CHICAGO, Illinois - An arctic blast is spreading potentially life-threatening cold across the Midwest as millions of Americans gear up for what could be the lowest temperatures in more than two decades.

The bitter weather has already arrived in much of the Midwest. But meteorologists warn this is just the tip of the iceberg.

A polar vortex will cause bone-chilling temperatures to plummet even lower, bringing threats of hypothermia and frostbite into next weekend.

The winter weather has already wreaked havoc on roads.

Whiteout conditions led to multiple wrecks on I-90 in Pennsylvania.  The interstate was closed so emergency officials could clear the scene.

As the polar vortex plunges into the U.S., meteorologists say it will be warmer in parts of the Arctic than in Chicago and Minneapolis.

"We're about to experience a seven-day period of time of both snow and freezing weather," said Chicago Mayor Rahm Emanuel. 

With the wind chill, parts of Illinois and Iowa could hit -35 degrees. Parts of Wisconsin and Minnesota could dip as much as -45.

The polar vortex rarely plunges as far south as the U.S. The last big plunge was January 6th, 2014, when Chicago's temperature dipped to -16.