DALLAS - Airlines are shifting the timing of thousands of flights, even adding dozens of redeyes, as they try to avoid delays while hauling millions of passengers from now through the Christmas weekend. Success or failure could all depend on the weather, and Mother Nature isn't making it easy on airlines.

Rain and fog in the Northeast caused delays and cancellations Wednesday in Boston, New York, and Washington - the busiest part of the country's airspace. Rain in Chicago and Atlanta caused some delays in those cities too. Severe storms that ripped through Mississippi and Alabama, however, had little impact on air travel since no major airports are in those states.

There were about 5,300 delays and 430 cancelations around 8:15 p.m., according to flight tracking site FlightAware. The majority of flights canceled were smaller regional jets that carry 50 to 76 passengers. More than 28,000 flights were scheduled for Wednesday and a typical day sees about 150 cancelations and 4,000 delays.

The catch: Flights are extremely full over the holiday period, with most travelers unable to make changes in their schedule.

Airlines expect about 38 million passengers over a 17-day period spanning Christmas and New Year's, an increase of about 3 percent, according to an industry trade group, Airlines for America. The group says the average flight could be 90 percent full.

Crowds like that mean any hiccup in the system - delays at a major airport, a technology glitch - can ripple across the country and leave tens of thousands of passengers standing in airport lines.

"The biggest factor is always weather," said American Airlines spokesman Ross Feinstein.

Back-to-back storms led to more than 4,300 canceled flights around Christmas 2012. This time the Northeast corridor not only should be free of snow and ice, it should be relatively balmy with temperatures on Christmas Day in the 60s from New York to Boston. But rain and snow are forecast through Thursday in parts of the West, and the South and Ohio Valley could see severe storms before Christmas.

Airlines have been helped recently by the El Niño pattern that has brought above-average temperatures to northern states. "We saw that through the Thanksgiving holiday season, and we've seen that through November and December," said Steve Hozdulick, Southwest Airlines' managing director of operational performance.

United posted its lowest flight-cancellation rate ever for a Thanksgiving week, and Southwest had its best on-time performance ever for the day before the holiday, which helps reduce other problems such as lost or delayed bags.

From 9 percent to 19 percent of flights were delayed over the peak five-day Thanksgiving period, according to tracking service FlightAware.com. A year earlier, when the weather was worse, delays ran between 12 percent and 31 percent.

Besides the vagaries of weather, airlines in recent years have done a better job of adjusting schedules for peak holiday periods.

According to Mark Duell of FlightAware, U.S. airlines added up to 700 flights a day on Monday, Tuesday, Wednesday, Saturday, and Sunday, compared with the same days last week. They cut about 4,400 flights on Christmas Eve and 5,700 on Friday, Christmas Day, when fewer people want to travel, he said.

The airlines say they will have enough employees on hand to handle the extra passengers.