Millions of people along the US mid-Atlantic region should be bracing for "perhaps crippling" snow, the National Weather Service says.
Forecasters say two feet of snow could fall over the weekend in some areas, with Washington to bear the brunt.
There are warnings the blizzard could cause power outages and bring road and air travel to a halt.
A light dusting of snow on Wednesday night caused gridlock in Washington, raising fears about what is ahead.
Routine commutes lasted up to three hours and some people abandoned their cars, after an inch fell - a fraction of what is expected on Friday and Saturday.
Washington Mayor Muriel Bowser tweeted an apology to her constituents, saying that officials had "failed to deploy the necessary resources in response to the snow - for that I am sorry".
Among those affected was President Barack Obama, whose motorcade spent an hour and 12 minutes trying to get him from an airport back to the White House - a trip that typically takes about a half hour.
But it is what lies ahead that is occupying the thoughts of millions of people living on the US east coast.
Long queues have formed at supermarkets, as people anticipate shop closures over the weekend.
"Heavy snow and blowing snow will cause dangerous conditions and will be a threat to life and property," the National Weather Service warned.
Ms Bowser declared a state of emergency in Washington, which will remain in effect for 15 days, allowing the District of Columbia government to request federal disaster aid.
Governors in Maryland, North Carolina, Pennsylvania and Virginia have also declared states of emergency.