Thousands of flights have been cancelled in the United States ahead of a major winter storm that has already knocked out power to more than 100,000 customers and snarled major roadways with dangerous ice.

Widespread heavy snow, sleet and freezing rain threatened nearly 180 million people – more than half the US population – in a path stretching from the southern Rocky Mountains to New England, the National Weather Service (NWS) said on Saturday night.

It warned people to brace for a string of frigid days.

US President Donald Trump called the storm “historic” and on Saturday approved federal emergency disaster declarations in the states of South Carolina, Virginia, Tennessee, Georgia, North Carolina, Maryland, Arkansas, Kentucky, Louisiana, Mississippi, Indiana, and West Virginia.

“We will continue to monitor, and stay in touch with all States in the path of this storm. Stay Safe, and Stay Warm,” Trump wrote in a post on Truth Social.

Seventeen states and the District of Columbia have declared weather emergencies, the Department of Homeland Security (DHS) said.

“We do have tens of thousands of people in affected states in the ⁠South that have lost power. We have utility crews that are working to restore that as quick as possible,” DHS Secretary Kristi Noem said late on Saturday afternoon.

She went on to warn Americans to take precautions.

“It’s going to be very, very cold,” Noem said. “So we’d encourage everybody to stock up on fuel, stock up on food, and we will get through this together.”

Forecasters said the damage, especially in areas pounded by ice, could rival that of a hurricane.

Flight tracking website FlightAware said that more than 4,000 US flights scheduled for Saturday had been cancelled, and that more than 9,000 US flights originally set for Sunday have also been cancelled.

Sunday’s cancellations, which are still growing, are already the most on any single day since the COVID pandemic, according to aviation analytics firm Cirium.

Major US airlines warned passengers to stay alert for abrupt flight changes and cancellations.

In an update ‌on its website on Saturday morning, Delta Air said it “continues to make schedule adjustments due to Winter Storm Fern”, with additional cancellations in the morning for Atlanta and along the East ‌Coast, including Delta hubs in Boston and New York City.

The airline said it ⁠was relocating experts from cold weather hubs to support de-icing and baggage teams at several southern airports.

JetBlue said that, as of Saturday morning, it had cancelled about 1,000 flights through Monday, with additional cancellations possible, as it was “closely monitoring” forecasts for the storm.

The number of power outages also continued to rise.

As of 6:30pm ET (23:30 GMT), more than 130,000 US customers had lost electricity, the bulk of them in Texas and Louisiana, according to PowerOutage.com.

In the Texas city of Dallas, which has typically mild temperatures in January, the mercury plummeted to -6 degrees Celsius (21 degrees Fahrenheit), while the mayor of nearby Houston urged residents to hunker down by late Saturday.

“Be where you need to be for the next 72 hours,” said Mayor John Whitmire.

Snow also hit Oklahoma and Arkansas, where some spots already recorded 15cm (6 inches) on the ground, the NWS said.

“This is a mean storm,” Jacob Asherman, a meteorologist at the US Weather Prediction Center in Maryland, told the Reuters news agency.

He said it was the biggest storm so far this season in terms of intensity and scope.

Life-threatening wind-chill readings had plunged to below -45C (-50F) in the central states of North and South Dakota, and Minnesota. The meteorologist warned that exposure to such cold without proper clothing “can lead to hypothermia very, very quickly”.

The worst was predicted for parts ⁠of southern Louisiana, Mississippi and Tennessee, where ice up to an inch thick (2.5cm) was likely to coat tree limbs, power lines and roadways, Asherman said.

After battering the country’s southwest and central areas, the storm system was expected to hit the heavily populated mid-Atlantic and northeastern states, dumping snow exceeding 30cm (1 foot), the NWS predicted.

“Please, if you can avoid it, do not drive, do not travel, do not do anything that can potentially place you or your loved ones in danger,” New York City Mayor Zohran Mamdani said on Saturday. “Instead, I urge every New Yorker who can to put a warm sweater on, turn on the TV, watch Mission Impossible for the 10th time, above all, to stay inside.”

In New Jersey, Governor Mikie Sherrill announced restrictions on commercial vehicle travel and a 56km/h (35 mph) speed limit on highways, and urged people to stay indoors.

“We are expecting a storm the likes of which we haven’t seen in years,” she said.

“It’s a good weekend to stay indoors.”