The U. S. could be in for a chillier and snowier winter. Why? The polar vortex is acting up. This big ring of winds up in the atmosphere usually keeps the cold air locked up near the North Pole. But when it gets weak or disturbed, that cold air can spill out and head south.

There are two polar vortices. One is in the troposphere, which affects winter weather every year. The other is in the stratosphere, which holds even colder air but doesn't usually impact surface weather. When the polar vortex is strong, the cold air stays put. But when it's weak, the cold air can escape and head towards the U. S.

Scientists are still figuring out why these disruptions happen. Sometimes, the polar vortex stretches out, causing quick cold snaps. Other times, a sudden warming in the stratosphere, called sudden stratospheric warming (SSW), can cause bigger disruptions. These events can take weeks to unfold and can lead to colder temperatures and more snow.

But it's not a sure thing. An SSW might not happen, and the polar vortex might just stretch out, causing a noticeable change in weather. Other factors, like La Niña, will also play a role in the winter forecast. La Niña can influence weather patterns and increase the chance of a colder December.

Long-range forecasts suggest below-average temperatures for most of the U. S. from late November into December. But these forecasts don't account for short-term variations. So, there could be a cold snap followed by warmer weather, with the average temperature leveling out.

These disturbances can also increase the chances of snow. The pattern change supports more winter-like conditions across the central U. S. and increases the potential for heavy snow, especially along the Great Lakes.

COLD OUTBREAKS

Why the U. S. Might Face a Colder, Snowier Winter

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