The millions of U.S. travelers who battled a nightmarish mix of winter weather en route to their Thanksgiving holiday destinations are encountering more of the same for the trip home as a coast-to-coast winter storm system brings snow, rain and heavy winds — and hours-long delays — to major travel hubs across the country.

The storm, which originated on the West Coast, headed to the upper Midwest and Northeast, with Chicago, Philadelphia, Boston and New York expected to experience the worst of the wintry precipitation early Sunday — just as the return leg of the busiest travel holiday of the year hits its peak. This year, AAA estimated that a near-record 55 million people hit the road, rails and skies for the holiday travel period, which it defines as the five days from Wednesday to Sunday.

“The amount of the population under a warning, advisory or watch is extensive — it’s in the millions,” said meteorologist Brian Hurley at the National Weather Service’s Weather Prediction Center. Hurley called the expanse of the storm “impressive”: What started as a deep cyclone off the West Coast has become a “coast-to-coast storm.”

By Sunday afternoon, a cluster of airports in the northeast were experiencing average delays of at least an hour because of wind or snow, according to the Federal Aviation Administration. The biggest delays were reported at Newark International Airport in New Jersey, where the typical arrival was slowed by about 2½ hours.

The delays were even longer across the country at San Francisco International Airport, where wind caused departures to stall more than four hours on average. The Bay Area is under a high wind warning through the early hours of Monday.

Several major airlines, including Delta and American Airlines, are waiving flight-change fees for passengers likely to be stranded by the storms.

In the central United States, weather-related delays were hampering post-Thanksgiving travel plans, even as early as Friday: Officials in Rapid City, S.D., issued a no-travel advisory, while Utah Highway Patrol troopers reported a “huge smattering” of wrecks across the state beginning Friday afternoon, according to the Salt Lake Tribune. In Duluth, Minn., a no-travel advisory was issued for those planning trips. They were advised to remain in place until at least midday Sunday.

The heaviest of the anticipated winter weather, including snow or ice, will generally be north of Interstate 80, which stretches east and west in the upper third of the country, while the heaviest snow accumulation will be the eight to 16 inches that could fall on portions of Minnesota, the northern two-thirds of Wisconsin and upper Michigan, Hurley said.

For travelers in the Chicago area, home to the busiest airport in the United States, wind will be the main concern.

“We’re not forecasting a lot of snow for Chicago,” Hurley said. “But air travel gets complicated when you have a wide array of precipitation and issues with visibility.”

As the storm system moved eastward, parts of Pennsylvania were under an ice storm warning Sunday, while snow fell in New York City following predictions of up to four inches. The National Weather Service in Boston predicted a “burst of snow” Sunday afternoon and evening in the area, warning that it would make for “treacherous travel.”

As airports struggled with reduced visibility, road travelers were urged to be careful amid a mix of snow, rain, ice and a drop in visibility. About 46 percent of all crashes involving bad weather occur in the winter, AAA spokeswoman Jeanette Casselano told The Washington Post via email.

“Snow and sleet can cause significant safety problems by reducing visibility and making it difficult to safely maneuver or stop, but by being vigilant behind the wheel, motorists can help to reduce the number of crashes and fatalities,” Casselano said. The travel group’s research found that fatal crashes peak during the first snowfall of the year.

The West Coast isn’t being spared from inclement weather: A storm system hanging off the coast of California was forecast to bring heavy rains in Northern California, especially in the mountains and foothills of San Francisco, Oakland and across the Sierras, Hurley said. An expected three to five inches of rain could also cause flooding in “burn-scarred” areas that were scorched by recent wildfires. In those spots, the land does not absorb water as quickly as normal.

On Saturday, some areas were still digging out or drying off from storms that hit in the lead-up to Thanksgiving.

Weather caused at least one planeload of passengers to miss Thanksgiving altogether when a United Airlines flight bound for Denver was rerouted early Thursday and was unable to take off again.

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