More than 1,000 flights were already canceled Monday, and New York City residents were among millions under a blizzard warning for a storm that hadn't fully formed yet.

A nor'easter was forecast to rage up the East Coast late in the day and through most of Tuesday, slamming some areas with more than a foot of snow and wind gusts of 60 mph or more. The storm will take shape as a strong area of low pressure develops off the coast late Monday.

“A fairly large area of the Northeast should see a foot or more of snow,” National Weather Service meteorologist Jim Hayes told USA TODAY. He said the bulls-eye for the most snow continues to be southeastern Pennsylvania, northern New Jersey and the Lower Hudson River Valley in New York, which could see 18 inches.

New England was also a target, with the Boston forecast calling for 10 to 16 inches.

President Trump weighed in on Twitter: "Everyone along the east coast be safe and listen to local officials as a major winter storm approaches. @NWS ." The U.S. House cancelled Tuesday votes and won't reconvene until late Wednesday.

The dire predictions brought more chaos to air travel, with almost 1,000 flights canceled Monday and more than 1,700 already grounded for Tuesday, according to the flight-tracking service FlightAware.

More than 19 million people are under a blizzard warning from the new storm. Hayes said strong winds, with potential gusts of 60 mph, could lead to outages since the heavy, wet snow will cling to power lines.

“This will easily be the most widespread winter event of the season for the northeastern part of the country,” weather service meteorologist Michael Musher noted in an online forecast.

In New York, a coastal flood warning covering the bays of western Long Island and the Atlantic Ocean beachfront will take effect Tuesday morning.

The Metropolitan Transportation Authority anticipates “substantial” service changes across the New York City subway and bus systems, as well as for the commuter rail systems serving the city and its northern and Long Island suburbs, Gov. Andrew Cuomo announced. New York was among cities that took a pre-emptive strike, announcing Monday that public schools will be closed for the city's 900,000 students.

In all, winter storm watches and warnings were in effect from the mountains of North Carolina to northern Maine, a distance of more than 1,000 miles.

Once the snow ends, it will likely stick around for awhile. Temperatures are forecast to remain quite chilly through the end of the week.

“Besides the snow, it will be cold,” Mayor Bill de Blasio warned. "We urge you to avoid unnecessary travel and help keep roads clear for sanitation crews and first responders.”

Heather McCready, spokeswoman for Whole Foods in the region, said bottled water and bread was flying off the shelves. Ice melting chemicals and firewood was also selling fast, she said.

"We knew in advance that the storm was coming, so we stocked up," she said.

Nelson Rodriguez, who manages the Associated Supermarket on Manhattan's Lower East Side, said the rush for staples such as bread, milk and toilet paper hasn't been too crazy yet. And he said he hopes to be open Tuesday, when the storm will be at its worst.

“We’re going to try," he said. "Some people probably won’t make it, but some people are still going to come and do some shopping.”

Contributing: Kevin McCoy; Ben Mutzabaugh