Snow-Liquid Ratios Over the Northern Sierra Nevada: Climatology and a Predictive Methodology

While Snow-Liquid Ratios (SLRs) are a crucial component of snowfall forecasting, historical SLRs for the Sierra Nevada of California are difficult to find. Thus, National Weather Service (NWS) forecasters are often left to use rules-of-thumb and previous experience in snow forecasting. To develop a better scientific basis for SLR forecasting, this study uses several years of precipitation data from two locations along a major Sierra interstate highway pass to a) Document climatological SLRs and b) Develop a forecasting tool to predict SLR based on in-cloud and surface conditions.

SLRs at the two observing sites are found to average around 9, roughly agreeing with existing local knowledge at the NWS office in Sacramento. Using the NARR (North American Regional Reanalysis), a modest correlation was found between 700 millibar temperatures and SLR. A relationship was also found between NWS forecasted surface temperatures and SLR, although less significant. A software tool was created to predict SLR based on these two relationships. This tool was found to increase the accuracy of SLR forecasts versus the archived official NWS forecast for data from 2011-2012.