The Polar Vortex Will Weaken Much Earlier Than Usual; Here's What That Could Mean In December

What La Niña Means For Your Winter Outlook

The polar vortex is forecast to weaken unusually early, and that could lead to more persistent cold weather in at least parts of the U.S. in December.

Last week, almost 100 daily record lows were tied or broken in the East and South, including some "coldest so early in season" record ties. That cold retreated quickly from most areas.

But there are hints in the atmosphere that the cold may take over, for at least a while.

What is the polar vortex: Almost 12 years ago, the "polar vortex" entered into pop culture during a January 2014 cold outbreak. But scientists have known about it for many years, even before the Civil War.

Located near the poles (hence "polar") this spinning cone of low pressure — a vortex — is always there in the colder months. But it's high up in the stratosphere, that layer of the atmosphere above which most of our weather occurs, from about 7 to 31 miles above the Earth's surface.

Sometimes it's a perfect cone: In the colder months, this polar vortex can vary in strength, shape and location.

Sometimes it's strong, almost perfectly circular and directly over the pole.

Other times, not so much: But occasionally, the polar vortex weakens, stretches or even splits in two, with its center shoved off the pole.

That's often caused by a sudden stratospheric warming (SSW). By sudden, we mean temperatures in the stratosphere can skyrocket 50 to 70 degrees or more in just a few days.

This sounds extreme, but it typically happens at least once every other cold season, according to data compiled by NOAA scientist Amy Butler.

Strange early weakening forecast: And that is what is forecast to happen soon. As you can see in the graphic below, the polar vortex is expected to be pushed off the North Pole by the warm air and stretched like pulling both ends of a rubber band, possibly as soon as the end of November.

If the warming in the stratosphere is strong enough to switch the typical westerly winds to easterly, it's called a major sudden stratospheric warming.

In records dating to the late 1950s, only twice before has a major SSW event happened as soon as November, in 1968 and 1958. And the last time one happened in December was in 1998. So, this could be the earliest such event in at least 27 years.

Why this all matters: Cold air can and does plunge into the U.S. and Europe without hiccups in the polar vortex.

When the polar vortex is strongest, circular and centered near the North Pole, such as the earlier image showed, you're less likely to see cold air plunge deep into North America or Europe. This stronger polar vortex effectively helps wall off cold arctic air from the mid-latitudes.

It's when the polar vortex is weaker, such as is forecast for late November or December, that blocking patterns may eventually set up over the higher latitudes that influence the persistence of cold air masses.

Decemberrrrrr: Despite this overall scenario, the forecast ahead for December may be tricky.

"This looks to be a prolonged event and I expect the weather models to struggle correctly simulating how it evolves and how it will impact our weather," Judah Cohen, MIT research scientist and director of seasonal forecasting at Atmospheric and Environmental Research told weather.com.

Cohen said this polar vortex weakening could increase the risk of severe winter weather over a prolonged period.

"The key for U.S. weather impacts is that the SSW event must end up altering the Pacific jet stream," University of Oklahoma associate professor Jason Furtado told weather.com in a 2019 interview, "...and that takes time."

"The U.S. usually has a delay — sometimes 2 to 4 weeks (after the polar vortex weakening) — in the effects, particularly the central and eastern U.S.," Furtado said.

That's just one factor — along with La Niña — in a generally colder December outlook for the northern tier, as you can see below.

How long into December or the rest of winter this colder pattern may last also remains unclear.

Previous January-March periods after an SSW have trended cold in the West and Plains, but milder in the Great Lakes and Northeast, according to Todd Crawford, Vice President of Meteorology at Atmospheric G2.

So, enjoy any milder spells in the northern tier of the U.S. while you can. That may change quickly in December.

(MORE: Winter Outlook)

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. Extreme and bizarre weather are his favorite topics. Reach out to him on Bluesky, X (formerly Twitter) and Facebook.