Snow is already coming down in Philadelphia and New York, as a storm the National Weather Service called "potentially historic" starts to make its way through the Northeast. Cities in the region that's home to more than 35 million people have begun shutting down.
The heaviest snow is expected to hit Monday night and into Tuesday, with one to three feet across much of the region. The National Weather Service has issued a blizzard warning for a 250-mile stretch of the Northeast—meaning heavy, blowing snow and possibly whiteout conditions.
More than 5,000 flights have already been canceled. In Boston, which is expected to get at least two feet of snow, officials at Logan Airport say there won't be any flights after 7 p.m. today—and the shutdown could last until late Wednesday. About half the flights out of New York's LaGuardia Airport today were called off, along with about 60 percent of the flights headed to that airport.
"This will be the strongest storm of the year," said NWS meteorologist Brian Hurley. "This Nor'easter is going to produce a wide swath of snowfall." New York City, Boston and Connecticut should expect whopping snow totals of up to 2 feet, he said.
The latest NWS forecast calls for 18 to 24 inches of snow for New York City, down from a forecast of up to 36 inches last night.
Connecticut's governor has ordered a travel ban on the state's highways. Officials in other states are asking residents to avoid any unnecessary travel.
New Jersey governor Chris Christie declared a state of emergency on Monday and said state offices would be closed on Tuesday.
New York Gov. Andrew Cuomo also declared a state of emergency and said the state is considering a travel ban on main roads starting at 11:00 p.m. ET. NY subways will operate on a limited schedule after7 p.m..
Towns and cities along the Northeastern and mid-Atlantic coast were under flood watches, warnings and advisories on Monday morning, according to a NBC News report. The mid-Atlantic region could see 2 to 4 feet of storm surge flooding, the report said.
A weekend storm that had brought snow and slush to the Northeast — the first real snow of the season for many areas — was just a warm up.
"Looks like our luck is about to run out," said John Paulsen as he gassed up his sport utility vehicle in New Jersey. "I can't complain too much since we've had a pretty mild winter, but I don't know if I'm ready for a foot or so of snow all at once."
The storm system driving out of the Midwest brought snow to Ohio on Sunday and was expected to ultimately spread it from the nation's capital to Maine.
Lesser totals were forecast for the Washington area — a coating or a bit more — with steadily increasing amounts expected as the storm plods its way north. Philadelphia could expect to see about 8 inches but points north and east should see more substantial snowfalls: 14 inches were possible in some parts of New Jersey, the National Weather service said Sunday afternoon.
The storm promised treacherous travel by both land and air throughout the busy northeast corridor.
At New York's Penn Station, Cicero Goncalves was waiting for a train to Vermont, where he's going snowboarding, because he expected the flight he had hoped to take would be canceled.
But the 34-year-old flight attendant from Queens counted himself and his travel partner as lucky. "We'll get there before it snows, and we're coming back when the storm is over, on Thursday," he said.
Preparations large and small were in effect elsewhere in New York. A Manhattan Home Depot store sold about twice as many shovels over the weekend as it normally does while transit officials hoping to keep the subways running smoothly planned to use modified subway cars loaded with de-icing fluid to spray the third rail that powers trains.
Farther north, a blizzard watch is in effect for Boston from Monday night through early Wednesday. Wind gusts of 60 mph or more are possible on Cape Cod, forecasters said.
Wyatt Baars, manager of the Charlestown Ace Hardware in Boston, sold out of his bags of ice-melting pellets. But he said a New Hampshire distributor is helping him and delivering more.
"Everybody is preparing for the storm," he said. "When we have something this big on the horizon, everybody comes in for the ice melt, snow shovels, snow brooms."
Snow plow driver Al Laplant expected to be out clearing roads of Simsbury, Connecticut this week, just as he has for more than two decades.
"We'll be out there until the storm's over and then at least three hours after cleaning up," he said as he attended a home show in Hartford. "We'll be out there through the whole storm."
But even for a plow driver, the snow is no cake walk.
"It's kind of exhilarating," he said. "But at the same time, I've been doing it for 27 years, so I'm kind of tired of it myself."
—CNBC contributed to this report.