January is shaping up to be the coldest in a decade as the polar vortex dips south across the eastern half of the United States — shocking a nation that has enjoyed an unusually balmy end of the year.

“It’s going to get cold, and then very cold,” a Fox Weather meteorologist told The Post Monday.

“Based on the latest long-range data, this January has the potential to be the coldest since 2014,” they added. Temperatures are expected to begin dropping from the Rocky Mountains to the East Coast around Thursday, but the real plunge will set in sometime between seven and 14 days from now.

The plunge will bring wind chills across the Midwest dipping below zero, “exceptionally cold” temperatures down to 20 degrees below average in some places, and freezing conditions setting in as far south as the Gulf Coast and even into Florida — though it is still too early to forecast exact temperatures. And there is a “pretty good chance” the vortex will bring with it “significant winter storms,” Fox Weather cautioned.

“The prime window for these winter storms would be the middle to end of next week,” they said, adding that the Southeast could even see snow or ice. While most polar vortex forecasts are often unpredictable, Fox Weather said this round of cold is different.

“This is a very highly confident forecast. Across the board the weather models are screaming at us that this really cold air is on the way,” the meteorologist said.

The cold is expected to persist throughout January, with relief coming once February rolls around.

Such plunges occur when the air currents that keep frigid air over the North Pole — the polar vortex — weaken, and the freezing arctic air slips south and pushes the vortex down with it. That means arctic air is blasting over the US, while pressure changes and the motion of the polar vortex whip up high winds and create a perfect recipe for wintry weather.

Cold subsides when the vortex restabilizes and drives the arctic air back north.

January’s freeze-out comes after December started cold, but finished out unusually warm across most of the country.

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