Concerns about illness or inflation aren’t stopping Americans from hitting the roads and airports this holiday season. But a massive winter storm might.

Forecasters predict an onslaught of heavy snow, ice, flooding and powerful winds from Thursday to Saturday in a broad swath of the country, from the Plains and Midwest to the East Coast. A surge of Arctic air will follow. The Christmas weekend could be the coldest in decades.

The National Weather Service said Wednesday the storm was so large and encompassing that about 190 million people are under some type of winter weather advisory.

Southwest Airlines said it has canceled 500 of its 4,000 scheduled flights today and Friday. The company said it wanted to maintain safe operations for passengers and crew members.

At least 145 flights into or out of Denver International Airport were canceled Wednesday as the city was hit with snow, gusty winds and freezing temperatures, according to FlightAware, a flight tracking company. At least 219 flights into or out of Denver were expected to be canceled Thursday.

FlightAware was also expecting at least 364 flights to be canceled Thursday at O’Hare and Midway airports in Chicago. Earlier this week, those two airports said they had 350 pieces of snow removal equipment and 400,000 gallons of pavement de-icing fluid on hand for the storm.

Delta, American, United, Frontier, Alaska, Southwest and other airlines were waiving change fees and offering travelers the option of choosing new flights to avoid the bad weather.

Jean-Paul Blancq got to Boston’s Logan Airport a day early for his flight home today to New Orleans. Blancq had to take a bus to Logan from his seasonal job in New Hampshire and was unsure of the storm’s path.

“I hope that my flight doesn’t get canceled because I don’t know what I’ll do,” Blancq said.

Bianca Thrasher-Starobin, a consultant and lobbyist in Atlanta, flew into New York on Wednesday morning for an event and planned to fly out the same night.

“I’m trying to get out of this weather. I would have stayed longer but I just can’t take that chance,” she said as she raced through LaGuardia Airport.

Bus and train travelers also were bracing for cancellations and delays.

As of late Wednesday, Amtrak had canceled train service on about 30 routes, some through Dec. 25. Greyhound canceled bus service on 25 routes for Wednesday and Thursday, including service from Las Vegas to Denver, Denver to St. Louis and Chicago to Minneapolis, Memphis and Nashville.

The weather added uncertainty to what was expected to be a busy travel season. This month, AAA estimated that nearly 113 million people would travel 50 miles from home or more from Dec. 23 to Jan. 2. That’s 4% higher than last year, although still short of the record 119 million in 2019.

Most planned to travel by car. About 6% will travel by air, AAA said. Either way, many travelers found themselves hastily changing their itineraries.

Joel Lustre originally planned to drive from Bloomington, Ind., to McGregor, Iowa, on Thursday. But he shifted his work schedule, and his wife canceled an appointment so they could leave Wednesday and beat the storm.

In Montana, several ski areas announced closures Wednesday and Thursday because of the extremely cold temperatures and sustained winds. Others scaled back offerings.

Authorities across the country are worried about the potential for power outages and warned people to take precautions to protect older people, the homeless and livestock — and, if possible, to postpone travel.

“If you don’t have to be out driving, especially on Friday, we ask that you don’t be out there,” said Ron Brundidge, Detroit’s public works director. Brundidge said 50 trucks will be out salting major roads on around-the-clock shifts once expected rain turns to snow Friday.

Kelli Larkin arrived Wednesday from Florida for a holiday trip to New York. She plans to fly back Saturday night but said she’ll watch the forecast and change her return flight if she has to.

“It’s a little concerning,” she said. “We’ve got to play it by ear.”

Kurt Ebenhoch, a consumer travel advocate and former airline executive, said fee waivers give airline passengers valuable time before a storm to figure out alternate days and routes. But consumers should read the fine print carefully. Airlines might charge the difference in fares if passengers book beyond a certain window, for example.

Ebenhoch stressed that passengers have the right to ask the airline to book them on a different airline’s flight if there are no options that meet their needs. And if the airline cancels the flight, consumers have the right to a full refund, not just credits for future travel.

The urge to travel and visit family and friends over the holidays appeared to outweigh concerns about illness. The Centers for Disease Control and Prevention said coronavirus cases and deaths have increased in recent weeks, and the trio of COVID-19, seasonal flu and respiratory syncytial virus, known as RSV, continues to stress the health care system.