The case presented was from an early season winter storm in the Sterling VA CWA, which resulted in the heaviest early season snowfall in most places since either the so-called Veterans' Day Storm of 1987 or the record snowfall of 30 November 1967. Short-range ensemble forecasts (SREFs) from NCEP were helpful in determining the degree of uncertainty for precipitation amount and type, when the limitations of the precipitation type algorithm used in the SREF are taken into account. This adjustment was made by adding the snow and ice pellet probabilities together to determine the snow probability, as per the precipitation type case study for this same event. The precipitation type diagnostic deficiency in the SREFs illustrates the point that model errors and biases should be taken into account whenever using probabilistic forecasts, just as with deterministic forecasts.
A significant risk of a winter storm criteria snow event was indicated over much of the Sterling CWA by ensemble runs starting at day 3 with the MREFs. Even at day 2, the SREFs were predicting a significant probability of snow (with freezing rain over the southeastern areas), and precipitation from 0.5-1.0" for the event. The day 1 forecast had slighly lessened the precipitation amounts (to about 0.4-0.8") and increased the probability for mixed precipitation further north and west, to include the major metropolitan areas.
The SREFs closest to the event did not do as well as earlier runs in predicting where there would be snow. The Eta SREF exhibited the error of overdevelopment of a frontal wave too much into the cold air, which resulted in a warm above-freezing layer at 800-hPa advecting in too rapidly and too far north and west.