A Climatology of Snow-to-Liquid Ratio Across The Burlington, Vermont County Warning Area

Arguably, one of the most difficult aspects of weather forecasting is accurately predicting snowfall accumulations. It involves determining a quantitative precipitation forecast (QPF) and a snow-to-liquid-equivalent ratio (SLR), both of which can be very challenging. Forecasters commonly start with an initial SLR value using the 10 to 1 rule, assuming a uniform snow density of 100 kg m-3. Previous research has shown that considerable variability of SLR exists across different regions of the country and across variable terrain. The goal of this study was to improve snowfall accumulation forecasts in the National Weather Service Forecast Office Burlington, Vermont (BTV) county warning area (CWA) by examining the variance of SLR, and offer a new methodology for using the results in operational forecasts. A 30-year climatology of SLR was developed and stratified by four variables -- temperature, QPF, time of year, and terrain--using National Weather Service Cooperative Observer Program data. An effort was made to eliminate mixed precipitation and provide SLR for snow-only precipitation events. Results indicate that all four variables play a role in forecasting SLR. The highest values of SLR occur during the months of December through January, with QPF less than 0.1 in., average surface temperatures between zero and 10 degrees Fahrenheit, and across elevated terrain. An operational tool for forecasting snowfall accumulations was created incorporating the results of this study. This study and forecast tool provide forecasters an improved understanding of SLR and a means to forecast snowfall accumulations across the BTV CWA.