For most of the U.S., the weather of early-to-mid January will be the polar opposite of what it was in December.

The East, which roasted like chestnuts on an open fire in December, is looking at multiple shots of cold, Arctic air and potential snowfall events during the next two to three weeks.

Meanwhile, in southern California, which was left high and dry during December, the long-advertised El Niño-related wave of storms has begun, bringing heavy rain, mountain snows and potential flooding and mudslides. More such storms are expected as the winter progresses.

Change in the shape of the polar vortex between the last week in December (left) and the second week in January (right). Image: Judah Cohen/AER

To what do we owe this reversal of atmospheric fortune?

The dramatic weather pattern shakeup has ties to a complex series of interactions between a near-record strong El Niño event in the Pacific Ocean, a series of atmospheric disturbances in the northern Pacific, and one freak storm in the North Atlantic that transported warm, moist air from the tropics to the North Pole, thereby disrupting one of the more famous weather phenomena there is: the polar vortex.

Piece of the Polar Vortex (circled) forecast to roll into the U.S. on Jan. 10, 2016, according to a computer model projection. Image: WeatherBell Analytics

It might seem far-fetched that a single, near-record strong storm that hit Iceland on Dec. 28 to 30 could, in large part, be responsible for tearing off a piece of relatively frigid air from the Arctic, and blasting it southward into the U.S. by the second week of January.

After all, these events are far apart in both time and distance.

But this was no ordinary storm, and the upcoming cold air outbreak may be exceptional as well (at least in the context of this winter so far), judging by computer model projections.

Storm Frank helps upset the polar vortex

First, there's the storm, which the UK Met Office named "Storm Frank." This storm was one of the strongest on record to form in the North Atlantic Ocean at this latitude, and it lashed Iceland and the North Sea with 90 to 100 mile per hour winds. Its minimum central pressure, which is the main indicator of the storm's intensity, bottomed out at 928 millibars on Dec. 30. In general, the lower the pressure, the stronger the storm.

By comparison, Hurricane Sandy had a minimum central pressure of 946 millibars when it came ashore in New Jersey on Oct. 29, 2012.

Storm Frank also blasted the UK with flooding rains, exacerbating an already dire flood situation there.

But most important of all for the U.S., the storm helped transport a huge amount of heat and moisture from the midlatitudes to the top of the world. In fact, the storm, which on satellite imagery resembled a coiled snake, temporarily boosted temperatures near the North Pole to just above freezing, despite the 24-hour winter darkness and average temperatures of about minus-20 to -25 degrees Fahrenheit.

That unusual warmth helped expel the coldest air in the Arctic southward, between Greenland and Canada. It's as if someone flushed the Arctic toilet, and all of the ultra-cold air that had been sitting at the top of the planet drained out to the North Atlantic, moderating in the process.

Temperature anomalies from the GFS computer model on Dec. 28, 2015, showing the plume of milder than average air flowing toward Greenland and the Arctic. Image: WeatherBell Analytics

This has left several spots of frigid — but not ultra cold air — scattered about the northern latitudes, some of which is slated to leak southward into the U.S.

"As the warm air went around the eastern side of Greenland, the cold, northern Canadian air was evacuated over the North Atlantic," said meteorologist Ryan Maue of the private weather firm WeatherBell Analytics, in a Twitter message exchange with Mashable.

The addition of that heat to the Arctic may also have disturbed the polar vortex, which is a weather feature that became a pop culture phenomenon during the winter of 2013-14.

The main polar vortex is a circulation of air enveloping a near-permanent area of low pressure that exists in the upper atmosphere, above typical cruising altitudes for commercial jetliners, over the Arctic. When these winds weaken, as has been happening recently, pieces of the vortex can break off, and drunkenly wander south into the U.S., Europe and parts of Asia. A weakened polar vortex can be likened to a spinning top that starts to wobble more as its speed decreases.

View of a wobbly polar vortex on Jan. 5, 2014. Image: NOAA

There is the main vortex that exists in the stratosphere, about 55,000 feet high, as well as a related circulation lower in the atmosphere, in the troposphere.

While the part of the polar vortex that resides in the stratosphere is not forecast to be significantly distorted during the next few weeks, the tropospheric vortex — which is the layer of air in which most weather occurs — is already starting to wobble.

Theme of the next two weeks is cold "shots" for US East. Tropospheric PV placement favors persistent cold in Plains. pic.twitter.com/DxqDMKTlzF — Jason Furtado (@wxjay) January 4, 2016

"To get the weakening of the polar vortex you had to get heat transport into the Arctic,” Cohen said. “Storms are the vehicles that get the heat out of the subtropics and into the Arctic.”

“Clearly Storm Frank was maybe the match that lit the fire,” said Judah Cohen, principal scientist and director of seasonal forecasting at AER in Massachusetts, in an interview, adding that the strong El Niño may have added even more heat energy to the system.

“We’re getting the kind of return, the round trip of this warm air,” Cohen said. “It displaces that cold air that’s there and that heads southward," he said of the unusually mild air the Iceland storm brought into the Arctic.

“It’s not so much breaking down as it is getting stretched and pulled in a north-south direction — it’s more like a distortion of the polar vortex,” Cohen said.

This is what has happened during the past few winters, as scientists have observed a wavier jet stream pattern that has often led to a milder than average Arctic and cooler than average midlatitudes.

Some scientists have hypothesized that the increased prevalence of unusually mild Arctic winters combined with colder than average midlatitude winters, known as a "warm Arctic, cold continents" pattern, is tied to global warming and the loss of sea ice in the Arctic. Cohen's own research has tied sea ice loss in the summer to the greater buildup of snow in Siberia and wintertime Arctic outbreaks across the northern hemisphere, for example.

However, this is still a subject of active scientific research.

Pacific ties

Maue, however, disagrees. Rather than looking at Storm Frank as the domino that set off a weakening of the polar vortex, he says, the storm itself should be seen as part of a domino effect originating in the heat being evaporated into the atmosphere from a hotter than average Pacific Ocean. Weather patterns around the world are being reshaped by the heat added to the atmosphere from one of the top three strongest El Niño events on record, which is centered in the tropical equatorial Pacific.

According to Maue, El Niño events can influence storms forming in the North Pacific Ocean, which can then reverberate downstream, all the way to the skies above the North Atlantic Ocean.

In this way, Storm Frank should be viewed as being tied to El Niño, Maue wrote. This would also mean the upcoming Arctic outbreak in the U.S. is, in some way, tied to El Niño, albeit via a circuitous path.

"The downstream impacts are surely being felt today — but the storm itself was a result of an upstream impact from something else," Maue wrote.

However, where the dominoes begin and end is not an easy question to answer when the weather is involved, especially now, when El Niño, manmade global warming and natural climate variability are all vying for influence. The combination of these three factors made 2015 the warmest year on record, and may make 2016 even milder on a global basis.

“We don’t have a good research basis for saying storms in the Atlantic are significantly influenced by the El Niño," said Richard Seager, a research professor at the Palisades Geophysical Institute at Columbia University's Lamont Doherty Observatory, in an interview with Mashable.

While there is more "connectivity" between the North Pacific and North Atlantic storm track in El Niño years, Seager said, “The El Niño impact on storms over the Atlantic sector is not known to be very strong.”

Either way, residents of the Plains states and parts of the Midwest are about to get blasted by record-challenging cold weather, with lows of well below zero Fahrenheit likely beginning late this weekend, possibly lasting for a week or more.