MISSION, Kan. (AP) — Thousands of flights were canceled and homeless shelters were overflowing Thursday amid one of the most treacherous holiday travel seasons the U.S. has seen in decades, with temperatures plummeting 50 degrees Fahrenheit in some areas and forecasters warning of an impending “bomb cyclone” that could make conditions even worse before Christmas.

The frigid air was moving through the central United States to the east, with windchill advisories affecting about 135 million people over the coming days, weather service meteorologist Ashton Robinson Cook said Thursday. Places like Des Moines, Iowa, will feel like minus 37 degrees, making it possible to suffer frostbite in less than five minutes.

“This is not like a snow day when you were a kid,” President Joe Biden warned Thursday in the Oval Office after a briefing from federal officials. “This is serious stuff.”

Forecasters are expecting a bomb cyclone — when atmospheric pressure drops very quickly in a strong storm — to develop late Thursday and into Friday near the Great Lakes. That will stir up blizzard conditions, including heavy winds and snow, Cook said.

In South Dakota, Rosebud Sioux Tribe emergency manager Robert Oliver said tribal authorities have been working to clear roads to deliver propane and fire wood to homes, but face a relentless wind that has created drifts over 10 feet in some places.

“This weather and the amount of equipment we have -- we don’t have enough,” Oliver said, noting that rescues of people stranded in their homes had to be halted early Thursday when the hydraulic fluid in heavy equipment froze amid a 41 below zero windchill.

He said five have died in recent storms, including a blizzard from last week.

In Texas, temperatures were expected to quickly plummet Thursday, but state leaders promised there wouldn't be a repeat of the February 2021 storm that overwhelmed the state's power grid and was blamed for hundreds of deaths.

The cold weather extended to El Paso and across the border into Ciudad Juarez, Mexico, where migrants have been camping outside or filling shelters as they await a decision on whether the U.S. will lift restrictions that have prevented many from seeking asylum.

Elsewhere in the U.S., authorities worried about the potential for power failures and warned people to take precautions to protect older and homeless people and livestock — and, if possible, to postpone travel. Some utilities were urging customers to turn down their thermostats to conserve energy.

“This event could be life-threatening if you are stranded," according to an online post by the National Weather Service in Minnesota, where officials reported dozens of crashes.

In Kansas City, Missouri, one person died after a vehicle overturned into an icy creek, police said.

Michigan State Police prepared to deploy additional troopers to help motorists. And along a toll road on Interstate 90 in northern Indiana, crews were braced to clear as much as a foot of snow as meteorologists warned of blizzard conditions there starting Thursday evening.

“If you’re looking to get to someone’s house for the holidays and you haven’t left by now it could get dicey soon," said Rick Fedder, the chief operating officer of ITR Concession Co., the toll road’s private operator.

The School District of Philadelphia, the largest in Pennsylvania, announced that Friday's final classes of the calendar year would be held online rather than in-person as scheduled. In Allegheny County in the western part of the state, public works spokesman Brent Wasko said officials would deploy 33 salt trucks but that pretreating the roads wasn't an option because expected rainfall Thursday night and Friday morning would wash the salt away.

More than 2,156 flights within, into or out of the U.S. had been canceled as of Thursday afternoon, according to the tracking site FlightAware. Airlines have also canceled 1,576 Friday flights. Airports in Chicago and Denver were reporting the most cancelations.

Among those with canceled flights was Ashley Sherrod, who planned to fly from Nashville to Flint, Michigan, on Thursday afternoon. Sherrod is now debating whether to drive or risk booking a Saturday flight she worries will be canceled.

“My family is calling, they want me home for Christmas, but they want me to be safe too,” said Sherrod, whose bag — including the Grinch pajamas she was planning to wear to a family party — is packed and ready by the door. “Christmas is starting to, for lack of a better word, suck.”

Amtrak, meanwhile, canceled service on more than 20 routes, primarily in the Midwest.

Some shelters in the Detroit area already were at capacity but still making room.

“We are not sending anyone back into this cold,” Aisha Morrell-Ferguson, a spokeswoman for COTS, a family-only shelter, told the Detroit News.

And in Portland, Oregon, officials opened four emergency shelters. In the city’s downtown, Steven Venus tried to get on a light-rail train to get out of the cold after huddling on the sidewalk overnight in below-zero temperatures.

“My toes were freezing off," he said, a sleeping bag wrapped around his head, as he paused near a flimsy tent where another homeless person was taking shelter.

Courtney Dodds, a spokeswoman for the Union Gospel Mission, said teams from her organization had been going out to try to convince people to seek shelter.

“It can be really easy for people to doze off and fall asleep and wind up losing their lives because of the cold weather."

In Montana, temperatures fell as low as 50 below zero (minus 46 Celsius) at Elk Park, a mountain pass on the Continental Divide. Schools and several ski areas closed, and several thousand people lost power.

Near Big Sandy, Montana, rancher Rich Roth said he wasn’t too concerned about his 3,500 pregnant cows weathering the cold snap, saying “they’re pretty dang resilient animals."

In Ohio, Gov. Mike DeWine warned of a “unique and dangerous” situation of flash freezing Thursday night statewide. He also urged people to check on their neighbors and loved ones.

In famously snowy Buffalo, New York, forecasters predicted a “once-in-a-generation storm" because of heavy lake-effect snow, wind gusts as high as 65 mph (105 kph), whiteouts and the potential for extensive power outages. Mayor Byron Brown urged people to stay home, and the NHL postponed the Buffalo Sabres' home game against the Tampa Bay Lightning.

Denver, also no stranger to winter storms, was the coldest it has been in 32 years on Thursday, when the temperature dropped to minus 24 (minus 31 Celsius) in the morning at the airport.

In Charleston, South Carolina, a coastal flood warning was in effect Thursday. The area, a popular tourist destination for its mild winters, braced for strong winds and freezing temperatures.

The wintry weather extended into Canada, causing delays and cancellations earlier in the week at Vancouver International Airport. A major winter storm was expected Friday into Saturday in Toronto, where wind gusts as high as 60 mph (100 kph) were predicted to cause blowing snow and limited visibility, Environment Canada said.