Using the Froude Number to Improve Orographic Snow Forecasts in the Green Mountains of Vermont

Orographic snow is a very challenging aspect of forecasting in the northeastern United States. This paper provides a method for improving these forecasts using the Froude number. Past research has shown that variations in the non-dimensional Froude number during upslope snow events can be correlated to the nature of the orographic snow bands, and the distribution of snowfall accumulations. Twenty-five observed northwest flow orographic snowfall cases from 2007 to 2012 in the Green Mountains of Vermont are examined with a focus on the Froude number during the events. Composite synoptic analyses show 500-hPa northwest flow and cyclonic north to northwest flow at 850 hPa. In these cases, the Froude number has a critical level of 1.0, at which events lower than this critical level, the flow becomes blocked and precipitation backs up into the lower elevations of the western slopes of the Green Mountains. With Froude numbers less than 0.5, precipitation affects the Champlain Valley and the Burlington metropolitan area. At Froude numbers above 1.0, air flows more freely over the Green Mountain spine, and the majority of the precipitation falls along the mountains and downwind. The forecast Froude number, in addition to other fields from numerical weather prediction models, can assist forecasters in determining the potential for and possible distribution of orographic snow.