A polar vortex broke, and severe temperatures are set to dominate most of the United States, including the Midwest, in the coming days.

Bitter cold is taking hold across the U.S. on Thursday, with millions waking up to freezing temperatures and in some areas, sub-zero wind chills.

Tens of millions of Americans are under winter storm warnings as a "major" system is dumping snow and delaying flights.

The active December winter weather pattern continues across the United States. The Northern Tier will remain the primary focus for multiple rounds of clipper systems heading into next week. These systems will keep snowy conditions in place as cold air lingers across the region.

Chicago O'Hare International Airport is the most impacted airport.

Multiple states in New England received up to a foot of snow, causing hundreds of car accidents.

Temperatures across Indiana are forecast to plunge into some of the coldest in the U.S., dipping well below zero as a surge of Arctic air sweeps in.

Such events distort the polar vortex, which causes Arctic air to descend into the U.S. and causes temperatures to plummet.

Forecasters have said parts of the U.S. can expect frigid temperatures in December. A first round has arrived, and more cold air could be coming.

Another clipper system swings through the Midwest on Saturday afternoon, which will bring some quick snow accumulation to southern Minnesota.