A man and woman look at the arrivals and departures board Thursday in the international terminal at San Francisco International Airport.

New York ? Fair weather helped make the holiday sojourn a not-so-painful experience in much of the country Thursday, even with more people on the move than last year, but travelers’ luck might be running out.

A storm was expected to bring snow and ice to parts of the heartland today, deliver a rare white Christmas to Nashville on Saturday, and perhaps sock swaths of the Northeast on Sunday.

“People that are going to Grandma’s house,” said Bobby Boyd, a meteorologist for the National Weather Service in Nashville, “need to get going.”

Eric and Tatiana Chodkowski, of Boston, were driving Thursday with their kids, ages 2 and 4, to see relatives in New York. They said forecasts for snow on Sunday made them wonder whether they’d make it back then, as planned.

They deemed the roads congested but manageable Thursday, and most people found the nation’s airports to be the same way.

Planes took off into windy but accommodating skies at New York’s LaGuardia Airport as Steve Kent prepared to fly to Denver for a family ski trip, scoffing at the puny lines.

“I don’t find it that difficult,” he said. “I think Thanksgiving is harder.”

At airports, the long security lines feared over Thanksgiving, when practically everyone is on the move the same day, never materialized, and aren’t expected to now. The spread-out nature of the year-end holidays means things won’t be quite so cramped.

Travelers may notice that airport screeners are taking a closer look at empty insulated beverage containers like thermoses because air carriers have been alerted about a potential terror tactic involving them, an administration official said.

The official, who spoke Thursday on condition of anonymity to discuss sensitive security matters, stressed that there is no intelligence about an active terror plot. The Homeland Security Department regularly alerts law enforcement about evolving terror tactics.

The Air Transport Association expects 44.3 million people on U.S. flights between Dec. 16 and Jan. 5 — up 3 percent over the same period a year ago but still below pre-recession travel volume. The average ticket price is $421, up by 5 percent.