A massive winter storm has swept into the southern and eastern United States, with thousands of flights cancelled, emergencies declared in five states and Washington braced for 2ft of snow.
At least eight people have already died in road accidents blamed on the icy conditions.
The National Weather Service (NWS) said the winter storm could rank near the top 10 to ever hit the region, and meteorologist Paul Kocin compared it to "Snowmageddon", the first of two storms that "wiped out" Washington in 2010.
Weather service director Louis Uccellini said: "It does have the potential to be an extremely dangerous storm that can affect more than 50 million people."
The snowfall, expected to continue from late on Friday into Sunday, could easily cause more than $1 billion in damage and paralyse the eastern third of the nation, he added.
So far, the snowstorm is looking just like the forecasts promised, and Washington could get one of its top three storms in history, the NWS said.
Mr Uccellini said all the elements have come together to create a blizzard with brutally high winds, dangerous inland flooding, white-out conditions and even the possibility of thunder-snow, when lightning strikes through a snowstorm.
Snowfall as heavy as 1-3in an hour could continue for 24 hours or more. In addition to Washington, 12-18in of snow is predicted for Philadelphia and 8-12in for New York.
A state of emergency has been declared in Tennessee, North Carolina, Virginia, Maryland, Pennsylvania, the District of Columbia and parts of other states.
Blizzard warnings or watches are in effect along the storm's path, from Arkansas through Tennessee and Kentucky to the mid-Atlantic states and as far north as New York.
As far south as Atlanta, people are urged to go home and stay there.
Schools and government offices are closed, thousands of flights have been cancelled and millions of people have been stocking up on supplies.
All major airlines have issued waivers for travel over the weekend, allowing passengers to rebook on to earlier or later flights to avoid the storm.
Flight tracking service FlightAware said airlines have cancelled more than 2,400 flights on Friday to, from or within the US, and another 2,400 have been cancelled for Saturday. By Sunday afternoon, airlines hope to be back to full schedule.
Washington's subway system said it will shut down late on Friday night and remain closed until Sunday. About 1,000 track workers will be deployed to keep New York City's subway system moving and 79 trains will have "scraper shoes" fitted to reduce the icing on the rails.
White House spokesman Josh Earnest said President Barack Obama will hunker down at the White House.
The US Capitol Police said sledging on Capitol Hill, which only recently became legal after an act of Congress, would be welcome for the first time in decades, as long as conditions are safe.