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A strange winter mix of blinding snow, sleet, tornadoes and high winds posed hazards to travelers in many sections of the country on Christmas Day and threatened more woes on Wednesday - one of the biggest travel days of the year.

Some mountainous areas of Arkansas' Ozark Mountains could get up to 12 inches of snow amid warnings travel could become "very hazardous or impossible" in the northern tier of the state from near whiteout conditions, the National Weather Service said.

"This storm will be ramping up as we move into the evening and into tomorrow. It's two-day threat," said Chris Vaccaro, spokesman for the National Weather Service. "We are just getting started."

Wednesday is the third busiest day to fly this Christmas and New Year holiday season, according to the travel comparison website KAYAK.

"It will be a horrific day for travel in Buffalo, Cleveland, Detroit, Indianapolis and Pittsburgh. There will be major problems, major delays for airports," said AccuWeather meteorologist Mark Paquette.

The snow will be a problem for those traveling in the Midwest and Northeast in the next two days, especially on the roads, Vaccaro said. On Wednesday, the severe weather - thunderstorms, damaging winds and tornadoes - will move from the Gulf to the Carolinas and part of Georgia.

"There are two sides to this storm, the winter side and the tornado side," said Greg Cardin, meteorologist with the NOAA National Weather Service Storm Prediction Center. "They are both pretty good sized."

By Tuesday night, there will be very heavy snow in southern Illinois and central Indiana, and Wednesday morning there will be snow in Detroit, 12 inches in Cleveland and heavy snow and sleet in Pittsburgh, Paquette said. AccuWeather forecast a major snow storm for upstate New York later in the day Wednesday.

Southwest Airlines has weather accommodation policies for airports including Akron, Buffalo, Cleveland, Detroit, Indianapolis, Oklahoma City, Pittsburgh and Tulsa. "We still serve there," Southwest spokesman Paul Flaningan said. "The accommodation is for if they can't make it to the airport."

Airlines are on alert for delays and cancellations in northern Texas and Oklahoma.

"It's too early to tell, there might be some intermittent delays," Flaningan said. "At this time we are just keeping a really close eye."

Airlines expected 42 million passengers to fly during the holidays. That's down about 300,000, or 1%, from last year, the industry group Airlines for America says.

More than 240 flights nationwide were canceled and more than 2,000 delayed by 2:30 p.m. Tuesday, according to the flight tracker FlightStats.com. Most were out of the Dallas/Fort Worth International Airport.

A blizzard warning stretches across northeast Arkansas, southeast Missouri, southwest Indiana, western Kentucky and southern Illinois amid predictions of up to 6 to 12 inches of snow tonight into tomorrow, said Marshall Moss, vice president of forecasting at AccuWeather.

Much of Oklahoma and Arkansas braced under a winter storm warning of an early mix of rain and sleet forecast to eventually turn to snow.

Icy roads already were blamed for a 21-vehicle pileup in Oklahoma, where authorities warned would-be travelers to stay home. Fog blanketed highways, including arteries in the Atlanta area where motorists slowed as a precaution. In New Mexico, drivers across the eastern plains had to fight through snow, ice and low visibility.

"Travel is going to get rough," Moss said. "They haven't quite gotten to the bad part - it's still above freezing now."

Auto club AAA has predicted that 84.4 million people - 1.3% more than last year - have hit the road over the Christmas holiday for trips of at least 50 miles.

At least two tornadoes were reported in Texas, though only one building was damaged, according to the National Weather Service.

The Harris County Sheriff's Office said a 25-year-old man died after strong storms that moved through the Houston area knocked a tree onto his pickup truck.

Along the Gulf Coast, keep a close eye on warnings for tornadoes, Paquette said. "You don't want to mess around with tornadoes. That threat continues over night. Anytime you have a storm that is creating tornadoes and snow at the same time is a big deal."

Contributing: The Associated Press

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