December could bring extra cold weather to Maine thanks to sudden stratospheric warming above the North Pole that could disturb the polar vortex.

When the polar vortex is displaced or elongated, it can cause very cold air to spill southward into the United States. Long-range forecasts for this winter predict below-average temperatures across much of the northern parts of the country.

Sudden warming in the stratosphere, the second layer in Earth’s atmosphere, is uncommon in November — it happened once in the 1950s — and its occurrence this week could become the earliest instance on record.

These events typically occur every other year or so, most often in January or February, according to the National Oceanic and Atmospheric Administration.

WHAT IS SUDDEN STRATOSPHERIC WARMING?

Sudden warming in the stratosphere can disrupt the polar vortex, slowing or even reversing the strong band of winds that flow counterclockwise around the North Pole, and send arctic air southward with the jet stream, said Amy Butler, a research scientist with NOAA’s Chemical Sciences Laboratory.

Polar vortexes, which exist at both poles, are not something new, although the term has only recently been popularized. The only danger to humans is how far temperatures will drop when the polar vortex expands and sends icy air southward, according to NOAA.

Many times during winter in the Northern Hemisphere, the polar vortex will expand, according to NOAA. This happened in January 2014, with earlier notable outbreaks in 1977, 1982, 1985 and 1989.

In 2014, the polar vortex episode led to the Midwest’s coldest winter in over 30 years.

WHY DO THESE WARMING EVENTS MATTER?

These events matter for two reasons, Butler said.

For one, the polar vortex is a hemispheric-scale feature of the atmosphere, “so when it becomes disturbed or slows down, it can have a significant impact across the Northern Hemisphere,” she said.

In general, sudden stratospheric warming events are followed by cold air outbreaks over the Eastern United States, northern Europe and Asia. (In Greenland and parts of the Arctic, they are followed by warmer-than-normal conditions.)

Also, Butler said, these events lead to persistent changes in the atmospheric circulation that can last for weeks or months.

“There are very few sources of weather predictability once you get out beyond seven to 10 days,” Butler said. “But the stratospheric polar vortex can provide information that improves forecast skill even out to five or six weeks later.”

Michael Clair, a meteorologist with the National Weather Service office in Gray, said the sudden warming events give an indication of when the cold around the pole could be disturbed and come farther south, but they aren’t a reason to worry.

“The fact it’s happening now gives us a heads-up that in a few weeks, we should be on the lookout for some cold air,” Clair said. “It’s an early sign, but it’s not a sure thing. That cold air could end up dumping into Europe.”

HOW WILL THIS IMPACT TEMPS?

The NOAA Climate Prediction Center predicts the upcoming atmospheric pattern supports a setup what will likely drive winter-like weather across large portions of the U.S., with below-normal temperatures in the northern and central regions of the country.

This pattern is expected to be widespread from Thanksgiving to Monday, the prediction center said.

In northern areas, colder temperatures will persist into at least mid-December, according to the prediction center.