Thousands of flights were canceled and Tostitos were in high demand Monday as a swath of the East Coast awaited what could be the season's most devastating storm.

The National Weather Service issued a blizzard warning affecting almost 20 million people, including New York City. 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. As of 8 p.m. ET, a few flurries had arrived in the Washington, D.C. region as residents prepared for the worst.

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. Much of the storm's energy will be transferred from a weakening storm that pasted the Midwest with snow Sunday and 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 canceled Tuesday votes and won't reconvene until late Wednesday.

Nicole Chevalier was restocking shelves with chips at a Harris Teeter supermarket in McLean, Va. Chevalier, who works for Frito-Lay, said all the Tostitos were gone when she arrived at the store Monday.

"People want to eat chips and watch movies since they’ll have to stay at home," she said. "One day before snow, we order up. ... Party-size Doritos also run out fast when there’s snow.”

The forecast brought more chaos to air travel.

More than 350,000 airline passengers will be affected by flight cancellations this week from snowstorms in the Midwest and Northeast, according to, the online tracking service. Airlines have already canceled 6,408 flights this week, mostly on Tuesday and Wednesday in Chicago and along the East Coast from Washington, D.C., to Boston, according to FlightAware.

View: Flight tracker

“The storm will cripple air travel in the Northeast” on Tuesday, with 4,611 cancelations, including nearly half of those scheduled for Washington’s National and Dulles airports, two-thirds of the flights involving Boston and more than 80% of the flights involving Baltimore-Washington and Newark airports, according to FlightAware.

“We expect a much worse picture tomorrow as the storm heads east,” FlightAware CEO Daniel Baker said of the flight disruptions Monday in Chicago.

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. 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. 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.

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.”

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.”

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.

Though some New Yorkers appeared to be in panic mode over the approaching storm, others said they were looking forward to settling down indoors and catching up on movies.

Karin Alexis left the grocery store Monday afternoon with frozen pizza and a few other odds and ends and explained she persuaded her husband, an actor with a recording gig in Connecticut, to stay home on Tuesday.

"I look at it as like the days when my kid was little - it's a snow day," said Alexis, 61. "Nothing is so important that it can't wait a couple of days," she said.

Shopper Cara McCaffrey speculated that many New Yorkers take advantage of the 24-hour nature of the city and grocery shop all the time, therefore having less need to stock up in an emergency.

McCaffrey exited a grocery store with a couple of her own bags that contained lettuce and a few other items.

She said she was not worried about the storm.

"I'm excited about it," said McCaffrey, 49, of Manhattan, explaining she planned to make chicken soup. "I get to stay home and cook food and be with my kids."

In Central Park in Manhattan, dog owners, bikers, runners and walkers faced Monday's cold temperatures as they got in one last adventure outside before Winter Storm Stella hit.

Frances Bridges was led along by her boisterous 5-year-old lab mix, Mocha, and said she expects any walks outside the rest of the week with her pet would be as short as possible.

"She has a lot of energy so I am trying to wear her out a bit," said Bridges, 29, of Manhattan.

Contributing: Kevin McCoy; Melanie Eversley; Ben Mutzabaugh