'Winter is far from over': Polar vortex reversal could bring springtime snow to US

A shift in the polar vortex could bring a blast of Arctic air to millions of Americans in the second half of March, forecasters have warned.

The polar vortex usually keeps cold Arctic air confined above the North Pole, but experts believe it is starting to shift and stretch. That disruption could create the conditions for springtime snow and storms in the central and eastern U.S., according to a statement released by weather website AccuWeather.

"Don't put your winter jacket and gloves away just yet," Paul Pastelok, the lead long-range forecaster at AccuWeather, said in the statement. "Winter is far from over. Many parts of the central and eastern U.S. will see a surge of springlike warmth next week, but the polar vortex could contribute to a sharp drop in temperatures across parts of the U.S. during the week of March 17."

The polar vortex disruption will cause Arctic air to flow south into North America. However, it's not yet clear how severe its impact will be. With the warmer temperatures of spring approaching, any Arctic air flowing into the U.S. won't be as cold compared to if this happened in January, according to the National Oceanic and Atmospheric Administration (NOAA) Polar Vortex Blog.

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The polar vortex is an area of low pressure and cold air circulating around the poles in the stratosphere — a layer in the atmosphere hovering between 7.5 and 31 miles (12 and 50 kilometers) above the surface. The Northern Hemisphere's polar vortex regularly expands over winter, sending a blast of cold air south with the jet stream, according to the National Weather Service.

There are a variety of different factors that contribute to cold weather, so not all cold snaps are related to the polar vortex. This winter, the vortex has actually been strong and stable for the most part, with rapid west-to-east winds keeping Arctic air confined to the Arctic. However, a sudden warming will buck that trend.

The temperature difference between the North Pole and the equator maintains the polar vortex's strong west-to-east winds over winter. However, sudden warming in the stratosphere can bring this whirling to a halt and cause the winds to reverse direction. Such a warming event can displace the polar vortex or split it in two, according to the Polar Vortex Blog.

The polar vortex might not recover its strong west-to-east winds after the warming, which would make this the final stratospheric warming event of the season. The polar vortex reversal happens every spring (usually later in mid-April) as sunlight returns to the north pole and the temperature difference between the North Pole and the equator decreases, according to the Polar Vortex Blog.