WASHINGTON — More than 800,000 customers in the U.S. as far west as New Mexico were without electricity and over 10,000 flights were canceled on Sunday ahead of a monster winter storm that threatened to paralyze eastern states with heavy snowfall.

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

DHS Secretary Kristi Noem, at a news conference on Saturday, 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.”

The number of power outages continued to rise. As of 10:18 a.m. EST on Sunday, more than 800,000 U.S. customers were without electricity, according to PowerOutage.us, with at least 300,000 in Tennessee and over 100,000 each in Mississippi, Texas, and Louisiana. Other states affected included Kentucky, Georgia, North Carolina and Alabama.

Energy Department Orders Backup Resources

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

On Sunday, the DOE issued an emergency order to authorize grid operator PJM Interconnection to run “specified resources” in the mid-Atlantic region, regardless of limits due to state laws or environmental permits.

The National Weather Service warned of an unusually expansive and long-duration winter storm that would bring widespread, heavy ice accumulation in the Southeast, 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 by Monday.

More than 10,100 U.S. flights scheduled for Sunday were canceled, according to flight tracking website FlightAware. Over 4,000 flights were canceled on Saturday.

Airlines, Grid Operators Scramble to Prepare

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

Delta Air Lines adjusted its schedule on Saturday, with additional cancellations in the morning for Atlanta and along the East Coast, including in Boston and New York City.

It would relocate experts from cold-weather hubs to support de-icing and baggage teams at several southern airports, the airline said.

JetBlue said that as of Saturday morning it had canceled about 1,000 flights through Monday.

United Airlines said it had proactively canceled some flights in places with the worst expected 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 centers in the world, said if its ice forecast held, the winter event could be among the largest to affect the company.

(Reporting by Phil Stewart and Lewis Krauskopf; Additional reporting by Chandni Shah, Devika Nair, and Preetika Parashuraman in Bengaluru, and Blake Brittain in Washington, DC; Editing by Sergio Non, Chizu Nomiyama, Edwina Gibbs, Bernadette Baum and Nick Zieminski)