(CNN) -- Millions of travelers headed home Sunday at the end of the Thanksgiving holiday weekend, braving crowded roads and airports on one of the busiest travel days of the year.Mother Nature cooperated across much of the nation, with some travelers saying their journeys were surprisingly smooth.But conditions were less hospitable across the heartland, with a line of storms prompting tornado warnings in Arkansas and Missouri.Further west, the National Weather Service said an "intense" winter storm was moving slowly northeast across the central Plains, causing wind gusts near 60 mph in Colorado.The combination of snow and high winds "may produce brief whiteout conditions," the weather service said, with motorists in Nebraska and Kansas advised to expect widespread blizzard conditions.Weather closed parts of Interstate 70 in Kansas and Colorado, where the Department of Transportation also blamed the weather for multiple accidents.Travel group AAA projected that more than 37.29 million Americans would travel at least 50 miles from home during the Thanksgiving holiday weekend, an increase of 0.8 percent over the 37 million who did so a year earlier.According to the group's telephone survey, about 31 million of those were traveling by car, nearly 5 million by airplane and almost 2 million by other means, such as train or bus.Mantill Williams of AAA said even more people would travel over the Christmas and New Year's holidays, but those journeys were "spread out over a two- or three-week period."He said the organization wasn't surprised at the number of people hitting the roads for Thanksgiving."What we found historically is that high gas prices alone do not prevent people from traveling," he said. "It may alter their travel somewhat, but you'll be hard-pressed to tell grandma or your mother-in-law that you can't come see them because you can't pay that extra $10 at the gas station."Meanwhile, weather conditions contributed to delays at some of the nation's busiest airports on Sunday.Departure traffic destined for O'Hare International Airport in Chicago, Illinois, faced delays averaging nearly two hours early Sunday evening, according to the Federal Aviation Administration.Arriving flights were delayed an average of one-and-a-half hours at Hartsfield-Jackson International Airport in Atlanta, Georgia. And flights bound for Dallas/Fort Worth International Airport in Texas were temporarily suspended during the afternoon.Chicago's O'Hare wasn't expecting Sunday to be the busiest Thanksgiving travel day; that came Wednesday, when an estimated 217,000 passengers passed through the airport.Travels were smooth for some who departed early and allowed time for long lines.Passenger Jonathan Dorfman said the start of his journey home to Boston, Massachusetts, from Reagan National Airport in Washington, D.C., was "incredibly easy.""This morning was a breeze. We left a little bit early, but encountered no traffic," he said. "We have no anxiety at all."On the roads, two deadly accidents led to delays in California and Arizona.Driver fatigue likely contributed to the crash of a Greyhound bus in Santa Maria Sunday morning that left two people dead, a spokesman for the California Highway Patrol said. (Full story)Northbound lanes of U.S. Highway 101 were closed, and vehicles crashing into the slowed traffic caused two other accidents in southbound lanes.In Arizona, two people died and four others were seriously injured when a passenger van rolled over on Interstate 17 north of Phoenix, closing the highway for several hours, according to the Arizona Department of Public Safety.Williams advised drivers to pack a "winter survival kit" when heading out."Make sure you have your cell phone, extra blankets, and something to mark your location just in case you do break down," he said."The best thing to do is to make sure you take your time and make sure you buckle up, because this time of year the weather can change in an instant," Williams said.Travelers were sharing the road with millions of shoppers hunting bargains on the first big shopping weekend of the holiday season.The National Retail Federation estimated that 22 million people were expected to be out Sunday, after more than 60 million shoppers headed to stores on Friday and 52.8 million on Saturday. (Full story)CNN's Jeff Green contributed to this report.