More than 4,000 flights were cancelled in the US on Saturday ahead of a monster winter storm that has already cut power to more than 160,000 electricity customers as far west as Texas, and threatened to paralyse eastern states with heavy snowfall.

Forecasters said snow, sleet and freezing rain, accompanied by dangerously frigid temperatures, would sweep the eastern two-thirds of the nation on Sunday and into next week.

Calling the storms “historic,” President Donald Trump on Saturday approved federal emergency disaster declarations in 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 said.

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

The number of power outages continued to rise. As of 10:17 p.m. EST, more than 160,000 U.S. customers had no electricity, the bulk of them in Louisiana and Texas, according to PowerOutage.com.

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The US Department of Energy on Saturday said it issued an emergency order authorising the Electric Reliability Council of Texas to deploy backup generation resources at data centres and other major facilities, aiming to limit blackouts in the state.

The US National Weather Service warned of an unusually expansive and long-duration winter storm that will bring widespread, heavy ice accumulation in the southeast US, where “crippling to locally catastrophic impacts” can be expected.

Weather service forecasters predicted record cold temperatures and dangerously cold wind chills descending further into the Great Plains region of the U.S. by Monday.

As of 10:21 p.m. EST, more than 4,000 U.S. flights scheduled for Saturday had been cancelled, according to flight tracking website FlightAware. More than 9,400 U.S. flights originally set for Sunday have been cancelled.

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

In an update on Saturday morning, Delta Air Lines (DAL.N), opens new tab said it was continuing to make schedule adjustments, with additional cancellations in the morning for Atlanta and along the East Coast, including in Boston and New York City.

It added 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.

United Airlines said in an email that its weather preparations included proactively cancelling some flights in places with the worst weather.

U.S. electric grid operators on Saturday stepped up precautions to avoid rotating blackouts.

Dominion Energy, whose Virginia operations include the largest collection of data centres in the world, said if its ice forecast holds, it could be among the largest-ever winter events to affect the utility’s operations.

Noem, speaking at a news conference about U.S. government preparations for the storm, warned 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.”