A “potentially historic blizzard” could dump up to 90cm (3ft) of snow on the US north-east coast, the National Weather Service (NWS) has warned.
New York’s mayor said the storm could be the city’s biggest ever, telling residents to “prepare for the worst”.
Heavy snowfall is forecast from Philadelphia to Maine. More than 3,000 flights have already been cancelled.
A blizzard warning has been issued for New York and Boston from Monday afternoon until Tuesday.
More than 1,800 flights on Monday and more than 1,600 on Tuesday had been cancelled, according to the website FlightAware.
Airports in New York, Boston and Philadelphia have already been affected.
Speaking on Sunday, New York City Mayor Bill de Blasio showed journalists a list of the city’s top 10 snowstorms, dating back to 1872.
“This could be the biggest snowstorm in the history of this city,” Mr de Blasio said, telling New Yorkers to “prepare for something worse than we have seen before”.
“Prepare to be safe, take every precaution. Now is the time to get ready for this extreme weather.”