More than 50 million people have been warned a “potentially paralyzing storm” will drop up to 30ins of snow in hours.
Southern states are already blanketed under 8ins and snow began falling in Washington DC at lunchtime.
Thousands of flights have been cancelled and people told to stay home.
The weather system affects a huge swath of the country, from Arkansas in the south to Massachusetts in the north-east, with eight people killed so far.
A rush for supplies led to long queues and empty shelves at supermarkets on January 22.
Washington is expected to be worst-hit. Mayor Muriel Bowser warned residents of 30ins of snow – that would beat the city’s record of 28in that fell during a two-day period in 1922.
“I want to be very clear with everybody, we see this as a major storm. It has life and death implications,” she said.
High winds could compound problems, with 30mph winds forecast for Manhattan on January 23, and even stronger gusts in Washington and Baltimore.
As the weather system approached New York City, Mayor Bill de Blasio urged residents to be ready.
“We’re bracing for the first big storm of the winter. I want to let my fellow New Yorkers know we’re prepared, the agencies here are ready for what’s coming up ahead,” he said.
Facing criticism for saying he would be staying on the presidential campaign trail, New Jersey Governor Chris Christie reversed course, tweeting: “I’m sorry, [New Hampshire] but I gotta go home – we got snow coming.”
National Weather Service director Louis Uccellini said the system had “the potential of being an extremely dangerous storm that could affect over 50 million people”.
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