Tens of millions of Americans were digging out on a bitterly cold Monday in the aftermath of a monster winter storm that dumped a foot of snow from New Mexico to New England, paralysed much of the eastern United States, caused at least 18 deaths and scuttled thousands of flights.

From New York and Massachusetts in the northeast to Texas and North Carolina in the south, roads were frozen slick with ice and buried under often more than a foot of snow.

At least 25 governors declared states of emergency.

In some southern states, residents faced winter conditions unseen for decades, with inch-thick ice bringing down trees and power lines.

The storm was blamed for at least 18 deaths across multiple states.

In Frisco, Texas, a 16-year-old girl died in a sledging accident on Sunday; another youth died in Saline County, Arkansas, while being pulled by an ATV vehicle over snow and ice when it struck a tree, authorities said.

In Pennsylvania, three people died while shovelling snow, local media reported.

In Austin, Texas, a person died of apparent hypothermia while trying to shelter at an abandoned gas station, authorities said.

At least five people died in New York City from exposure to the cold, Mayor Zohran Mamdani said on Sunday, urging residents to call for help if they saw anyone out on the street in need.

While the storm system was drifting away from the East Coast into the Atlantic on Monday, a blast of Arctic air was rushing in from Canada behind it, prolonging sub-freezing temperatures for several more days, the National Weather Service said.

“This storm is exiting the East Coast now, with some lingering snow squalls,” said Allison Santorelli, a meteorologist with the NWS’s Weather Prediction Centre.

“But the big picture story is the extreme cold, it’s lasting into early February.”

Almost 200 million Americans were under some form of extreme cold alert, from along the Canadian border to the Gulf of Mexico, forecasters said.

Lubbock, Texas, had a low of minus 4 degrees Fahrenheit (-20 degrees Celsius) on Monday, and New York City, Washington DC and Boston all faced single-digit temperatures through much of the week ahead.

Nearly 800,000 customers, including both homes and businesses, across the southeastern US were facing the cold weather without power, according to the tracking site PowerOutage.us, including 246,000 in Tennessee.

The storm snarled air traffic, with more than 12,500 US flights cancelled on Sunday - the most of any day since the onset of the COVID-19 pandemic in 2020.

About 3,900 flights within, into or out of the United States had already been cancelled on Monday as of 9:15am, according to the tracking website FlightAware.

US Transportation Secretary Sean Duffy told CNBC he hopes airports will be “back to normal” by Wednesday.

SCHOOLS SHUT DOWN

The storm’s mix of snow, ice and freezing rain turned many roads and highways dangerously slick.

In Tulsa, Oklahoma, Ryan DuVal spent part of Sunday driving his vintage fire truck through the city’s icy streets, looking for anyone who needed help.

“I just saw a need for getting people out of the cold,” he said. “You know, just cruise the streets, see someone, offer a ride. If they take it, great. If not, I can at least warm them up in the truck and just get them a water, a meal, something.”

In Bonito Lake, New Mexico, residents were shovelling out after 31 inches of snow. New York City’s Central Park received 11.4 inches, while Logan Airport in Boston saw 18.6 inches, Santorelli said.

New York Governor Kathy Hochul said she had mobilised National Guard troops in New York City, Long Island and the Hudson Valley to assist with the state’s emergency storm response.

Announcing that schools would be shut for a remote school day, Mamdani quipped: “I know that this may disappoint some students, so if you do see me, feel free to throw a snowball at me.”

Still, despite the disruptions, the winter conditions were fun for many, including in Washington DC, where a huge crowd gathered on Sunday for a raucous impromptu snowball fight in Meridian Hill Park.

Families brought sledges to Capitol Hill, where children zoomed down the steep slope below the white-domed seat of the US Congress.