(NEW YORK) — A significant change in the atmosphere means the weeks after Thanksgiving could be colder and snowier than normal across portions of the United States.

It’s all thanks to a disruption in the polar vortex. Here’s what you need to know.

Why is this happening?

Our atmosphere consists of many layers of moving air; the troposphere is where our weather happens and above it is the stratosphere. Over both poles, a ring of strong mid-level winds called the polar vortex traps cold, sub-arctic air.

There are two of these polar vortexes — one in the troposphere that affects weather every winter, and one in the stratosphere that contains much colder air but only affects the surface weather during winter less frequently.

When polar vortex winds are strong, this frigid air remains near the poles. But when the polar vortex is weak and is disturbed, the frigid air can spill out from the North Pole, down south towards the surface.

Scientists are still trying to understand these disturbances and why they occur, but they can drive the most intense cold-air outbreaks and winter weather bursts in the U.S.

Sometimes, the polar vortex simply stretches out, bringing quick bursts of cold air and some wintry weather to the U.S. In more extreme cases, a phenomenon called “sudden stratospheric warming” — or SSWs — can trigger more significant disruptions to the polar vortex.

While it may seem counterintuitive, a sudden warming in the stratosphere above the poles often weakens these winds, disrupting the polar vortex and allowing cold air to spill out from the North Pole and down into places like the U.S., Europe and Asia.

These events can take weeks to unfold and can have cascading impacts. Some of these events can lead to strong cold outbreaks and snowier spells, while others have little impact near the surface.

According to Dr. Amy Butler, a meteorologist and lead of the Stratospheric Modeling & Analysis program at National Oceanic Atmospheric Administration, SSWs occur once every other winter, But “there are only one to two major events in late November in the record back to 1958.”

“We have this unprecedented disruption in the polar architecture where it’s so early in the season … There’s not many past analogs to draw from and say [with confidence] about how this is going to go,” said Dr. Judah Cohen, a climatologist at MIT and the director of seasonal forecasting for Atmospheric and Environmental Research (AER) at the JANUS Research Group.

Cohen added that an SSW is not a given yet and that we could simply see the polar vortex be stretched out past Thanksgiving, but it would still result in a noticeable change to colder and possibly more wintry weather for parts of the U.S.

La Niña will also play a role in this pattern shift, as will the forecasted weather conditions for the winter ahead.

“With North America, we have a weak La Nina and other patterns in the Pacific that have a direct influence on the weather as well,” said Dr. Jason Furtado, associate professor in the School of Meteorology at the University of Oklahoma. Furtado also added that these, consequently, lead to a higher chance of a colder December.

What does it mean for the forecast?

Long-range forecasts from NOAA’s Climate Prediction Center favor below-average temperatures for the majority of America from late November into December.

However, this outlook deals with average temperatures over the period, so they don’t account for short-term variations that occur over days or weeks — meaning there could be a cold snap for a few days then warmth for the rest of the period, with the average leveling out or warmer.

What does it mean for snow?

These disturbances to the polar vortex can plunge cold air south and often create more chances for snow. However, the relationship between these two is not as straightforward.

According to NOAA, this pattern change supports more winter-like conditions across the central U.S. and increase the potential for heavy snow. Specifically, the increased potential for heavy snowfalls along the Great Lakes, a region historically known for lake-effect snow.

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