Millions of travelers could face delays and potentially dangerous driving conditions Sunday as winter weather hits parts of the country on one of the busiest travel days of the year.

(CNN) -- Millions of travelers could face delays and potentially dangerous driving conditions Sunday as winter weather hits parts of the country on one of the busiest travel days of the year.

"A strong cold front is triggering showers and thunderstorms from the Great Lakes to the Gulf Coast," Meteorologist Bonnie Schneider reported for "CNN Sunday Morning."

Drivers should be aware of changing conditions as rain could turn to snow showers Sunday evening, she said.

At one of the busiest airports, Atlanta's Hartsfield-Jackson International, a different kind of problem arose due to an aircraft with three blown tires on a runway, according to the Federal Aviation Administration. In a news alert, the FAA said there was a ground stop in effect, but video from the airport showed flights continued to land, and the FAA website showed no delays reported for Atlanta a few minutes later.

The FAA website reported that flights due into Philadelphia and Teterboro, New Jersey, were delayed, as were flights into San Francisco.

Continental Airlines reported average delays of 67 minutes in San Francisco due to "low ceilings," which the FAA defines as "the lowest layer of clouds when over half of the sky is obscured."

Continental was also reporting possible delays due to weather in Cleveland, Cincinnati, Detroit, and Memphis, citing "reduced visibility and rain."

Schneider cautioned that airport delays are possible in the South and Midwest later Sunday. "#HavePatience," she tweeted.

Nearly 42.5 million people were expected to travel for the Thanksgiving holiday this year, an increase of 4% from last year, the travel organization AAA projected.

Slightly fewer people are choosing to get to their destinations by plane compared to last year.

About 23.2 million travelers are flying during a 12-day period surrounding Thanksgiving, a 2% drop from last year, according to a forecast by the Air Transport Association of America.

The busiest air travel days for the Thanksgiving holiday period are expected to be Sunday and Monday, the association said.

The Transportation Security Administration says it has prepared its work force for a "smooth holiday travel experience for travelers."