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Disturbed polar vortex bringing delayed dose of winter to eastern U.S.

Four weeks after a disruption to the polar vortex, chillier and stormier weather is trickling toward the mid-latitudes

The American GFS model simulates cooler than average temperatures over the eastern U.S. over the next two weeks. (WeatherBell)
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In mid-February, air temperatures 10 to 30 miles above the North Pole spiked as much as 50 degrees, setting in motion a chain-reaction process that’s already influencing the weather over North America. Known as a “sudden stratospheric warming” event, the abrupt warm-up of the stratosphere — the second layer of Earth’s atmosphere — disrupted the polar vortex and led to its demise.

Since that episode three weeks ago, dominoes have been toppling that have shuffled weather patterns over North America. Now, it’s looking like parts of the eastern United States — which have largely skirted Old Man Winter’s wrath to date — may be in for a spell of cold and increased winter storm chances.

Already, the development of high pressure over Greenland — a textbook response to polar vortex disruptions — is helping to displace cold air from the Arctic and transplant it south. The Arctic has suddenly turned unseasonably mild in many areas, while below-average temperatures are arriving in the eastern United States, all while a pair of winter storms prepare to march from the Upper Midwest into the interior Northeast and Mid-Atlantic.

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It comes as the calendar inches ever closer to astronomical spring, but winter may not be fully ready to relinquish its grip yet. Experts believe the polar vortex’s disruption is already having an impact — and winter’s final hurrah could be noticeable.

What happened to the polar vortex?

The stratospheric polar vortex is like a fierce atmospheric whirlpool located at two or three times the altitude that commercial aircraft fly. Frigid temperatures over the North Pole, brought on by a lack of sunlight, cause columns of air to become more dense and contract toward the ground. That creates a void of sorts in the upper atmosphere, which induces an inward vacuum-like effect. As the surrounding air spirals counterclockwise into the vortex, it too cools, repeating the same process.

The stronger the vortex, the faster it whirs. That keeps Arctic cold bottled up at the high latitudes. But if something happens to weaken the vortex or disrupt its circulation, then lobes of bone-chilling air can slosh toward the mid-latitudes.

Why do we care?

It takes weeks, but after each polar vortex disruption or sudden stratospheric warming event, impacts can begin to manifest in the lower atmosphere.

“It’s pretty typical that there [is] a long lag between its onset and maximum apparent impact on the weather,” wrote Simon Lee, an atmospheric scientist at Columbia University, in an email.

Lee said that the sudden stratospheric warming came in two spurts — an initial reversal of the stratospheric polar vortex’s winds on Feb. 16, and then a doubling down of that vortex weakening late in the month.

“[The Feb. 16 event] was initially too brief and too high up to have much impact, but once this was reinforced, things changed,” wrote Lee.

That’s why only now are the effects fully being realized.

Impacts on North America

Researchers point to a several telltale signs that the sudden stratospheric warming is starting to shape mid-latitude weather patterns.

First, the weakening of the polar vortex helped flip the AO, or Arctic Oscillation, to a negative state. When the AO is positive, frigid air tends to be bottled up over the Arctic as the polar vortex is strong and stable. But when the AO swings negative, it signals that the vortex has become disturbed and floodgates are open for Arctic air to spill south.

Judah Cohen, an atmospheric scientist with Atmospheric and Environmental Research, likened the polar vortex’s piecemeal influence on mid-latitude weather to “dripping paint,” with each drip marking the emergence of cold air and potential wintry weather. He says one is already affecting northeastern North America, and wrote in a Twitter direct message that models “also show another drip or downward influence in about a week, [suggesting] there may be more.”

Second, a “Greenland block” has become established. One of the symptoms of a classic sudden stratospheric warming event is often the establishment of this block, which is a stagnant dome of high pressure near Greenland. It deflects the jet stream south in eastern North America, allowing cold air from the Arctic to spill southward.

Although much of the eastern United States has seen record warmth since the December holidays, colder weather is starting to trickle in. In the next six to 10 days, the National Weather Service Climate Prediction Center is highlighting the East as having “very likely” odds of below-average temperatures, and calls for cooler-than-average weather all the way into early April.

Moreover, the presence of a stalled high-pressure system can spur an atmospheric logjam of sorts, maintaining a period of above-average storminess over the East Coast. While that has yet to happen, signs point to an unsettled stretch in the next one to two weeks. Forecast models show the potential for a mix of rain and snow from back-to-back storm systems late this week and early next week in parts of the Mid-Atlantic and Northeast.

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On a phone call, Cohen explained that the effects of the vortex disruption probably began at least a week ago.

“I do think we’ve already seen impacts,” he said. “We had a closed low [pressure system] over western Russia and Eastern Europe. It kind of got cold and snowy there. We also saw a closed low tried to develop in the Canadian Maritimes. Boston, New York and D.C. haven’t seen much snow, but the New England ski resorts did.”

He attributed part of northern New England’s snow to an immediate shake-up in late February following the actual polar vortex disruption itself.

“Portland, Maine, is practically at normal [snowfall] for the season,” he said. “I think that was a quicker impact from the polar vortex disruption.”

That said, one thing is certainly working against snowfall odds, much to the dismay of winter lovers — time. The days are growing longer, the sun angle is getting higher and whispers of spring are beginning to sneak into the atmosphere.

“The clock is definitely about to strike midnight,” Cohen said. “After a week or two, the polar vortex influence might wane or disappear … but that’s still a question mark.”