Plummeting temperatures across the country have claimed at least 21 lives since the beginning of this week — but the weather-related danger won’t necessarily subside as soon as things warm up.

According toThe New York Times, dozens of people have been hospitalized across the country for symptoms of hypothermia and frostbite sustained as a result of the bitter cold.

A frozen Lake Michigan in front of the Chicago skyline.Scott Olson/Getty

Polar Vortex Brings Extreme Cold Temperatures To Chicago

Millions of other lives have been disrupted because of the temperatures, which have broken at least 160 records in the Midwest and Northeast in the past two days alone, according toUSA Today. In these regions, school and business closures, canceled flights and closed roads became standard this week. In nine states, mail delivery has stopped.

A homeless man in New York City on Jan. 30.Drew Angerer/Getty

Polar Vortex From Upper Midwest Brings Extreme Cold Temperatures To New York City

While temperatures in much of Midwest (including Minneapolis and Chicago) and Northeast (including Buffalo and Vermont) were hovering around zero degrees Fahrenheit, the weather is expected to warm up during the weekend.

RELATED VIDEO: Extreme Cold in Midwest Leads to Postal Service Cancellation, Residents Advised Not to Talk Outside

This time of year, “I don’t think there’s ever been a case where we’ve seen [such a big] shift in temperatures,” Jeff Masters, meteorology director of the Weather Underground, toldUSA Today. “Past record-cold waves have not dissipated this quickly … Here we are going right into spring-like temperatures.”

It’s not all good news, however.

The drastic switch — as much as an 80-degree change in some cities,according to Fox News— could cause water main breaks and overflowing rivers. The massive amount of snow and ice melting so rapidly could lead to “dangerous flooding,” FEMA Michigan warned,according to theDetroit Free Press.

“It’s important for individuals to monitor their local forecast closely and always heed the direction of local officials to stay safe when hazardous weather conditions exist,” said James Joseph, FEMA Region V administrator.

Still, according to the National Oceanic and Atmospheric Administration, this January has yielded twice as many record highs as record lows,USA Todayreports.

source: people.com