(Bloomberg News) Passengers stranded when airlines canceled more than 6,000 flights amid a winter storm in the eastern U.S. may face lengthy waits to rebook their trips as carriers move aircraft and search for seats on crowded planes.

"It's a mess," Jay Sorensen, president of consultant Ideaworks and a former airline marketing executive, said yesterday. "It takes a long time for this to sort out. With every day of cancellations, the problem just compounds itself."

Six U.S. carriers canceled at least 3,299 flights yesterday, primarily in Philadelphia and the New York area, where all the major airports were closed. That's in addition to 3,334 flights grounded Dec. 26 by snowfall of as much as 20 inches (51 centimeters) and winds gusting to 30 mph.

Teresa Harlow, who had planned to return to Manchester, England, said she didn't learn her Dec. 26 flight with Delta Air Lines Inc. had been canceled until she checked the website for New York's John F. Kennedy airport.

Carriers struggled to relocate aircraft and crews while factoring in airport employees unable to travel to work after New York's heaviest December snowfall since 1948 blocked streets and canceled some local train service.

"An airline schedule, when it's connected with staffing and equipment allocation, is like a picture puzzle," Sorensen said. "What has happened is this storm has essentially blown into the room and tossed all the pieces up into the air."

'Man A Phone'

AMR Corp.'s American Airlines added reservations agents yesterday, after a flood of calls from passengers on canceled flights Dec. 26, said Ed Martelle, a spokesman for the Fort Worth, Texas-based carrier.

The volume of calls to American's reservation lines from people trying to rebook flights is more than double the normal level, he said.

"We are calling people in off their vacation, trying to find people who can man a phone anyway we can," Martelle said. "We're extending part-timers so there will be additional manpower."

The time needed to accommodate affected passengers will depend on the length of the shutdowns, he said. "It's a moving target. I don't know how fast we can get them on planes."

New York's LaGuardia airport reopened yesterday afternoon and departing flights were scheduled to resume at John F. Kennedy and Newark Liberty airport later in the evening, according to the Federal Aviation Administration.

Mandatory Overtime

US Airways Group Inc. has "several hundred" reservation agents working mandatory overtime for the next three days to help rebook travelers, said Valerie Wunder, a spokeswoman for the Tempe, Arizona-based company.

Continental Airlines, a unit of United Continental Holdings Inc., said its Web site was the "fastest and most convenient way" for its passengers to change their travel plans.

Carriers waived penalties for changing flight reservations in cities affected by the storm and some offered refunds for canceled flights. Southwest Airlines Co. said it notified more than 30,000 customers in advance that their flights were called off.

JetBlue Airways Corp., which canceled more than 300 flights yesterday, posted a blog entry on its website about the problems carriers face after a major storm.

"We know it's frustrating that you can't get from point A to point B right now because airports have been closed since last night and may remain closed through a good portion of today," the New York-based carrier said in the posting.

'Severe Disruption'

Harlow, the Delta passenger, said that when she called the Atlanta-based airline, she was greeted by a recording that the carrier couldn't answer because of adverse weather conditions. On a later call, she was told she'd have to hold for an hour to get help.

A Delta spokesman said he couldn't respond specifically to Harlow's experience. Generally, the airline notifies customers in advance if their flights have been canceled, the spokesman, Trebor Banstetter, said in an e-mail yesterday.

"Because of the severe disruption the weather has caused, we're experiencing an extremely high volume on our reservations assistance line," he said. "While our call centers are fully staffed, some customers have encountered a wait as we work to rebook customers, and particularly those with travel plans occurring within the next 24 hours."

Airlines should keep passengers informed, said Harlow, who went to a store offering Internet service to book a different Delta flight via the Web after hers was canceled.

"If you know what's happening, you can plan," she said. "If you don't know what's happening, you can't."

'Given Away'

Both JetBlue and Delta have been "quite responsive" to customers, reaching them through social-networking Web site Twitter, said Genevieve Shaw Brown, a senior editor with Southlake, Texas-based travel website Travelocity.com. Travelers should check in for flights as soon as possible, up to 24 hours before the departure time, she said.

"There are going to be a lot of stranded people trying to get your seat," Brown said. "If you're late to the airport and miss the cutoff for check-in, it'll be given away."

Rebooking passengers is more difficult because the holidays are a high-demand travel time, and because carriers have spent the past two years cutting flight capacity to better match demand eroded by the recession.

Load factor, or the percentage of seats filled, already was on pace to finish the year at its highest since 1944, according to data from the airlines, the Air Transport Association and the U.S. Transportation Department.

'Mother Nature'

Carriers may have to add flights to help clear the backlog of passengers, said Sorensen, of Ideaworks, based in Shorewood, Wis. The airlines will count on some seats being freed as travelers switch to another mode of transportation or cancel trips altogether, he said.

That's what Beth McCuen, an elementary school teacher from Diamond, Ohio, did after her flight from New York's LaGuardia was canceled Dec. 26 and then postponed to today. McCuen booked a train to Pittsburgh and hoped to catch a ride from there to Diamond, about 90 minutes away by car.

"What are you going to do?" she said. "Mother Nature, you can't fight her."