Ice Storm or Snow Storm?

With cold air firmly in place over Vermont and a strong front well to the southeast of Vermont, the forecast question for tomorrow is whether tomorrow's precipitation will fall as freezing rain or snow. Here are a few pointers for predicting precipitation type using a modern forecasting technique known as ensemble forecasting.

Ensemble forecasts are numerical (i.e. computer) forecasts that have slight differences in the the way the forecast parameters such as temperature, precipitation, and humidity are calculated. Nobody knows ahead of time which precise method is best, so several (21 to be precise) computer model runs are made to see how variable these predictions are. They give forecasters a feel for how prone a numerical forecast are to error. Taking the average gives you a "best guess" at what may happen .

The image at top left (click on image for larger image) shows a 27 hour forecast of the WRF model ensemble forecast for the NE U.S. It is initialized 12 Z (7 AM) Thursday, 11 December, 2008; it is valid 3Z Friday, 12 December, 2008(10 PM Thursday night). Shown is the precipitation type (snow (blue); freezing rain (pink); rain (green). An ice storm will occur somewhere along a large swath extending from Maine to Pennsylvania. The question is whether this swath will make it to the NEK in Vermont. About 1/3 of the runs at 10 PM Thursday show no precipitation, another 1/3 show snow, and yet another 1/3 show freezing rain.

As the blue band of snow is very narrow, skiers and snowboarders will be disappointed. Any snow we get at Burke will likely be mixed with all sorts of sleet and freezing rain. Because there will be a strong inversion, it could be that mountaintops will even experience mostly rain. New Hampshire ski areas are almost certain to get freezing rain, and Jay is likely to get only a little snow early in the day, major snowfall occuring late in the day. The best skiing will likely be at Jay on Saturday morning, if you can make it up there in strong winds and ice cold temperatures.

Three hourly forecasts out to four days are available at the Penn State Meteorology Map Wall. Mouse over the forecast time (F03, F06, F09, ... ) for the forecast of the given time. This will be a major event that will intensify throughout the afternoon. Stay tuned for possible class cancellations Friday morning!