Cold air damming is a weather pattern that is (somewhat) unique to the Mid-Atlantic.

Dr. Gary Lackmann at NC State devotes an entire chapter to it in his textbook about large-scale meteorology.

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What is cold air damming?

Cold air damming (CAD) refers to a pattern in which the air is dammed/wedged up against a mountain range. In our case, that would be the Appalachian Mountains.

In the winter, it can provide us with freezing rain, sleet, snow or a cold rain.

How cold air damming often leads to winter storms and ice storms in our area
How cold air damming often leads to winter storms and ice storms in our area (© WRAL)

High pressure is usually at the root of it. Without the presence of the Appalachian mountains, we'd be sunny and dry (like we usually are beneath high pressure).

Oftentimes in a CAD setup, the high is north of here or toward New England. Because the flow around high pressure in the northern hemisphere is clockwise, the air has an easterly (coming from the east) component to it.

Once that air rises up the mountains, you get this dome of cooler air and overcast that spreads eastward across parts of Virginia, North Carolina, South Carolina (and sometimes as far south as Florida, Georgia and Alabama).

A layer of cool air resides closer to the surface
A layer of cool air resides closer to the surface (© WRAL)

Sometimes, you get wind above to come from the west and southwest. That locks in some moisture above the cold dome.

Moist air rides above a shallow layer of cooler air
Moist air rides above a shallow layer of cooler air (© WRAL)

This will then translate to precipitation at the surface. Depending on the setup or intensity, that determines how light/heavy the precipitation is and the type you see falling.

Precipitation usually develops in a cold air damming setup
Precipitation usually develops in a cold air damming setup (© WRAL)

How does this apply to our weather now?

We're seeing a pretty typical cold air damming pattern Tuesday. In the case of Tuesday, the rain is heaviest toward the mountains and foothills of North Carolina. It's lighter and more drizzly closer to the Triangle/west of I-95.

Locally, our temperatures will range from about 5 to 10° below average.

Temperature departure from average in the WRAL viewing area Tuesday
Temperature departure from average in the WRAL viewing area Tuesday (© WRAL)

If you back out the view more, you can see the affect the CAD is having on other parts of the state and in parts of Virginia and South Carolina.

Temperature departure from average Tuesday across NC, SC and VA
Temperature departure from average Tuesday across NC, SC and VA (© WRAL)

How often does cold air damming happen?

The most recent study I could find is from 2016, but its results confirm what was previously studied.

Cold air damming is most frequently seen in the fall and winter months, and it is pretty rare in the summer. On average, it happens less than once per August.

How often we see cold air damming in the Eastern U.S. according to a 2016 study
How often we see cold air damming in the Eastern U.S. according to a 2016 study (© WRAL)

For now, it provides us with a break from the heat and humidity and a preview of the next season to come.

In the winter, CAD can be potentially more impactful.

Have questions about the weather and how it works?

Send me an email with the subject line 'Ask the Meteorologist:' to cmichaels@pugongyingbo.com.