Asheville News

Why this winter snow storm forecast is so tricky: ‘cold air damming’

Jason Sandford January 16, 2008

Jason Sandford

Jason Sandford is a reporter, writer, blogger and photographer interested in all things Asheville.

With weather forecasters basically calling for everything – rain, snow, sleet, freezing rain – we get the sense that they don’t quite know exactly what is going to happen tonight and tomorrow. That’s because it’s so damn tricky to make a weather call in the mountains. There are many variables, including something called “cold air damming” that could make or break our forecast tonight.

A USA Today blog has a good explanation here, with links.

Here’s a snippet:

Cold air damming can result in sleepless nights for forecasters in the Mid-Atlantic and Appalachians. While the basic ingredients of high pressure in the Northeast or eastern Canada and low pressure moving up from the Southeast are fundamental to cold air damming, no two events are exactly alike.

Since a significant cold air damming event can result in a crippling ice storm along the eastern slopes and foothills of the Appalachians, using the term “cold air damming” can get the public’s attention. But then the forecaster has to come to terms with the following — how strong is the cold air push, how long will it linger, how thick will the cold air be? All of these factors can make or break a forecast.

Are the weather-guessers at WLOS and around our area up to the challenge? We shall see…

Jason Sandford

Jason Sandford is a reporter, writer, blogger and photographer interested in all things Asheville.

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1 Comment

In The Know January 17, 2008

Western North Carolina is one of the hardest places in the country to forecast a winter weather event. For one, Asheville sits in a valley. You have the Appalachian mountains to the south and east of town and the Great Smokies to the north and west of town. If a system comes in from the north, the TN border counties get it. If the system comes from the south, Asheville has a better chance for accumulation, but so the do counties south and east of Buncombe.

Add to the recipe, the dreaded "Wedge". This occurs when an area of high pressure sets up shop in the Northeast. This funnels cool dry air down the eastern side of the Appalachians setting up the "Cold Air Dam" – because it is damming the air up against the mountains. The dam will actually force the cooler, more stable air, all the way south through north Georgia.

Add even more fun to the recipe by talking about temperature profiles. You have to remember that the atmosphere is 3D. You must look both at the surface and aloft. So, picture a vertical column of air. If the entire column is below freezing, it will be all snow. If the column of air is above freezing aloft but below freezing at the surface, you will see a mixed bag of sleet and freezing rain. If the entire column, both aloft and at the surface, is above freezing, it will be all rain.

Are you keeping up?

Now, maybe you can see the fun in forecast winter weather.

Oh yeah…one more complication. The nearest OFFICIAL weather balloon site, which measures upper air temperatures, is either Greensboro or Atlanta. This does WNC no good for winter weather. Therefore, we must rely on computer models for this information. While they are fairly reliable, they aren’t perfect, so neither will WNC forecasts.

There are a few studies taking place in WNC with weather balloons that the National Weather Service is taking into consideration, but they are permanent launch sites.

Hope this information helps!

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