Much of the contiguous US could be very cold for Thanksgiving as a wobbly polar vortex threatens to send a blast of Arctic air southward.
According to the European Center 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 cold air that swirls around the Arctic. When disrupted, this air creeps toward lower latitudes, causing temperatures to drop 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 much of the western US during the week of Thanksgiving. After the holidays, CPC forecasts the cold pattern could last through early December across much of Central and North America.
“If you have plans to go skiing to the Rocky Mountains over the Thanksgiving holiday, this is looking great,” MIT climatologist Judah Cohen told Gizmodo.
Nevertheless, it is important to remember that these approaches are based on modeling. The further away the prediction is, the less accurate it will be. With that being said, let’s look at what the data currently shows.
going cold turkey
The polar vortex rotates from west to east around the Arctic. The stronger its rotation, the more tightly this cold air closes around the poles – like when a figure skater locks her limbs together to spin faster.
When the normal westerly flow of the vortex is reversed due to a sudden warming of the stratosphere, it is like a figure skater who has extended her arms in the middle of a spin. The vortex slows and wobbles, and cold air locked tightly around the North Pole leaks southward in what is known as an Arctic blast.
The ECMWF chart above projects a rapid 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 dip below the dashed line) persist for a long time, it indicates a sudden stratospheric warming event that could trigger an Arctic eruption.
Each blue line represents a different forecast. Many of these forecasts predict a shift toward easterly winds around Thanksgiving – and therefore a possible polar vortex disruption.
Beyond that point, forecasts vary considerably, ranging from continued strengthening of easterlies to a quick return of westerlies and a stable polar vortex. This underlines the low accuracy of long-term predictions. That said, it’s entirely possible that a large portion of the US could experience a blast of arctic air on Thanksgiving.
role of la niña
According to the CPC Outlook, these cold temperatures will primarily affect the western US. To understand why this is so, we need to consider a different weather phenomenon: La Niña.
The CPC confirmed that La Niña conditions had emerged in September, as indicated by an extension of below average sea surface temperatures in the central and eastern equatorial Pacific Ocean. According to previous research by Cohen and colleagues, La Niña diverts cold air from the rotating polar vortex to western North America.
That explains why forecasters expect western states to bear the brunt of next week’s cold temperatures, Cohen said, but that could still change. Much remains uncertain about the timing, intensity, and geographic reach of this potential Arctic eruption. Still, if you’re worried about cold temperatures and blizzards impacting your Thanksgiving plans, the data suggests you should keep a close eye on your local forecast next week.