Cold Air Damming

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By Steve MacLaughlin on December 3, 2012, 2:45pm Last modified: December 3, 2012, 10:40pm

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Although "Cold Air Damming" is frustrating enough to make a meteorologist swear like a sailor, the "dam" is actually like the Hoover Dam or the thing beavers build...to hold back or close up or obstruct or constrict.

The image above is the surface map from Sunday, December 2nd just before lunchtime. Although Sunday was not the best example of this weather pattern (which actually causes the most headaches in the winter), we can use it as an example to why Cold Air Damming happens and why it can be so tricky.

The first piece of the puzzle is the weight and density of air. Cold air is very heavy and dense while warm air is much lighter. This is why warm air rises. This is why in a fire we are always taught to get as low to the ground as possible since the hot smoke will rise. This is also why the best heaters in our homes are on the ground while the most efficient air-conditioners are closer to the ceiling.

On a day like Sunday, the forecast was for a warm front from the south to move through CT and to our north during the day. That would have meant some fog or showers early followed by a huge warm-up and tons of sunshine. But that did not happen and Cold Air Damming is the culprit.

Check out the warm front and how it buckles sharply toward Washington, DC and Virginia. Instead of the warm front moving north (as it is doing near New Jersey and the Atlantic Ocean), a little notch starts forming and almost pushes the warm front back to the south - But only in the valley east of the Appalachian Mountains. When a meteorologist sees this pattern, he or she knows that Cold Air Damming is trying to happen.

So what exactly is going on? Warm air from the south is trying to move to the north with the warm front. But at the same time, much colder air from Canada is building up and trying to move south. This cold air is very heavy and dense and low to the ground so it starts wedging itself into the low-lying valley east of the moutains. So warm air is moving north at the same time that this wedge of cold air starts building in.

On Sunday, the result was clouds and fog. That warm air from the south (full of moisture) was trying to move to the north and it hit this cold wedge of air...when that warm, moist air cools down, it condenses into clouds and fog and since the cold air just won't budge, we never get into the sunny, warm weather that is just a few miles south. It's the difference between a sunny, 60 degrees and a foggy, cloudy, damp 40 degrees.

But where Cold Air Damming really gets us is in a winter storm. Imagine the image above but in January when the air temperature is cold enough for snow. You have this warm front coming up from the south triggering the snow to start falling. But at the same time you have very cold air seeping into the valley and into Connecticut. The warm air is lighter so it lifts up above the very cold air pouring in on the ground level. This "Over-Running Precipitation" can cause major problems because the snow falls into the warm air and melts into rain and then keeps falling into the much colder air and re-freezes into either sleet or freezing rain. It can mean a forecaster predicts a foot of snow but it ends up being all ice...or a forecaster predicts mainly rain and it ends up being all frozen precip.

Cold Air Damming will usually (if we are lucky) begin to show up on our computer models a couple of days before the storm which will help us prepare for it...but we never know how strong the damming will be until it actually starts happening - and that can lead to one of the trickiest types of weather for the East Coast.

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Steve MacLaughlin

Town: New Haven, CT  

Reporting for WXedge since January 2012.

Articles: 138

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