Temperatures plunged far and fast Thursday as a winter storm formed ahead of Christmas weekend, promising heavy snow, ice, flooding and powerful winds across a broad swath of the country and complicating holiday travel.

The National Weather Service reported that temperatures across the central High Plains plummeted 50 degrees Fahrenheit in just a few hours. In much of the country, the Christmas weekend could be the coldest in decades.

“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.”

The frigid air will move 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.

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.

Already, roads in rural stretches of western South Dakota were blocked, leaving people stranded with dwindling supplies of food and heating sources.

“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 Rosebud Indian Reservation because snow drifts made it impossible for a delivery driver to re-supply him.

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.

Gov. Greg Abbott, in a news conference Wednesday, was confident the state could handle the increased demand for energy as the temperatures dropped.

“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,” he said.

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.

“This event could be life-threatening if you are stranded with wind chills in the 30 below to 45 below zero range,” 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. About 150 National Guard members also have been deployed to help snow-bound Indiana travelers.

More than 1,700 flights had been canceled Thursday morning within, in or out of the U.S., according to the tracking site FlightAware, with Chicago O’Hare and Denver airports seeing the most. Freezing rain forced Delta to halt departures from its hub in Seattle.

Amtrak, meanwhile, canceled service on more than 20 routes, primarily in the Midwest. Service between Chicago and Milwaukee, Chicago and Detroit, and St. Louis and Kansas City, Missouri, was suspended through Christmas Day.

Some shelters in the Detroit area already were at capacity. The Detroit News reported that the 140 beds at COTS, a family-only shelter in Detroit, were full. The facility is hoping to make room for others, though, spokesperson Aisha Morrell-Ferguson told the newspaper Wednesday.

“We are not sending anyone back into this cold,” Morrell-Ferguson said. “It does not matter if we have to pull out air mattresses. We are doing everything we can, looking at alternative spaces to support the needs that may arise.”

In Montana, temperatures fell as low as 50 below zero at Elk Park, a mountain pass on the Continental Divide. Several ski areas announced closures Wednesday and Thursday because of the extreme cold and winds. Others scaled back offerings. Schools also closed, and several thousand people lost power.

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, whiteouts and the potential for extensive power outages. The NHL postponed the Buffalo Sabres’ home game against the Tampa Bay Lightning and rescheduled it for March 4.

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, 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 were predicted to cause blowing snow and limited visibility, Environment Canada said.