【Sexy Sisters】

The Sexy Sistersweather pattern across North America and into the Arctic is volatile and bizarre for this time of year, with unusually mild conditions continuing to surge into the Arctic while frigid air is being pulled south.

It's as if someone opened the door to the refrigerator that is the North Pole and let all the cold air drain away. For the Arctic, this means record-low sea ice and all-time high air temperature records.

But for a huge swath of more populated land, from Alberta to Quebec in Canada, and the Pacific Northwest to the Northeast in the U.S., a polar vortex-linked series of frigid blasts await.

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The forecast details are still being sorted out, with some computer models flip flopping by as much as 50 degrees Fahrenheit or more from one run to the next (I'm talking about you, GFS model).

However, ensemble runs, which are the result of running a computer model many times while varying initial conditions slightly, show that at least two frigid Arctic blasts are in store for Canada and the U.S. after this weekend.

Via Giphy

But we have to get through this weekend first.

The cold has been exceptional in the Rockies. On Thursday morning, Casper, Wyoming, hit minus-33 degrees Fahrenheit, which was its coldest temperature since 2006. Denver hit minus-10 degrees, which was a few degrees milder than the daily record there.

Temperatures will be as low as 30 degrees colder than average from the Rockies to the East Coast this weekend, with freezing temperatures reaching all the way into Florida.

Original image has been replaced. Credit: Mashable

But here's the unsettling thought: this cold blast isn't the polar vortex-related severe one everyone is talking about (and that Mashablefirst warned you about on Dec. 6).

After this cold snap, there are likely to be two more surges of Arctic air, each one of them as intense if not more so than the one currently boosting winter coat sales from coast to coast.

Original image has been replaced. Credit: Mashable

Exactly how severe the cold will be and when it'll hit is not set in stone just yet, but expect the first of these cold snaps to begin in Alberta and move southeast into the northern Plains and Midwest by midweek next week, with an extremely cold weekend in store for the Northeast starting on Dec. 16.

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Temperature departures from average are likely to be between 15 and 35 degrees Fahrenheit below normal for this time of year in areas that are exposed to the frigid air mass.

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High temperatures may not make it out of the single digits in parts of New England on Saturday, Dec. 17 if current projections hold true.

Subzero lows may occur from the Plains to the Midwest and the Northeast next week, especially if any snow falls during the period, which will help drive temperatures even lower.

Yes, this is a legit polar vortex event

For those readers who are suffering from a case of polar vortex fatigue and think the term is being overused, in this case at least, its use is justified.

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. 

Original image has been replaced. Credit: Mashable

When these winds weaken, as has been happening recently, filaments of the vortex can break off, and meander south into the U.S., Europe and parts of Asia. Last month, parts of the vortex set up shop over Siberia, causing exceptionally cold and snowy conditions there while driving unusually mild air into the Arctic and melting sea ice.

Now, some of that Siberian cold is headed for Canada and the U.S., with the assistance of a massive ridge of high pressure across Alaska and the North Pacific Ocean. That high pressure area is drawing Arctic air southward on its eastern flank.

Uncertainty about long-term pattern

"Our weather models are having severe complications with resolving the extent of the North Pacific ridge in the medium range. This is causing big swings in our medium-range weather models prediction over the U.S," said Michael Ventrice, a meteorologist with The Weather Company, in a Twitter message conversation.

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"I do think that the North Pacific ridge is going to be a staple point for this Winter season," he said, noting its connection to conditions in the tropical Pacific Ocean.

"This should keep the pattern volatile across North America, keep much of western-central Canada well below average on the temperature front, with periodic Arctic air mass intrusions sliding down the Rockies and spreading eastward into the eastern states," Ventrice said.

After the first punch of cold air exits the Northeast at the end of next weekend, long-range models suggest that the cold could regroup in western Canada, slide into the Pacific Northwest -- potentially delivering more snow to Seattle and Portland -- and then that cold may slosh south and east with time, eventually making it back to the Northeast.

Original image has been replaced. Credit: Mashable

An even greater uncertainty concerns the storm activity that will accompany these temperature transitions. It is likely that the Pacific Northwest will be targeted by repeated bouts of storminess, and that the Great Lakes will see round after round of lake effect snows. But whether the East Coast has a significant snowstorm to go along with the cold remains unclear.

Although it will be cold, there may not be many record lows set or tied during these outbreaks. This is partly because climate change has reduced the odds of historic cold snaps, according to a recent non-peer reviewed analysis from the research group Climate Central.

"Such cold air outbreaks are, in fact, decreasing in intensity both in observations and climate models primarily because the source of the cold air, the Arctic, is warming strongly,” the analysis found.

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