Bitter cold will sweep through the U.S. It'll be a frigid Valentine's Day.

Arctic air will plunge deep into the U.S.

Unusual cold, even for the heart of winter, will grip a great swath of the U.S. by the weekend. That's largely because the polar vortex — comprised of lofty winds that circle around the Arctic during winter — has weakened in 2021, ultimately allowing frigid northern air to spill...