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Much of the U.S. is in the deep freeze as of Dec. 15, with wind chills near or well below zero from the Plains to Washington, D.C. Power lines crashed to the ground Wednesday night in Portland, Oregon, due to the weight of snow and ice.

In Minnesota, the statewide average temperature as of 9 a.m. local time was a brutally cold minus-14.3 degrees Fahrenheit. The coldest temperature in the lower 48 states this morning was minus-22 degrees Fahrenheit, set at both Aberdeen, South Dakota and Fosston, Minnesota.

And don't even ask what the wind chills were in these locations.

SEE ALSO:The polar vortex is going to punch you in the face, repeatedly

In Chicago, the temperature was a big round goose egg at 9:30 a.m. CT (i.e. 0 degrees Fahrenheit), with a wind chill of minus-18 degrees Fahrenheit.

In the East, the cold is peaking on Thursday and Friday, with a brutal blast of cold air expected to sweep across New England on Thursday night, with 60 mile per hour winds causing power outages from New York City to Maine as wind chills plummet to dangerous levels of at least 25 degrees below zero in some locations.

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And this is just the first wave of Arctic air associated with a displacement of the dreaded polar vortex.

The next polar plunge will take place this weekend, and will be squarely targeted at the Upper Plains and Midwest. According to the National Weather Service, a significant snowstorm will precede the second surge of polar air, which will arrive on Saturday.

Widespread temperatures in the minus-20s Fahrenheit on Saturday night, with wind chills down to minus-45 degrees Fahrenheit are predicted for the Minneapolis area by early Sunday, with similarly cold weather in nearby states.

Polar vortex redux

As is usually the case with extreme weather events, there is not one single factor to blame.

Weather and climate events as far away as the North Pacific Ocean have helped create a wavy jet stream, with steep dips or troughs as well as sharp ridges of high pressure, which is conducive for allowing bitterly cold Arctic air to seep out of Siberia and Alaska -- like a freezer door left open all night.

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This weather pattern is related to the part of the polar vortex residing in the troposphere, which is the lowest layer of the atmosphere in which most weather takes place. There are currently two areas of intense cold around the world that are tied to a displaced polar vortex.

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, with the center of the low typically located above the central Arctic. 

When these circumpolar winds weaken, as has been happening recently, filaments of the vortex can break off, like a figure skater wobbling a landing, or a drunk person walking home from a bar.

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Temperature anomalies showing unusual cold across Siberia and North America, with warm temperature anomalies in the Arctic.Credit: climate reanalyzer

These pieces of ultra-cold air can then meander south into the U.S., Europe and parts of Asia.

 One of these polar vortex pieces is pinwheeling above central and southern Canada, and the other has gone on sabbatical over much of northern Eurasia, keeping Siberia locked in a freeze that is unusual even for a region synonymous with frigid temperatures.

The Siberian cold has been accompanied by unusually warm conditions in the Arctic itself, which scientists have linked to long-term global warming as well as short-term weather variability.

During the next week, warm temperature anomalies of at least 15 degrees Fahrenheit above average are predicted to sweep across the central Arctic, further hindering sea ice growth, which has already been moving at a record-slow pace.

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While the polar vortex is plaguing large parts of the U.S., it is not forecast to maintain its grip on the Northeast or Mid-Atlantic throughout the rest of the winter, or even the rest of the week.

Meteorologists will be closely watching what happens to round two of this cold snap. Right now, it appears the cold will peak in the Midwest, and then temperatures will moderate as the cold weather spins eastward, possibly affecting the East Coast early next week.

This is not record cold

What's remarkable about this cold snap, however, is the absence of cold temperature records being set. This is where the impact of global warming comes into play.

Polar vortex events in the U.S. originate in the Arctic, and the Arctic has been warming sharply -- at twice the rate of the rest of the world -- in recent years, with the past year going down as the warmest year since at least 1900.

Core of the polar vortex-related cold air located across the Northeast on Dec. 15-16, 2016.Credit: weatherbell analytics

Because the source regions for Arctic outbreaks are seeing warming, it is less likely that the U.S. will experience cold snaps that you may have grown up with, or that your parents may talk about as being common during their younger days.

During the past week, for example, there have been 90 warm temperature records set or tied across the U.S., with 126 cold temperature records, according to preliminary data from the National Centers for Environmental Information.

None of the cold records have set monthly or all-time records, which would signify a truly remarkable cold air outbreak.

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During 2016, the ratio of record highs to record lows has been highly skewed, at 6-to-1, according to NCEI data.

In November, that ratio was astoundingly skewed, with 48 record highs set or tied for every record low. The climate research group Climate Central found that 98 percent of all the daily temperature records set last month were record highs.

What matters more with global warming is long-term data over many decades, and there too, the ratio of record highs to record lows has increased during every decade since the 1970s.

According to a recent study, a 3-degree Celsius, or 5.4-degree Fahrenheit, increase in average U.S. temperatures could increase the ratio of daily record highs to lows to 7-to-1.

Arctic temperature departures from average in 2016.Credit: noaa

An analysis from a multinational team of scientists that Climate Central is affiliated with found that cold air outbreaks like the one underway right now have been decreasing in intensity both in observations and in computer modeling studies due in part to Arctic warming.

Even in Alaska, recent cold has not set records. Fairbanks saw a low temperature of minus-36 degrees Fahrenheit during the period from Dec. 3 to Dec. 5, for example, but that city is accustomed to lows below minus-40 degrees Fahrenheit, which have been growing rarer with each passing decade.

This knowledge may not be much comfort when you're walking home against the fierce wind, hoping your eyelids don't freeze together. But the long-term perspective is important when putting this event into context.

(责任编辑：熱點)