WASHINGTON (AP) — American Airlines briefly grounded flights nationwide Tuesday due to a technical issue just as the Christmas travel season kicks into overdrive and winter weather is threatening more potential problems for those planning to fly or drive.

American flights were cleared to fly by federal regulators about one hour after a national ground stop order was issued by the Federal Aviation Administration.

There were 1,447 delays for flights entering or leaving the U.S. early in the day, with 28 cancellations. Snow was falling early in New York and Dallas-Fort Worth International, which is American Airlines' main hub, was getting hit with rain.

Dallas-Fort Worth had the most delays, followed by Charlotte, North Carolina, Washington, New York, Chicago and Miami

Because the holiday travel period lasts weeks, airports and airlines typically have smaller peak days than they do during the rush around Thanksgiving, but the grind of one hectic day followed by another takes a toll on flight crews. And any hiccups — a winter storm or a computer outage — can snowball into massive disruptions.

That is how Southwest Airlines stranded 2 million travelers in December 2022, and Delta Air Lines suffered a smaller but significant meltdown after a worldwide technology outage in July caused by a faulty software update from cybersecurity company CrowdStrike.

Many flights during the holidays are sold out, which makes cancellations even more disruptive than during slower periods. That is especially true for smaller budget airlines that have fewer flights and fewer options for rebooking passengers. Only the largest airlines, including American, Delta and United, have "interline agreements" that let them put stranded customers on another carrier's flights.

This will be the first holiday season since a Transportation Department rule took effect that requires airlines to give customers an automatic cash refund for a canceled or significantly delayed flight. Most air travelers were already eligible for refunds, but they often had to request them.

Passengers still can ask to get rebooked, which is often a better option than a refund during peak travel periods. That's because finding a last-minute flight on another airline yourself tends to be very expensive.

Just before 7 a.m. Eastern time, the Federal Aviation Administration ordered all American Airlines flights grounded in the U.S. at the airline's request. American had reported a technical issue affecting its entire system with millions traveling for the holiday.

American said in an email that the problem Tuesday morning was caused by a vendor technology issue that "impacted systems needed to release flights."

The groundings couldn't come at a worse time for the millions of travelers expected to fly over the next 10 days. The Transportation Security Administration expects to screen 40 million passengers over the holidays and through January 2.

Airlines expect to have their busiest days on Friday and Sunday, and on Dec. 26, Dec. 27 and Dec. 29.

Many flights during the holidays are sold out, which makes cancellations more disruptive than during slower periods. Even with just a brief outage, the cancellations have a cascading effect that can take days to clear up.

About 90% of Americans traveling far from home over the holidays will be in cars, according to AAA.

"Airline travel is just really high right now, but most people do drive to their destinations, and that is true for every holiday," AAA spokesperson Aixa Diaz said.

Gasoline prices are similar to last year. The nationwide average Thursday was $3.04 a gallon, down from $3.13 a year ago, according to AAA. Charging an electric vehicle averages just under 35 cents per per kilowatt hour, but varies by state.

Transportation-data firm INRIX says travel times on the nation's highways could be up to 30% longer than normal over the holidays, with Sunday expected to see the heaviest traffic. Boston, New York City, Seattle and Washington, D.C., are the metropolitan areas primed for the greatest delays, according to the company.

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AP Reporters David Koenig, Mae Anderson and Mike Pesoli contributed to this report.