Climatology and prediction of snow-to-liquid ratio in the central Wasatch Mountains

Contemporary snowfall forecasting is a three-step process involving a quantitative precipitation forecast (QPF), determination of precipitation type, and application of a snow-to-liquid ratio (SLR). The final step is often performed using approximate climatology or algorithms based on surface temperature or vertical velocity distribution. Based on a long period of consistent and professional daily snowfall measurements, this study 1) presents general characteristics of SLR at Alta, Utah, a high-elevation site in interior North America, 2) diagnoses factors that control SLR using reanalysis data, 3) determines differences between significant (> 25 mm liquid equivalent in 24 h) high and low SLR events, 4) develops a statistical method for predicting SLR at the study location, and 5) evaluates the potential for improving SLR and snowfall amount forecasts relative to existing methodology.

The distribution of SLR at Alta is found to be similar to that observed at lower elevations in the surrounding region, with substantial variability throughout the winter season. Significant high- and low-SLR events are characterized by notably different temporal evolutions of the local thermal and wind profile and regional synoptic pattern. Using data from the North American Regional Analysis, temperature, wind speed, and mid-level relative humidity at various levels are found to affect SLR, with the strongest correlation occurring between SLR and mean crest level (650 hPa) temperature. A stepwise multiple linear regression equation is constructed to account for a large portion of the SLR variance.

Finally, archived National Centers for Environmental Prediction 40-km Eta 12-36 h forecasts are used to develop model output statistics for predicting SLR. This approach is used to produce hindcasts of SLR at Alta for multiple winter seasons and yields an improvement over several existing operational SLR prediction techniques, although errors in model QPF over complex terrain are still shown to limit skill in forecasting snowfall amount.