The polar plunge, which will sweep across the U.S. this week, has commenced.

The much-anticipated arrival of Arctic air has arrived in Montana and parts of the Northern Plains, where temperatures are 10 to as many as 40 degrees colder than they were at the same time yesterday.

Big 24-hr temp change in Montana and Wyoming. pic.twitter.com/PUsML95DTb — Tim Ballisty (@IrishEagle) November 10, 2014

In Montana and parts of the Dakotas, temperatures currently stand in the high single digits to mid-teens, with wind chills below zero. This gateway region for the Arctic blast will feature the most brutal winter conditions for the entire week with highs not escaping the teens and overnight lows frequently dipping below zero.



U.S. temperatures at 10 a.m. ET (Oklahoma Mesonet)

But locations to the south and east will not escape the wrath of the cold.

The Arctic front will steadily drive towards the East Coast by Wednesday night, with temperatures plummeting it in its wake.

Denver expects a jaw-dropping 40 degree temperature free-fall between this morning and tonight, from the 60s into the teens.

In the Midwest, major cities are bracing for a 20 to 30-degree drop in temperatures between today and Tuesday.

Talk about a polar plunge! Air 20-40° below average will spill ALL the way 2 the Mexico border. Brr #PolarAirInvasion pic.twitter.com/xLW3w30g2X — Kathryn Prociv (@KathrynProciv) November 10, 2014

As front progresses East, it will lose some of its punch. For example, temperatures forecast to be 20 to 40 degrees (or even more in a few areas) below normal in the northern Rockies and Northern Plains (for much of the week) will ease to 10-20 degrees below normal by the time the front clears the East Coast Thursday.



Animation of temperature difference from normal at 7 p.m. ET each day Monday-Friday, as simulalted by the GFS model (WeatherBell.com)

But while the intensity of the Arctic cold will wane as it presses east, it will be still be plenty chilly. Afternoon highs will struggle to reach the low 40s along the I-95 corridor from D.C. to Boston Thursday as the core of the subfreezing cold holds true in the Great Lakes and Northern Plains.



GFS model simulation of temperatures at 1 p.m. Thursday (WeatherBell.com)

All indications are that this cold pattern will not relent, but rather reload this weekend into next week. Temperatures from the Rockies to the East Coast are forecast to remain well below normal for the next 8-14 days.



National Weather Service Climate Prediction Center 8-14 day forecast

The primary driver of this cold pattern is a massive bulge or ridge in the jet stream over western North America, in part pumped up by the weekend’s record-challenging North Pacific storm. The ridge is forcing the jet stream to then drive southward over the eastern two-thirds of the U.S., transporting south unseasonably cold air from the Arctic.



Upper level weather pattern shows massive ridge in the jet stream over western North America forcing the jet stream and Arctic air southward. Simulation from GFS model, valid Thursday afternoon. (WeatherBell.com)

An indicator of the strength of an Arctic air mass is its surface pressure. The higher the pressure, typically the more intense the associated cold air. On Tuesday night, the GFS model forecasts the high pressure center sitting over the northern Rockies to have a high pressure center exceeding 1050 millibars, which is an impressively high value in January, and rare for the month of November (3 to 4 standard deviations above normal).



Surface pressure forecast and difference from normal forecast by GFS model Tuesday night (WeatherBell.com)

Here’s a refreshed video explainer I worked on (last week, updated this morning) covering this cold air outbreak:

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