October 26, 2018

Can't Stand the Gloomy Weather in the Southeast? Blame Cold Air Damming.

Happy CAD season! Today is the first installment of many gray, soggy days that will cast a shroud of gloom over the southeastern United States through next spring. Cold air damming (CAD) is a frequent phenomenon east of the Appalachian Mountains during the chilly months, and it's something that—despite how common it is—you never quite get used to when you live around these parts.

Cold air damming is one of those great weather terms that doesn't leave much to the imagination. It describes exactly what's going on. The atmosphere is a fluid. Cold air is denser than warm air, so it tends to stay close to the surface. When easterly or northeasterly winds blow cool air across the Mid-Atlantic and Carolinas, the cooler air gets dammed up on the eastern side of the Appalachians because it's too dense to simply flow up and over the ridges. The cold air pools up east of the mountains and leads to chilly, dreary days like we're experiencing today.

Other parts of the country can experience cold air damming—especially in Montana along the eastern base of the Rocky Mountains—but the region between north-central Georgia and the Mid-Atlantic is where this type of weather is most common and most prominent.

Today's bout of cold air damming is about as classic as it gets. Northeasterly winds are blowing chilly air across the Piedmont as a result of high pressure near New Jersey and a developing low-pressure system entering Georgia. That area of low pressure will grow into a full-fledged nor'easter this weekend, bringing heavy rain and gusty winds to the Mid-Atlantic and Northeast. Some of the wind gusts could reach 60 MPH in coastal parts of New Jersey and New York.

The weather on cold air damming days is usually harmless—today, it's just heavy rain with temperatures in the 40s—but it can have significant impacts depending on the overall setup that led to the event. The most common feature is the unshakable gloominess that can blanket the southeast from Atlanta through the Washington D.C. area. Areas to the south could have a warm, sunny day, and even communities just on the other side of the Appalachian Mountains could see temperatures as much as 20-30°F warmer than towns stuck in the pool of cold air. Locations stuck under the wedge of cold air, however, remain chilly and socked under a thick deck of clouds—sometimes even thick fog—until the wind shifts direction and begins to scour away the cold air.

Cold air damming can have major implications on precipitation type when temperatures are hovering around the freezing mark. The wedge of cold air at the surface can lead to sleet or freezing rain when precipitation moves into the area, creating a travel nightmare in areas that aren't used to wintry precipitation or where the ice wasn't particularly well-forecast.

We probably won't have to deal with the winter weather aspect of cold air damming anytime soon. Today's weather is a dreary preview of what we can expect as we head deeper into the fall. Whenever you can't stand that winter-like gloom looming over the southeast or Mid-Atlantic, just look to the west and blame the mountains.

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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I contribute to The Weather Network as a digital writer, and I've written for Forbes, the Washington Post's Capital Weather Gang, Popular Science, Mental Floss, and Gawker's The Vane. My latest book, The Skies Above, is now available. My first book, The Extreme Weather Survival Manual, arrived in October 2015.