The Perils of Forecasting Winter Weather in the 7 Day Outlook

By: Dan Bickford | News Channel 7
Published: December 31, 2008 | 0 Comments | Post a Comment

All weather forecasters use something called “numerical weather prediction”…”computer models” is a simpler term…to assist us in our forecasting. These models take current weather conditions across the country (and globe) as input, and then predict where all of the weather features are going to be over the coming days. There’s more than one model; each model goes about its calculations differently to try to best grasp what the atmosphere may do. As a result, we may get multiple “answers” from these models…it’s up to the meteorologist to figure out if one, or more, or any, of the models has the best idea.

This is especially true when there’s a general shift in the overall weather pattern. The models will have a harder time converging on one solution. This becomes more apparent the farther into the future one looks.

I mention this because of what the models are indicating for us for the first full week of January. As I write this on New Year’s Eve, these models have been consistent in their inconsistency…they agree on a strong weather system to move through the Southeast early in that week; they do not agree on the details. Model solutions have varied over the past day from a cold rain for much of the area to forecasts that would yield a possible major winter storm with snow and ice, or a “minor” storm with a light to moderate mix of precipitation types. Even looking at just one model, the most recent “solution” may be radically different from one gathered just six hours before.

This makes long-term winter forecasting especially difficult; a model flip-flop can change the position of a storm by a hundred miles or alter the intensity. This can move the all-important 32-degree mark farther north or farther south, having a huge impact on the forecast as far as precipitation type is concerned.

This time of year in particular, anytime we show a storm system in the later days of our 7-Day forecast, keep a close eye on how the forecast evolves, especially with respect to those temperatures. It doesn’t take much in winter to move that freezing mark around, creating big changes in the weather we see.