The scope of the storm has been nearly unprecedented, stretching from the Great Lakes near Canada to the Rio Grande along the border with Mexico.
At least 18 people have been killed, and a million left without power, after a deep overnight freeze and snowstorm swept across the US.
Forecasters said a bomb cyclone -- when atmospheric pressure drops very quickly in a strong storm -- had developed near the Great Lakes, stirring up blizzard conditions, including heavy winds and snow.
The scope of the storm has been nearly unprecedented, stretching from Canada to the Rio Grande along the border with Mexico. About 60% of the US population faced some sort of winter weather advisory or warning, and temperatures plummeted drastically below normal from east of the Rocky Mountains to the Appalachians, the National Weather Service said.
More than 2,600 domestic and international flights were cancelled on Sunday, with more than five thousand delayed, according to the FlightAware tracking site.
The storm unleashed its full fury on Buffalo, with hurricane-force winds and snow causing whiteout conditions, paralyzing emergency response efforts. The state governor said almost every fire truck in the city was stranded, and the airport will be shut down until Monday.
Freezing conditions and day-old power outages had Buffalonians scrambling to get out of their homes to anywhere that had heat. But with city streets under a thick blanket of white, that wasn't an option for people like Jeremy Manahan, who charged his phone in his parked car after almost 29 hours without electricity.
“There’s one warming shelter, but that would be too far for me to get to. I can’t drive, obviously, because I’m stuck,” Manahan said. “And you can’t be outside for more than 10 minutes without getting frostbit.”
Mark Poloncarz, executive of Erie County, home to Buffalo, said ambulances were taking more than three hours to make a single hospital trip and the blizzard may be “the worst storm in our community’s history.”
The storm knocked out power in communities from Maine to Seattle, and a major electricity grid operator warned 65 million people across the eastern US of possible rolling blackouts.
Across the six New England states, more than 273,000 customers remained without power on Saturday, with Maine the hardest hit. Some utilities said electricity may not be restored for days.
In North Carolina, 169,000 customers were without power Saturday afternoon, down from more than 485,000. Utility officials said rolling blackouts would continue for the next few days.
Storm-related deaths were reported in recent days all over the country: Four dead in an Ohio Turnpike pileup involving some 50 vehicles; four motorists killed in separate crashes in Missouri and Kansas; an Ohio utility worker electrocuted; a Vermont woman struck by a falling branch; an apparently homeless man found amid Colorado's subzero temperatures; a woman who fell through Wisconsin river ice.
In Mexico, migrants camped near the US border were facing unusually cold temperatures as they awaited a US Supreme Court decision on pandemic-era restrictions preventing many from seeking asylum.