After more than a week of warning, a frigid Arctic airmass is finally sweeping across the eastern half of the United States. Tonight and tomorrow, dangerously cold temperatures will encase the country from the Plains to the Atlantic and down to the Gulf of Mexico. This will probably—hopefully—be the coldest air we'll see this winter.

The airmass is so potent that it will flood down into Mexico and Florida—in fact, wind chill advisories are in effect for almost the entire eastern half of the country, stretching from northern Minnesota to southern Texas and from northern Maine to southern Florida. While it won't be as brutal as what we saw last year, temperatures will bottom out more than 20 degrees below normal in many spots overnight tonight and during the day tomorrow. Many people in the south, especially, are not equipped (clothing- and heating-wise) to deal with this kind of cold weather.

The Setup

As I've reiterated for the past week, people are incorrectly calling this "the second coming of the polar vortex." The cold snap is influenced by the polar vortex, but it's nothing like what we experienced last year. The polar vortex is a persistent, large-scale cyclonic circulation in the upper levels of the atmosphere that sits over (or near) the North Pole. The circulation acts like a moat that keeps cold air confined to the far northern latitudes; when the circulation breaks down and becomes wavy or splits into several different upper-level lows, it can allow that cold, Arctic air to spill south into North America.

Last year, the main circulation in the polar vortex dove south and sat over the Great Lakes for a couple of days, bringing that horrendously cold weather that many of us remember. This year, the main circulation of the polar vortex is sitting between Greenland and Canada, centered over a large island in eastern Nunavut known as Baffin Island. As you'll see in a second, temperatures underneath the polar vortex's circulation are going to be unimaginably cold tonight—down to -50°F in some higher spots in northern Quebec.

A deep upper-level trough extending off of this circulation is cutting its way across the eastern half of the United States and Canada, allowing bitterly cold air to spill south. The above animation shows the jet stream at the 300 millibar level of the atmosphere (the height jets usually fly)—contours show heights in decameters, while the shading shows the jet stream's winds in knots. The animation runs from 7:00 AM EST today until 1:00 PM EST tomorrow.

It's a little hard to see everything because the map is so busy, but the closed contours near the top-right part of the map is the polar vortex over Baffin Island. The sharp dip in contours and shading across the eastern U.S. and Canada (all the way down to the Florida Keys) is the trough extending off the upper-level low, allowing the cold air to spill south for a day or two.

Temperatures

The coldest weather will remain along the Canadian border across interior parts of New England, where lows tonight will bottom out between -20°F and -30°F in many locations. This region is under a wind chill warning, as gusty winds will make those brutal temperatures feel as cold as -45°F on exposed skin. Frostbite will develop in a matter of minutes in temperatures and wind chills that extreme, and hypothermia will also develop if one is outdoors for a prolonged period of time without the proper protection.

Subzero lows are also a good bet across the western and southern Great Lakes region, with single-digit-below-zero lows expected from Ohio to the north and west through the Dakotas. Elsewhere, low temperatures in the single digits (above zero) are likely from New Jersey southwest through Tennessee and west through the Plains.

Temperatures down to freezing are possible as far south as Tampa, Florida. Tomorrow's record low temperature for Tampa is 28°F set back in 1970. The record low in Mobile, Alabama for January 8 is 19°F, which was set during last year's apocalyptic panic-a-thon; tonight's forecast low for Mobile is 18°F, which will be a new record for the date if the forecast verifies. Again, this cold outbreak is neither unprecedented nor the worst we've ever seen, but it's still a shock to the system for people who aren't used to such cold weather.

Highs will struggle to climb above freezing during the day on Thursday for many locations outside the Deep South.

Wind Chills

The threat of wind chills cannot be understated. Much like the heat index conveys the danger of combined heat and humidity (well, really dew point) on your body, the wind chill combines temperature and wind speed to tell you how cold the air will feel on your skin. Moving air helps to cool surfaces down more quickly (think: blowing on hot food before you eat it), and wind on an extremely cold day causes greater heat loss than you would otherwise experience if the air were perfectly still.

Say you're standing outside on a -10°F morning with a 25 MPH sustained wind, and your face is completely exposed to the elements. -10°F is cold enough, but the combined effect of that numbing airmass and the 25 MPH wind makes it feel like the air temperature is -37°F. Standing outside with a -10°F air temperature and a 25 MPH wind has the same effect on your exposed skin as a -37°F air temperature. Frostbite will begin to develop in just 10 minutes under those conditions.

Wind chills at 5:00 AM will be in the low teens as far south as northern Florida, with readings far below zero the farther north you go. Again, interior parts of the Northeast will see wind chill readings approaching -45°F during the worst cold tomorrow morning. Anyone caught outside—especially the homeless and those who work outside—who aren't wearing proper clothing will develop frostbite in less than an hour, and possibly even hypothermia in the worst cases.

The Day After Tomorrow

What happens after Thursday? The cold will stick around for at least five days, but moderate a little with each successive day. The GFS model, pictured above, paints subzero lows on Saturday morning from the upper Midwest into Pennsylvania, with single digits across the major I-95 cities in the Northeast. It will get better as we head into the middle of the month. Both the European and GFS models indicate that most of the next two weeks will feature temperatures at or above normal, with a few short-lived cold shots spliced in between.

It's great to see January acting like January should, even if temperatures are on the extreme side of the equation. It'll get better, and if you're counting, there are only 72 days until astronomical spring.

[Images (in order): Tropical Tidbits, NWS, WeatherBELL x2, NWS x2, WeatherBELL]

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