A polar vortex has brought dangerously low temperatures to a wide stretch of the Midwest in the United States, forcing schools and universities to close. A state of emergency has been declared in Illinois, Michigan and Wisconsin as temperatures dropped, reported The New York Times.

A polar vortex is a stream of cold air that spins around the stratosphere over the North Pole. The vortex’s current has been disrupted and is now pushing south into the United States.

The National Weather Service said a “record arctic air mass” will remain over central and eastern US over the next several days. “Wind chill values of 30 to 60 degrees below zero will be common across the northern Plains, Great Lakes, and upper Midwest,” it said.

At least 50 million people are likely to be affected as large expanses extending from Dakotas in the midwest to western Pennsylvania have received weather warnings.

The temperature is expected to remain below zero Fahrenheit in Minneapolis until Friday morning and below zero in Chicago from Tuesday to Thursday afternoon, according to ABC News. Weather advisories have been extended to the southeast with parts of Georgia, Alabama and Mississippi receiving light snow.

Cities in the midwest opened warming shelters as temperatures dropped below zero degrees Fahrenheit, or -17.8 degree Celsius, Reuters reported. Temperatures dropped to as much as minus 46 degree Celsius in the Chicago area and northern Illinois on Tuesday evening.

“This storm poses a serious threat to the wellbeing of people around the state, and we will use every tool at our disposal to keep our residents safe,” said Illinois governor JB Pritzker in a statement on Tuesday.

As much as six inches of snow was forecast in Illinois and two feet of snow in Wisoconsin state. Snow was also in the Great Lakes region and New England on Wednesday.

Regional governments have closed hundreds of schools while airlines cancelled more than 1,000 flights. Georgia’s governor Brian Kemp shut government offices in 35 counties on Tuesday, and schools across the state were also shut. Parts of north and central Georgia are expected to receive two inches or more of snow in the coming days, along with freezing rain.