Why North Carolina Winter Weather Is So Tough To Predict
We’re a state of multiple elevations and dueling fronts
Charlotte, January 2018.
North Carolina is a state where the catchall term “winter weather” can mean anything from 70 degrees and sunny to imprisonment within a slippery carapace of ice—sometimes on the same day, depending on time or elevation. People who have lived in this state for a while generally grasp the topographical escalator of the state’s terrain as it unfolds westward from the Atlantic, from Coastal Plain to Piedmont to Mountains; they get that, as a rule, the farther west (and therefore higher) you go, the colder it’s likely to be, and the more likely that the rain you drove through in Fayetteville will be a wintry mix in Charlotte and sleet or snow in Asheville.
But it’s a little more complicated than that, and the North Carolina mountains in particular sit at an intersection of elevation, latitude, and temperature that can make winter precipitation forecasts—the kind we’re all following as a storm hits this weekend—nearly impossible to pin down. A meme making the social media rounds this week joked about the certainty of North Carolina winter weather uncertainty. “NC Weekend Weather Advisory,” it reads. “Zero to 158 inches of snow. Starting Saturday. Or Sunday. Or Monday. Or not at all.”
But first, let’s zoom out a bit. Climatologists refer to the most common type of winter storm—the type that’s descending on us now—as a “simple” storm, so named because its catalyst is one rather than two (“complex”) low-pressure systems. During a “simple” storm, a low-pressure system swings down from the northeast, carrying cold, dry air as it hugs the coastline. Often, when it reaches us, the system’s air curls clockwise and meets two barriers: a high-pressure system with warm, moist air moving northeast from the Gulf of Mexico; and, to the northwest, the mountains, which block the cold and trap it. This is called Cold Air Damming (CAD), and when it happens, among other things, Charlotte gets really cold.
“It’s like a river running into a dam,” says Corey Davis, a climatologist at the state Climate Office, on the N.C. State University campus in Raleigh. “It's a pretty good way to keep cold air on the ground. But it's only as effective as the mountains are tall.”
Here’s where the simple storm gets complicated. “You can think about it as a tug-of-war, basically,” Davis said. Cold air is heavier and denser than warm air, so the moisture-laden Gulf air slides above the trapped “cold dome” like a brook running over a boulder. It starts to dump rain into the cold air below. If it’s below freezing all the way down, it freezes into snow. If it’s below freezing up high, then a little above freezing midway down, then below freezing again, the snow melts and refreezes into sleet. If it’s above freezing in the air but below freezing on the ground, it’s freezing rain, which falls as rain but freezes when it hits a surface. There is, of course, always the possibility of a combination of two or all three.
And it’s not like there’s some fixed geographical pattern of lines where all this happens. Air moves from spot to spot and elevation to elevation. But, Davis said, the spot in the sky that tends to straddle the 32-degree mark during simple storms happens to reside around the peaks of most North Carolina mountains—somewhere between 4,000 and 5,000 feet. (Virginia is just north enough for its mountains to remain below freezing during such a storm; the opposite is the case for South Carolina.) It’s around here where 50 feet of elevation or 100 feet of highway can make the difference between rain and sleet, or mix and snow; that’s why sometimes it’s clear in Blowing Rock and snowing like it’s Siberia in Boone, eight miles north.
Charlotte, in the Piedmont—the word comes from the Latin and literally means “foot of the mountains”—rarely encounters the drastic winter precipitation that mountain residents have to get used to. A winter storm is even more unusual for the city this early in the season. Since the end of World War II, the Climate Office has recorded only one appreciable snowfall in Charlotte in the first half of December—on December 3, 1971, when a storm dumped more than seven inches on what was then Charlotte Municipal Airport. Statewide, Davis says, January is the peak month for winter weather. But he declined to issue any long-range predictions for more winter storms aside from a general, and maybe ominous, forecast. “What we can say is that El Niño is in place,” he says. “That gives us pretty high confidence that it'll be wetter than normal.”