(Newser) – A weather phenomenon called a "polar vortex" has tens of millions of Americans shivering in the lowest temperatures in nearly 40 years, and forecasters believe it's going to get colder until nearly half the country is in below-zero conditions by Wednesday.

"The coldest temperatures in almost two decades will spread into the northern and central US today," the National Weather Service says. "Combined with gusty winds, these temperatures will result in life-threatening wind chill values as low as 60 degrees below zero."

Minnesota has closed all its public schools today and cities including St. Louis and Chicago have followed suit, saying it is too dangerous to send kids out in conditions so cold that exposed skin can become frostbitten within minutes, Reuters reports.

Just how cold is it? Forecasters believe temperatures will be as much as 30 to 50 degrees below average in some cities, with Atlanta and Nashville currently colder than Anchorage, Alaska, CNN finds.

Thousands of flights have been canceled and some cities have come to a standstill but the freakishly cold conditions didn't stop 70,000 Green Bay Packers fans from turning up for their team's loss to the 49ers in conditions that felt as cold as minus 15. "It's what Green Bay is all about—it's cold weather and hard-hitting football," a fan says.

The cold is expected to reach a long way into the South, with fears rising for Florida citrus crops and record, single-digit lows expected in Alabama, the AP reports.

But America won't be in the deep freeze all week. The cold is "not going to be long-lived," a National Weather Service meteorologist says. "By the end of the week the temperatures definitely start to moderate across the whole of the country. "

(Read more winter weather stories.)