NEW YORK — Millions of people across the United States on Monday braced for a “life-threatening” bitter Arctic blast that could send temperatures plummeting to their coldest in 20 years.

The northeastern United States and parts of Canada have endured heavy snow and deadly sub-zero conditions since the start of the year, but the deep freeze is now moving through much of the country east of the Rocky Mountains.

The wind chill from the rare “polar vortex” could make it feel as cold as -51oC in places, weather forecasters say, prompting authorities to warn residents to stay indoors and stock up on food.

“Cold temperatures and gusty winds associated with an arctic air mass will continue dangerously cold wind chills as far south as Brownsville, Texas [on the border with Mexico] and central Florida,” the National Weather Service (NWS) said.

“This arctic air mass will affect the eastern two-thirds of the country on Monday as a sharp cold front moves towards the East Coast.”

Exposed skin could suffer frostbite in as little as five minutes in such conditions, experts warn.

The NWS described the weather as “life-threatening” and “the coldest temperatures in almost two decades”.

Combined with wind gusts, the low temperatures in some areas “will result in life-threatening wind-chill values as low as 60oC [Celsius] below zero,” it said.

Authorities are warning people that they could face issues like cars that won’t start and flooded roads that quickly ice over caused by water pipes that freeze and burst.

Thousands of flights out of major airports like Chicago O’Hare International and New York’s John F. Kennedy were delayed or cancelled Sunday due to weather-related problems.

A Delta passenger airplane skidded into a snowbank at JFK on Sunday. Nobody was hurt, but the plane had to be towed to its gate.

At O’Hare, one of the country’s busiest airports, officials said that more than 1,300 flights were cancelled.

The states of Minnesota and North Dakota were expected to experience the worst weather.

Minnesota Governor Mark Dayton announced that schools will be closed Monday “to protect all our children from the dangerously cold temperatures”.

In Wisconsin, Sunday’s National Football League playoff showdown between the Green Bay Packers and the San Francisco 49ers at the open-air Lambeau Field was one of the coldest NFL games in history.

The Packers helped the more than 70,000 football fans at the stadium battle the freeze by handing out free coffee, hot chocolate and hand warmers. Despite fan support, the Packers lost the game to the San Francisco 49ers.

In Indianapolis, Indiana, Mayor Greg Ballard urged residents to simply stay indoors.

Freezing rain is forecast to hit the south and east, affecting the New England region, New York and Washington, with the extreme weather expected to continue into the early part of the week in many places.

“The heavy snowfall and freezing temperatures have combined to create hazardous conditions in many areas of the state,” said Michigan Governor Rick Snyder. “Residents should heed advisories and stay off the roads if at all possible.”

Illinois Governor Pat Quinn described the storm as “one for the record books, and we want to make sure everyone stays safe and warm until it passes”.

The southern city of Atlanta will likely experience record low temperatures, while in normally balmy Florida, the NWS issued a hard freeze warning for much of the northern part of the state.

The NWS said that across the country “the cold temperatures will remain in place through mid-week before a warming trend begins”.

Deaths blamed on the frigid weather include a worker crushed by a 30-metre pile of salt being prepared to treat roads in the Philadelphia area on Friday, and a 71-year-old woman suffering from Alzheimer’s disease who froze to death after getting lost in New York state, officials said.

In Colorado, a small passenger plane burst into flames upon landing at a snow-blanketed airport near Aspen on Sunday, though it was unclear if the accident was weather related.