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.
"We do have tens of thousands of people in affected states in the South that have lost power," DHS Secretary Kristi Noem said on Saturday.
"We have utility crews that are working to restore that as quick as possible."
The number of power outages continued to rise.
Some 230,000 US customers had no electricity on Sunday morning, the bulk of them in Louisiana, Mississippi, Texas, Tennessee and New Mexico, according to PowerOutage.com.
The Department of Energy on Saturday 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.
On Sunday, the DOE issued an emergency order to authorise 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 4000 US flights scheduled for Saturday had been cancelled, according to flight tracking website FlightAware.
More than 9400 US flights originally set for Sunday have also been cancelled.
Major US airlines warned passengers to stay alert for abrupt flight changes and cancellations.
On Saturday, Delta Air Lines continued to adjust its schedule and made additional cancellations in the morning for Atlanta and along the East Coast, including in Boston and New York City.
It said it was relocating experts from cold-weather hubs to support de-icing and baggage teams at several southern airports.
JetBlue said as of Saturday morning, it had cancelled about 1000 flights through Monday.
Dominion Energy, whose Virginia operations include the largest collection of data centres in the world, said if its ice forecast held, it could be among the largest-ever winter events to impact the company.
Noem warned Americans to take precautions.
"It's going to be very, very cold," she said.
"So we'd encourage everybody to stock up on fuel, stock up on food, and we will get through this together."
