How the polar vortex could deliver one last blast of wintry weather

In a year without much winter weather, there’s a chance of a bitter blow before spring takes hold. Experts say a disruption of the polar vortex could unleash cold, stormy conditions to the eastern United States around mid-March.

There are lots of variables at play complicating the forecast, and winter has generally been tamer than many experts expected. But there’s a connection between breakdowns of the polar vortex and episodes of wintry weather in the Lower 48 states.

The stratospheric polar vortex is like a whirlpool of cold swirling over the Arctic. The stronger it is, the faster it spins, and the more bottled up its frigid air remains.

But in about two weeks, weather models indicate the vortex will abruptly weaken and/or collapse. The vortex disruption will be “fairly large,” Judah Cohen, a long-range forecaster at Verisk Atmospheric and Environmental Research, told The Washington Post.

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That means the gyrating eddy of cold air will fill in, heat up and potentially displace frigid air to the lower latitudes.

Depending on where those lobes of Arctic chill go, the stage could be set for at least a one-to-two week period of cold, stormy weather east of the Rocky Mountains by around mid-March.

To this point, winter weather has largely been absent in much of the Midwest, Great Lakes and Northeast. Snowfall has been considerably below normal and, it’s been so mild in some areas that this season is being described as a “lost winter.” Record-warm weather is forecast early next week and it could well end up as the mildest winter on record in the United States.

At the same time, the clock is ticking on how much longer wintry weather can even occur. The behavior of the vortex is all that stands between the mild weather in place now and a transition to spring.

How the polar vortex disruption could unfold

Cohen said the polar vortex’s disturbance could be so major that its winds will reverse — switching from strong westerlies to a gentle easterly flow. That’s not set in stone yet, but the odds are increasing.

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“A major sudden stratospheric warming is likely to occur during the first week of March,” Simon Lee, a research scientist at Columbia University, wrote in an email. “These events do, on average, bring about a negative North Atlantic Oscillation pattern at the surface.”

The negative phase of the North Atlantic Oscillation (NAO) is often associated with a zone of high pressure near Greenland which can force the storm track over eastern North America to shift southward and also cause cold air to migrate south.

The NAO is closely related to the Arctic Oscillation (AO) which describes how pent-up the Arctic’s cold air is. When the AO is positive, that cold air remains banked up north. When the AO flips to its negative phase, the frigid air sloshes south. Long-range weather models indicate the Arctic Oscillation will turn negative sometime around March 7.

Laying the groundwork

This season already produced one sudden stratospheric warming and associated polar vortex disruption in January.

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“What is remarkable is we have a second disruption to the stratospheric vortex happening right now,” Andrea Lang, a professor of atmospheric sciences at the University of Wisconsin at Madison, said in an email. “Two major disruptions to the polar vortex in one season is not common. It has happened before, but it is not something that you expect to happen in any given winter season.”

She noted that, when one sudden stratospheric warming event occurs, there’s a 50 percent chance of a cold air outbreak somewhere in the Northern Hemisphere. The shift in placement of cold air masses can also establish “storm tracks,” which favor corridors of more active weather.

The vortex disruption in January, however, did not result in particularly harsh winter weather in the weeks that followed.

Timing a possible return to winter

In the lead-up to the looming polar vortex disruption, the eastern United States is favored to be milder than average.

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“But leading up to and during [the disruption], Siberia and East Asia are the regions most likely to be cold,” Cohen said. “That is consistent with the weather model forecasts.”

It usually takes a week or two for the signal to translate from the upper atmosphere — known as the stratosphere, where the disruption comes with a pronounced warming — to the lower atmosphere, or troposphere. It also needs to initiate a chain reaction that takes time to reach the United States.

“The Eastern U.S. often does turn colder eventually but about two weeks later,” Cohen said. “That would put a return of colder weather in mid-March.”

However, if, during the sudden stratospheric warming, the vortex becomes stretched out, the cold may be more pronounced. Cohen likened the stretched vortex shaped to that of a rubber band, which could snap more cold air southward.

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“That can result in a quicker return of colder air to the Eastern U.S. and more intense cold (but not necessarily),” Cohen said. “This is something that I am watching but for now I don’t see any signs of it in the weather models. If we do get a stretched PV that piggybacks on the [sudden stratospheric warming], then colder weather can return to the Eastern U.S. sometime during the first two weeks of March.”

This is all working against the arrival of springtime though — the longer days, higher sun angle and overall warming of the Northern Hemisphere could be an obstacle for cold air outbreaks and snow.

“At this time of year, winter is fighting a losing battle against the seasonal cycle (and that’s in addition to the warming climate),” Lee said. “So, while spring might start on a relatively cool and wintry note, the reality is that the occurrence of a [polar vortex disruption] in March doesn’t mean the same in practice as one in the depths of winter.”

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David Tolleris, a consulting meteorologist based in Richmond who provides forecasts to the public and businesses, wrote on X that he’s unconvinced the polar vortex disruption will bring about meaningful winter weather.

“I am not saying those forecasting a late winter return in mid March are wrong,” he said. “Indeed it could very much happen especially across the Great Lakes/ New England and the interior portions of places like New York and Pennsylvania where it can snow into late March or early April. What I am saying is you need to treat this with a great deal of skepticism.”

Jason Samenow contributed to this report.

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