Much of the contiguous U.S. could be in for a very chilly Thanksgiving as a wobbly polar vortex threatens to send a blast of Arctic air southward.
According to the European Centre for Medium-Range Weather Forecasts (ECMWF), a sudden stratospheric warming (SSW) event may be just around the corner. These rare, rapid temperature increases in the upper atmosphere weaken the stratospheric polar vortex—a large area of low pressure and frigid air that swirls around the Arctic. When disrupted, this air creeps down to lower latitudes, causing temperatures to plummet in parts of the northern hemisphere.
The Climate Prediction Center’s outlook for November 24 to 30 reflects these conditions, with below-normal temperatures expected across the majority of the western U.S. during the week of Thanksgiving. After the holiday, the CPC forecasts a colder pattern across much of the central and northern U.S. that could last into early December.
“If you have skiing plans for the Rocky Mountains over the Thanksgiving holiday, it’s looking pretty good,” MIT climatologist Judah Cohen told Gizmodo.
Still, it’s important to remember that these outlooks are based on modeling. The farther out the prediction is, the less accurate it becomes. With that being said, let’s break down what the data currently shows.
Going cold turkey
The polar vortex rotates west-to-east around the Arctic. The stronger its rotation, the tighter this cold air is locked around the poles—sort of like when a figure skater tucks in their limbs to spin faster.
When the vortex’s normal westerly flow reverses due to sudden stratospheric warming, it’s like a figure skater who has extended their arms mid-spin. The vortex slows and wobbles, and the cold air that was locked tight around the North Pole leaks southward in what’s known as an Arctic blast.
The ECMWF chart above forecasts a sharp weakening of the polar vortex over the next two weeks. The dashed horizontal line at 0 m/s marks the boundary between westerly winds (above the line) and easterly winds (below the line). When strong easterly winds (deep dips below the dashed line) are sustained over an extended period of time, this indicates a sudden stratospheric warming event that may trigger an Arctic blast.
Each blue line represents a different forecast. Many of these forecasts predict a shift toward easterly winds—and therefore a potential polar vortex disruption—around Thanksgiving.
Beyond that point, the forecasts diverge considerably, ranging from continued strengthening of easterly winds to a quick return to westerlies and a restabilized polar vortex. This underscores the reduced accuracy of longer-term predictions. That said, it’s entirely possible that a wide swath of the U.S. could experience an outbreak of Arctic air over Thanksgiving.
The role of La Niña
According to the CPC outlook, these cold temperatures will primarily impact the western U.S. To understand why that is, we need to consider a separate weather phenomenon: La Niña.
The CPC confirmed that La Niña conditions had emerged in September, as indicated by the expansion of below-average sea surface temperatures across the central and eastern equatorial Pacific Ocean. According to previous research by Cohen and his colleagues, La Niña tends to isolate the cold air from a meandering polar vortex to western North America.
This explains why forecasters expect western states to bear the brunt of next week’s cold temperatures, but that could still change, Cohen said. Much remains uncertain about the timing, intensity, and geographical reach of this potential Arctic blast. Still, if you’re worried about colder temperatures and snowstorms affecting your Thanksgiving plans, the data suggests you should keep a close eye on your local forecast next week.
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