A whirlpool of frigid, dense air known as a “polar vortex” descended Monday into much of the U.S., pummeling parts of the country with a dangerous cold that could break decades-old records with wind chill warnings stretching from Montana to Alabama.

For a big chunk of the Midwest, the subzero temperatures were moving in behind another winter wallop: more than a foot of snow and high winds that made traveling treacherous. Officials closed schools in cities including Chicago, St. Louis and Milwaukee and warned residents to stay indoors and avoid the frigid cold altogether.

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