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At a Glance The polar vortex was broken apart in early January.

This could lead to a blocked jet-stream pattern by the end of January.

Persistent cold is likely to be the result in Europe and the U.S. The stratosphere above the North Pole warmed dramatically in early January, and that is now leading to a colder, snowier end to January and possibly well into February.

You've probably heard of the polar vortex before. It became a social media and pop-culture sensation during the frigid January of 2014.

The polar vortex is a whirling cone of low pressure over the poles that's strongest in the winter months due to the increased temperature contrast between the polar regions and the mid-latitudes, such as the United States and Europe.

This isn't like a storm you might think of in the lower atmosphere, with cold and warm fronts producing rain or snow. Instead, the polar vortex occurs primarily in the stratosphere , a layer of the atmosphere about 6 to 30 miles above the ground – above the troposphere, where most of the weather with which you're familiar occurs.

Typically, the polar vortex looks like what is shown below – a circle of strong winds around colder air.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/strong_polar_vortex_2.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/strong_polar_vortex_2.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/strong_polar_vortex_2.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Example of a strong polar vortex near the North Pole in winter.

When the polar vortex is strongest, cold air is less likely plunge deep into North America or Europe (by strongest, we mean the generally west-to-east flow around the vortex is stronger than average).

The easiest way to think of this is in terms of a wall. A stronger polar vortex helps to wall off cold, arctic air from the mid-latitudes.

In early January, that vortex weakened dramatically , but a weaker polar vortex allows the cold air typically trapped in the Arctic to spill out into parts of Canada, the U.S., Asia and Europe.

This weakening was caused by a sudden stratospheric warming (SSW) over the North Pole, seen as the rapid spike in temperatures below.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/SSW_Jan2019.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/SSW_Jan2019.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/SSW_Jan2019.jpg?v=ap&w=980&h=551 800w" > The black line shows daily temperatures 25 miles above the North Pole, and the gray line indicates the average temperature for the 30-year period between 1981 and 2010. (Japan Meteorological Agency)

Any sudden 60-degree rise in temperatures will have ramifications, but the warmth covers much of the stratosphere over the Arctic and even encroaches into northern Asia, Greenland, Canada and northern Alaska .

This SSW is defined as a major event by the Japanese Meteorological Agency since winds across the poles reversed their orientation as the polar vortex fell apart.

Despite the magnitude of this warming, these events aren't unusual, typically occurring at least once every other cold season, according to Dr. Amy Butler, an atmospheric scientist specializing in stratospheric warming at the University of Colorado's Cooperative Institute for Research in Environmental Sciences.

This warming allowed the polar vortex to split into three distinct circulation areas in the first 10 days of the year.

Each of these circulations allows more air to flow across the polar region, which means more cold air is able to travel southward into Canada, Russia and Europe.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/011019_SSW_wind.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/011019_SSW_wind.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/011019_SSW_wind.jpg?v=ap&w=980&h=551 800w" > Early January circulation in the stratosphere over the Northern Hemisphere.

When this happens, the weaker polar vortex and the effects of the stratospheric warmth trickle down to make the jet-stream pattern from parts of North America to Europe more prone to blocking, such as the Greenland block, known by meteorologists as the negative phase of the North Atlantic Oscillation.

This blocking influences the location and persistence of cold air masses.

Since this blocking happens in the colder months, that can break down the cold-air wall, leading to a persistently cold pattern for weeks.

(MORE: 5 Extreme Weather Patterns to Look for in Winter )

The Outlook

Typically, an SSW event will turn into an increase in cold air over North America and Asia weeks after the peak warmth, and since that event occurred in early January, we're now seeing the effects.

The cold risk is greater as the month progresses, with increasing confidence in below-average temperatures arriving toward the end of January and into February.

(MORE: Below Zero Temperatures Expected Early Next Week in Northeast, Midwest)

Signals are increasing that cold air and increasing snowfall will be common in the last 10 days of January.

Along with the cold, increasing storminess is also expected in the eastern half of the country, beginning with Winter Storm Harper .

Much of the East Coast and northern tier will likely be wetter than average through the end of the month, according to the Climate Prediction Center. Much of this moisture will fall as snow.

A similar polar vortex split happened in February 2018, which eventually triggered a pair of “Beast From the East ” March cold outbreaks in Europe, then a parade of four nor’easters that hammered the East Coast in March, followed by persistent cold, which lagged in the nation’s midsection into April .

The effects of SSWs can last four to eight weeks as atmospheric information drips from the stratosphere down closer to the ground.

We've already seen an artifact of the SSW in Europe.

Several weather systems have piled up more than 7 feet of snow in the Alps of Austria and southern Germany in recent days, causing more than 20 deaths and increasing the risk of avalanches across the region. Much of southeastern Europe saw bountiful arctic air as well.

And the cold air isn't quite done there yet either.

Europe and parts of Asia are typically the first places where the cold is unleashed.

A 2010 study of 51 winter seasons in the Quarterly Journal of the Royal Meteorological Society found polar vortex weakening events trigger cold-air outbreaks first in Eurasia, eventually propagating westward toward North America.

When exactly the cold arrives, how cold it will be and how long it will last in the U.S. is trickier.

"The key for U.S. weather impacts is that the SSW event must end up altering the Pacific jet stream," University of Oklahoma professor Dr. Jason Furtado told weather.com in December, "... and that takes time."

Essentially, it takes time for the jet-stream pattern – specifically, the blocks in the pattern that are key to unleashing cold air – to respond.

The Jan. 9-12 cold snap in New England can also be linked to the SSW, according to Judah Cohen , director of seasonal forecasting at Atmospheric and Environmental Research (AER), a Verisk Business. One of the polar vortices that split apart due to the SSW was responsible for allowing cold air to plunge southward.