Snow has blanketed parts of the northeast United States as a blizzard rolled in, with residents being advised to stay at home, airlines grounding flights and schools cancelling classes.
National Weather Service blizzard and winter storm warnings were in effect for the region including New York, Pennsylvania, New Jersey and Connecticut, with forecasts for up to 60cm of snow by early Wednesday.
Some 50 million people along the Eastern Seaboard were under storm or blizzard warnings and watches.
"If you must go out, do so for as limited time period as possible ... but the best thing to do is stay in," New York Mayor Bill de Blasio said.
Above-ground subway service was suspended as transit officials in the New York metro area warned that more bus and train routes might be suspended throughout the day.
"It's a good day to stay home," said New York Governor Andrew Cuomo on CBS Radio. "It is going to be a dangerous, dangerous situation."
The Port Authority of New York and New Jersey prepared hundreds snow-clearing vehicles at the three major New York area airports. Thousands of tons of salt and sand were prepared for airport roads, parking lots, bridges and tunnels.
Airlines cancelled more than 5800 flights across the United States, according to tracking service FlightAware.com. The airports with the most cancellations were Newark in New Jersey, LaGuardia in New York and Boston Logan International Airport.
American Airlines cancelled all flights into New York's three airports - Newark, LaGuardia Airport and John F. Kennedy International Airport - and JetBlue Airways reported extensive cancellations.
Delta Air Lines cancelled 800 flights for Tuesday for New York, Boston and other northeast airports. United Airlines said it would have no operations at Newark or LaGuardia.
Governors in New York, New Jersey, Pennsylvania and Virginia declared a state of emergency.
"When this thing hits, it's going to hit hard and it's going to put a ton of snow on the ground in a hurry," Massachusetts Governor Charlie Baker said, urging people to consider working from home if they could.
New York City public schools - the largest US school system - cancelled classes on Tuesday. Districts in Boston, Fairfax County, Virginia, and Philadelphia were also shut.
Federal agencies in Washington D.C. said they were opening three hours later than normal on Tuesday.
Boston was braced for up to a foot of snow, which forecasters warned would fall quickly during the storm's peak. Washington D.C., a city which functions badly with even small amounts of snow, was expecting 13cm and twice that in outlying areas.
