Assessing the sensitivity of Wasatch Mountain winter precipitation to present climate temperature variations

The observed sensitivity of daily precipitation in the high elevations of the Wasatch Mountains of Utah is examined as a function of mid-tropospheric temperature. The objective of this study is to estimate the daily variability in elevation of the rain/snow line during the past thirty winters over the range as a whole. Daily precipitation records at NRCS SNOTEL and other available stations throughout northern Utah are related to the twice-daily rawinsonde observations at the nearby Salt Lake City, Utah airport. Based on physical reasoning and observational evidence, we assume the precipitation is likely to be rain when the surface wet bulb temperature is greater than a specified threshhold. We estimate station surface wet bulb temperature by using 700 hPa wet bulb temperature from the nearby rawinsonde and extrapolating the wet bulb temperature downward to the station locations. The estimated fraction of seasonal precipitation at a station falling as rain vs. snow is then assessed. Year-to-year variations in fractional snowfall at each station are then related to interannual variations in seasonal 700 hPa conditions at the Salt Lake City airport. The resulting regression relationships are applied in a companion study to present and future climate simulations. Our proxy relationship helps to assess one of the many ways that regional precipitation may be affected by increased greenhouse gas emissions.