MISSION, Kan. (AP) — 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 Fahrenheit 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.

“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 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. Oliver offered no details, saying the families are mourning.

“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.

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.

“I think trust will be earned over the next few days as people see that we have ultra-cold temperatures and the grid is going to be able to perform with ease,” Gov. Greg Abbott said in a news conference Wednesday.

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 theirs 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.