For the second year in a row, a gentle winter has eased along in the Mid-Atlantic and Northeast, with near- to below-average snowfall and mild temperatures. Weeks ago, it appeared that an influx of snow and cold might accompany an ongoing disruption to the polar vortex, but the visit from Old Man Winter has yet to materialize.

Experts now say that although the bout of unsettled wintry weather is delayed, it’s too soon to count it out.

The possible spell of wintry weather in the pipeline is thanks to a series of disturbances that have shaken up the polar vortex, eating into its circulation and leaving the feature “wrecked,” according to Paul Newman, chief scientist for earth sciences at the NASA Goddard Space Flight Center. But the impacts have yet to be felt in the Lower 48.

Ever since a storm in mid-December, the winter has been tame for most East Coast residents. New York City has had 10.5 inches to date this season. Boston is near a foot and a half, while Philadelphia stands at 6.6 inches. Washington, D.C., hasn’t seen a stitch of measurable snowfall.

And while there hasn’t been a single solid blockbuster snowstorm that’s targeted any of these coastal cities, atmospheric scientists warn that surprises can be in store when the polar vortex is weak and wobbly — as is currently the case.

Why the polar vortex matters

The stratospheric polar vortex is a whirlpool of high-altitude cold air that swirls around a low-pressure area near the North Pole. It’s born as a result of polar night, a months-long dark spell that accompanies the dead of winter in the Far North. The lack of sunlight chills the air, jump-starting the gyrating icebox’s spin every winter. It vanishes once spring returns, only to re-form by September or October.

When the vortex is strong and stable, it keeps cold Arctic air bottled north and the Lower 48 beyond the reach of its icy grasp. But disturbances that knock the vortex off-kilter can unleash that frigid air, bringing stormy weather and increased snow chances to much of the United States and Canada, while also affecting Europe and Asia.

The polar vortex now

In late December, a record-setting high-pressure area in Mongolia and a hurricane-force low-pressure system in the North Pacific sent upward moving atmospheric waves into the stratosphere, disrupting the high-altitude stratospheric polar vortex and causing a sudden stratospheric warming event. This warming event was so strong that it caused the whirlpool of winds encircling the pole to slacken and then even reverse direction, as the cold was displaced to the south.

The stratosphere has transferred some of this energy into the lower atmosphere, via downward-moving atmospheric waves, where it can help determine which areas get buried in blizzards while others see unusually mild conditions. It’s not guaranteed that each polar vortex disruption or stratospheric warming event will yield unusually cold and snowy conditions, given how unique each event is.

The ongoing event, which kicked off in early January, has been complex. “Typically, when you get a large disruption, that’s it,” said Judah Cohen, a polar vortex specialist and director of seasonal forecasting at Atmospheric and Environmental Research. “[But this time] we got a split, then it recovered; it kind of split again, and now it’s displaced, and it could split a third or fourth time.”

“On January 4 and 5, a large-scale wave developed and propagated up into the stratosphere that basically wrecked the polar vortex,” explained Newman. “If you look at maps of the stratospheric polar vortex, you see two big chunks floating around.”

One of the lobes has already brought extreme cold to Siberia, where temperatures in Yakutia, in eastern Russia, haven’t climbed above minus-40 in more than a month, according to the Associated Press. Delyankir, in northeastern Russia, dropped to minus-73 degrees Jan. 18. The concentrated cold has been fierce and extreme, and looks to remain in place in eastern Russia through at least early next week.

What the disruptions mean

When the polar vortex is disrupted, its weakening can induce a “Greenland block,” or a stagnant regime of cold, high pressure over Greenland. That helps channel cold air along the Eastern Seaboard, helping tip the scales toward snowfall if a storm system brews. Blocking high pressure also translates to slow-moving storm systems, since the high pressure acts as a downstream logjam that allows other systems to become backed up.

That’s all predicated on an East Coast storm system actually forming, which hasn’t been the case this go-round. Moreover, the exact strength and location of blocking hasn’t quite been the norm.

“For this event, though we have seen fairly typical influence of the [polar] disruption on the surface in the form of persistent high-pressure systems (or ‘blocking’) over the Arctic, many locations have not yet seen extreme cold,” wrote Amy Butler, a research scientist at the National Oceanic and Atmospheric Administration’s Chemical Sciences Laboratory, in an email. “This seems tied to where in particular the blocking has occurred, which may also be affected by tropical influences.”

She also noted that the messy split of the polar vortex may be causing it to play out differently than other years when the vortex was displaced.

“While the polar vortex has become very stretched out and wobbly, it has not displaced as strongly southward or split as clearly into two lobes as in other events,” wrote Butler. “[That] means it might not be able to influence the underlying jet stream quite as well.”

What lurks ahead

Signs also point to the vortex remaining unsteady and unstable for the remainder of the season. That bolsters the odds of wintry weather in the northeastern United States, and some weather models agree on this outcome.

“Typically when we see a major warming like this, the stratosphere has a hell of a time recovering,” said Newman, who explained that it’s just too late in the season to expect the polar vortex to regain its former strength. “We will not get to the cold temperatures in Arctic [stratosphere]; the vortex will stay in a weakened warm state.”

“The GFS [forecast model] has been suggesting some further disruption … which could possibly lead to stretching of the polar vortex over North America,” Butler wrote. “If it becomes disturbed enough, it could help nudge the jet stream south, allowing cold air to spill from Canada into the U.S. But this prediction is for a couple of weeks from now, which is still too far in the future for any guarantees.”

Butler says that other influences, including thunderstorm activity in the tropics and other large-scale atmosphere and ocean cycles, could play a role in influencing the overall weather pattern.

For his part, Cohen thinks the chaos in the stratosphere hasn’t yet truly manifested itself, and there’s much more in the cards.

“Typically, when you have a [polar vortex] disruption, it takes about two weeks for the signal to get to the surface,” he said. “One consequence from these multiple disruptions is the signal is delayed.”

His gut tells him late January into February could be interesting.

“It’s not over,” Cohen said. “I think it’s just getting started.”

Andrew Freedman contributed to this report.