A Climatology of Snow-to-Liquid Ratios in Alaska

Snow to liquid ratios (SLR) continue to be a large source of error for forecasters in the prediction of snow amounts during the winter months. Current methods of predicting SLR in Alaska WFOs range from an empirical method based solely on surface temperatures, to model-derived SLR. Both of these methods are limited in that numerical models parameterize many of the complex micro-physical factors that drive SLR, while surface temperatures are likely insufficient as a predictor given that snow formation originates well above the surface. A climatology was developed using data from 116 COOP stations around the state of Alaska for the period of 1950 to the present. This yielded a robust data set of snowfall observations. SLR was calculated for each event following the methodology of Baxter et al. (2004) and their SLR climatology for the contiguous United States. The mean SLR for all Alaska WFOs was found to be much higher than the empirical method based on surface temperatures. Considerable variability was noted in both mean and median SLR values between sites located along the immediate Gulf coast and those in the interior. Additionally the mean SLR for Southeast Alaska was also higher in general than those found by Baxter et al. in locations along the Pacific Northwest coast. It is hypothesized that the frequency of events where warmer marine air over- runs cold, dry arctic air from northwest Canada modulates these variations in snow to liquid ratio. An observed sounding climatology was also developed for various low level thermal fields and was matched to observations of SLR at each site. For Southeast Alaska, it was found that the 1000-850mb thickness was a good predictor of SLR with mid level thickness (850-700mb) being the best predictor in general for other areas of Alaska. Finally, the results were then analyzed on a grid using the National Weather Service Graphical Forecast Editor (GFE) and a smart tool was developed which would allow operational forecasters to use this climatology as a starting point when making a prediction of snow to liquid ratio. Enhancements to the tool were then incorporated which allowed the SLR climatology to be adjusted towards the upper/lower quartiles as the selected low level thermal field fell/rose toward threshold values.