Worried shoppers grabbed groceries off store shelves and airlines started canceling flights Sunday as the Northeast prepared for a major storm that threatened to strike with coastal flooding and more than a foot of snow.

A mixture of rain, sleet and snow started moving into the region Sunday, but warm air from the ocean was expected to delay the changeover to all snow along the coast, complicating forecasts. Winter storm watches remained in effect from West Virginia to Maine, the National Weather Service said.

“I wouldn’t have travel plans,” said weather service forecaster Mike Evans. “If this thing pans out, it may be practically impossible to go anywhere.”

Delta, Continental and other airlines canceled dozens of flights into the New York metropolitan area’s La Guardia, Kennedy and Newark airports, said Ernesto Butcher, chief operating officer of the Port Authority, which runs the region’s airports.

“What the airlines are trying to do is prepare for the worst because it’s always problematic to keep passengers stranded,” Butcher said. “Rather than having people sitting in airports around the country, they can cancel flights in advance to control the situation.”

Even though the heaviest snow wasn’t expected until Monday and Tuesday, people heeded warnings and cleaned out hardware store supplies of snow shovels and stocked up on bread and milk.

“We just ran out of ice melt and rock salt,” Jerry’s Hardware owner Jerry LaComfora said Sunday morning in Worcester, Mass. “One lady bought eight snow shovels – I didn’t ask why.”

Parts of Massachusetts were warned of a possible 2 feet of snow by the time flakes stop falling on Wednesday.

“I’ve had numerous customers take two carts of stuff, like they’re going to get snowed in for the weekend, like we’re back in ‘Little House on the Prairie’ times,” said manager Joe Jancsarics at Redner’s Warehouse Market in Trexlertown, Pa.

The slow-moving storm system gathered strength Sunday off the mid-Atlantic coast after spreading rain and thunderstorms across the Gulf Coast states. Up to 10 inches of rain had fallen over the week in Louisiana and flooding chased hundreds from rural homes. Lightning in Alabama started a fire that killed five people Saturday.

Up to 2 feet of snow was possible from Pennsylvania into New England, but meteorologists moved their forecasts of heaviest anticipated snowfall inland. The expected delay in the change from rain to snow cut New York City’s forecast accumulation to 6 to 12 inches of snow over 48 hours instead of the expected 1 to 2 feet, the weather service said.

“We’re playing it by ear; the weather forecast keeps changing on us,” said Bob Hawkins, a manager for Virginia’s Department of Transportation in Winchester.

Onshore wind of 20 to 25 mph was expected to cause flooding along the Jersey shore, and communities on Long Beach Island had started making emergency plans.

Harvey Cedars’ police were placed on standby, and Long Beach Township authorities rounded up as many four-wheel-drive vehicles and trucks as they could in case they were needed for evacuations.

NJ Transit was prepared to use a jet-powered snowblower to clear commuter train rails, said spokeswoman Kelly Stewart Mayor.