BECKLEY, WV (WVNS) — During the past few winter storms the term “Cold Air Damming” (CAD) has come up a lot, in both our forecasts and in conversation for some. But what is CAD and why is it so important?
First let’s start at ground level. The general set-up of a CAD event features a strong high pressure to the north, which helps to transport cold air south. Now, the other piece shown is a low pressure tracking north from the south. This is part of the set-up that we’ve had recently but isn’t always necessary to help initiate a CAD event. In place of a northerly tracking low pressure system, southerly flow in general can also provide similar results.
While this set-up matures we see two distinct camps set up at the surface. On the western side of the mountains we see temperatures surging as southerly flow brings in warm and moist southern air. On the eastern side of the mountains cold air becomes entrenched as the northerly flow from the strong high pushes cold air further and further south. Now, the reverse of this can happen too with cold air to the west, and warm air to the east, but the example we’re working with here is the more common of the two.
That just covers what’s happening on the ground, but the atmosphere is three dimensional. In this case we’ll only be looking at the troposphere, the lowest layer of the atmosphere where weather occurs. One of the most important parts of working on any forecast is making sure you take into account conditions in the ENTIRE troposphere.
In the diagram above, you see we have cross section of the mountains with the west side on the left and the east side on the right. So first we set up our surface wind directions; southerly on the west side, and northerly on the east. Aloft the second set of winds appear a few thousand feet above the ground. Notice aloft BOTH sides of the mountains have southerly flow bringing warm air into the mid-levels of the troposphere. This is what we refer to as the warm nose, another term that comes up often.
Now this is where precipitation comes into play. On the west side of the mountains since the entire column of air is above freezing we see regular old rain falling. But on the eastern side, where we have that nose of warm air pushing in and the strong layer of cold air at the surface you’ll see either freezing rain or sleet. Which one you’ll see will be dependent on how deep the cold air is on the eastern side of the mountains. If it’s a thick layer of cold air you’ll see sleet, if the layer of cold air is thinner you’ll see freezing rain since the rain doesn’t have enough time as it’s falling to refreeze before hitting the ground.