MISSION, Kan. — Travelers across much of the eastern United States were bracing Thursday for one of the most treacherous Christmas weekends in decades, with forecasters warning of a “bomb cyclone” that will pack heavy snow and wind while sending temperatures plummeting 50 degrees in a matter of hours.

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.

There were already widespread disruptions in flights and train travel.

Forecasters are expecting a bomb cyclone — when atmospheric pressure drops very quickly in a strong storm — to develop near the Great Lakes, which will increase winds and create blizzard conditions, 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.

“It’s just kind of scary for us here, we just kind of feel isolated and left out,” said Shawn Bordeaux, a Democratic state lawmaker, who said he was running out of propane heat at his home near Mission on the tribe's reservation.

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 transportation and patrol officials reported dozens of crashes and vehicles off the road.

Michigan State Police prepared to deploy additional troopers to help motorists. And along 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.

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 cancellations.

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.

And in Portland, Ore., 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 temperatures that dipped to zero degrees with wind gusts of 40 mph.

“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 persuade 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 at Elk Park, a mountain pass on the Continental Divide. Schools and several ski areas announced closures, and several thousand people lost power.

Near Big Sandy, Mont., 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" and are acclimated to the weather.

In famously snowy Buffalo, N.Y., forecasters predicted a “once-in-a-generation storm" because of heavy lake-effect snow, wind gusts as high as 65 mp, white-outs 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 in the morning at the airport.

In Charleston, S.C., 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.