ARLINGTON, Va. − A massive wave of arctic air sweeping down on the jet stream gripped a wide swath of the nation Monday as temperatures plummeted 20 to 40 degrees below normal, fueled by howling winds that placed almost half of all Americans under wind chill watches and warnings.

The weather was blamed for four deaths in Oregon, where more than 90,000 people remained without power after being slammed by high winds, ice and snow. A snowmobiler also died Sunday night when hit by a semitrailer in Utah, where the mountains got nearly 4 feet of snow in a day. And in Wyoming, a backcountry skier was killed after triggering a 50-foot-wide avalanche.

Montana saw wind chills surpass minus 40. Texas called for energy conservation because of unrelenting freezing temperatures, and parts of Florida were bracing for the freeze.

About 150 million Americans − 45% of the nation's population − were under a wind chill warning or advisory for dangerous cold and wind, according to National Weather Service meteorologist Zack Taylor.

Weather-related flight delays again brought havoc to airlines and airports. More than 9,600 flights within, into or out of the U.S. had already been delayed or canceled by 5 p.m. ET, according to FlightAware. More than 10,000 flights were similarly disrupted each of the last two days.

"The main theme to our weather story over much of the Lower 48 (states) remains the brutally cold temperatures and associated significant wintry weather," National Weather Service meteorologist Jacob Asherman said. "Little has changed with the 'take-home' message regarding the dangerous cold entrenched over the U.S."

Asherman said subzero temperatures and colder winds will prevail through Tuesday as wind chills dip below minus 30 across the Plains and minus 50 in Montana and the Dakotas. Farther south and east, potentially dangerous winter storms driving a mix of snow, sleet and freezing rain were rolling toward the Tennessee Valley and Gulf Coast states all the way to Washington, D.C., Philadelphia and New York.

The good news is that by Wednesday the arctic air mass will moderate. The bad news is another surge of frigid arctic air is expected to plunge southward from Canada by week's end. That, Asherman said, could lead to more treacherous conditions across the Midwest and Deep South.

On ice:Millions face cold temps from Dakotas to Florida

Developments:

∎ L​ows in the 20s are expected along the northern Gulf Coast, from East Texas to North Florida. Temperatures could drop into the teens and perhaps a few single digit-lows in the Deep South, The Weather Channel says.

∎ Almost 80% of the nation could see below-freezing temperatures and over 140 daily cold records could be broken Monday and Tuesday from Oregon to Mississippi, CNN reported.

∎ In Oregon, high winds toppled a tree onto a house, killing a man. Two other people died of suspected hypothermia and a fourth died in a fire after a tree fell onto an RV.

∎ The snowmobiler who was killed in Utah was part of a group of four trying to cross U.S. Highway 40 in the Strawberry Reservoir area about 70 miles southeast of Salt Lake City, the Utah Highway Patrol said.

∎ Chicago Public Schools, the fourth-largest district in the country, canceled Tuesday’s classes because of the cold weather.

Deep freeze, snow shutting schools in Tennessee

Road conditions across much of greater Nashville were perilous Monday as crews worked to clear several inches of snow with up to 6 more inches possible in some areas. Monday's high temperature was only expected to reach 20 degrees, and temperatures were not expected to climb above freezing until Thursday afternoon, the National Weather Service office in Nashville said.

Schools were closed in honor of Martin Luther King Day, but Metro Nashville Public Schools and a dozens of other districts around the state announced they would be closed Tuesday − and many on Wednesday, too. Many businesses also closed their doors.

At her home in Williamson County, south of Nashville, Kristin Kelly, 41, watched the snow fall.

“It’s very tranquil and vibrant all at the same time,” she said. “If we're lucky, we get one to two decent snow events a year. ... I’m happy with one good snowfall. Then I’m ready for the pool.”

− Natalie Neysa Alund

Record chill to greet Iowans for caucuses

Below-zero wind chills throughout Iowa on Monday will test caucus-goers' willingness to head out for town halls across the state to support their favorite presidential candidates. The Republican caucuses officially kick off at 7 p.m., and temperatures won't be conducive. Monday is expected to be the coldest Iowa caucuses day since the process began in 1972, when the wind chill dipped to 26 degrees below zero. The high that day was 25 degrees.

The forecast for Monday called for a high of zero degrees, according to AccuWeather Meteorologist Kerry Schwindenhammer. But temperatures, he said, could feel like 20 to 25 degrees below zero − or even 30 degrees below zero in some spots.

“You can’t sit home,” former President Donald Trump told supporters Sunday. “If you’re sick as a dog, you say, ‘Darling, I gotta make it.’ Even if you vote and then pass away, it’s worth it.”

Latest forecast:Bundle up for subzero temps, coldest caucus on record

Texas, Mississippi slammed with ice

An area of ice extended from northeastern Texas to the northern parts of Mississippi, Alabama and Georgia early Monday. This "icy zone" will expand southeastward into the central and western Gulf coast by Monday night, AccuWeather said. In Texas, the Electric Reliability Council of Texas − ERCOT − issued a conservation appeal for Monday, citing record-breaking demand.

“The dangerously cold wind chills could result in hypothermia or frostbite if precautions are not taken,” the National Weather Service warned.

In Mississippi, the National Weather Service in Jackson said temperatures would fall into the single digits, with wind chills below zero through Tuesday night. The weather service urged residents to "protect people, pipes and pets" as the freeze is likely to last until late Wednesday morning.

"A wintry mix is ongoing across the Delta and parts of southeast (Arkansas) and northeast (Louisiana)," the National Weather Service said. "While accumulations will be greatest farther north and west, travel issues are expected across much of the area."

Buffalo gets break in snow, help from fans for playoff game

Buffalo, New York, got a break in the snow for the NFL playoff game that started late Monday afternoon. The game was postponed from Sunday because of weather conditions. Buffalo Airport received more than 8 inches of snow Sunday, breaking the daily record set in 1963. Workers and volunteers shoveled about 2 feet of snow from the seats at Highmark Stadium.

“You got to remember you’re a Bills fan. It’s all part of the deal,” said Bob Issacs, among the local residents who took up the team on its request for volunteers to clear the seats in temperatures in the teens for $20 an hour. Their efforts paid off as the snow had been removed by kickoff.

More lake-effect snow was forecast Tuesday through Thursday, the Weather Service in Buffalo said.

Snow, icy conditions in store for mid-Atlantic, Northeast

Though not as overwhelming as the amount of snow that has blanketed Buffalo, parts of the Northeast and mid-Atlantic are due for their first significant coating in two years.

Most of the stretch from Virginia to Maine is expected to receive 1-3 inches of snow from Monday night and Tuesday, Accuweather reported. The 2-3 inches forecast for Washington would be the largest amount in the capital city in at least two years.

New York City has a chance to break its spell of days without more than 1 inch of snow, which like Washington and Philadelphia sits at 700-plus. But the bigger concern may be icy conditions and freezing rain.

"Unlike the flurries and snow squalls that drifted across the area on Sunday and mainly melted on roads, lower temperatures in place for the storm from Monday night to Tuesday will lead to slippery and snow-covered roads and sidewalks in many cases," Accuweather senior meteorologist Matt Benz said.

Contributing: Anthony Robledo, USA TODAY; The Associated Press