How To Forecast a White Christmas

December 13, 2012; 5:20 AM ET
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How do meteorologists figure out who will be getting snow when there's no clear answer? AccuWeather forecasters usually look at two or three models when making a forecast, but there are dozens of models from which to choose. Meteorologists look for consistency in the forecast models. They're more confident when models give similar forecasts over time.

The GFS is "just one tool," AccuWeather meteorologist Meghan Evans said. "We can compare that to other forecast models, like the European."

Why can't forecasters just pick one model?

"Each of the computer models handle the atmosphere slightly differently-- different rules, different physics, different data points-- so they come out with different answers," AccuWeather forecasting legend Elliot Abrams said. "One problem with the models is, even though one model may handle today's storm better than another model, there's no assurance it will handle the next one better."

Computer models aren't a meteorologist's only tools.

"We can also look at general [weather] patterns," Evans said, "like whether blocking will be set up, which means cold shots could reach parts of the Midwest and Northeast."

For example, five days out from a possible storm in the Northeast, meteorologists have a good idea of general areas that may get snow for the predicted Christmas Eve storm.

"Once the storm starts to form," Abrams said, "we'll probably see the computer models start converging on what will turn out to be the right answer, but we're not there yet."


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