover and surface albedo as measured from satellites. Second, a comprehensive synoptic analysis was also conducted as an alternate, yet independent means that is distinct from surface observational analysis: Weather systems that have the potential to produce precipitation in Utah have decreased in number, with those leading to snowfall decreasing at a considerably greater rate. Further circulation analysis showed that the anomalous anticyclone that has developed over western North America acts to reduce the chance of cyclone waves from affecting the Intermountain West. The results suggest that declines in snowfall, snow depth, and snow cover in Utah (and much of the Intermountain West) are not only reflections of surface and upper air warming, but are also the result of longer term changes in the atmospheric circulation. Third, regional climate model simulations from the North American Regional Climate Change Assessment Program (NARCCAP) project a continuous decline in the snow-rain ratio toward the middle of the 21st century