• Airlines struggled to find empty seats for travelers because flights were so full and schedules limited.
• Snow, high winds and bitter cold hit much of the U.S. in the days leading up to Christmas.
• Cancellations began to ease on Saturday as weather improved.

U.S. flight cancellations eased on Saturday but federal officials warned that disruptions could continue as severe winter storms snarled air travel across the country ahead of Christmas.

U.S.-based airlines have canceled more than 10,000 flights since Wednesday, according to flight tracker FlightAware.

The bad weather and flight disruptions upended the holiday travel plans of hundreds of thousands of people during what airlines expect to be one of the busiest periods since before the pandemic.

Airlines and travelers were hard-pressed to find alternative flights before the holiday because planes were booked so full and schedules dropped sharply during the weekend. The Federal Aviation Administration said schedules peaked at 47,554 flights on Thursday, dropping to 30,875 on Saturday and just 27,997 on Sunday, Christmas Day.

That could force airlines to provide cash refunds to travelers who decide to scrap their trips altogether because of the delays.

FedEx and UPS warned holiday packages could arrive late due to the storms.

Airlines scrubbed about 5,600 flights on Friday alone, about a quarter of the schedule, when storms swept through cities from the Pacific Northwest to the East Coast, bringing life-threatening cold to many areas. Federal forecasters warned of dangerous road conditions from ice and low visibility.

"Wind and blowing snow could cause delays in Chicago, and wind could cause delays in and around Washington, D.C., New York, Boston and Philadelphia," the FAA said Saturday morning. "Seattle, San Francisco and airports serving Colorado ski resorts could see delays from low clouds and visibility."

More than 1,700 Saturday flights were canceled.

Delays are also possible in Florida because of the high number of seasonal flights, the FAA added.

Extreme cold and high winds also slowed operations at dozens of airports. Fifty-eight percent of U.S. flights arrived late on Friday, by an average of nearly 90 minutes, FlightAware data showed.

The disruptions stemmed from weather that hit all major U.S. carriers.

Southwest Airlines, which has a large operation in Chicago and in Denver, canceled more than 1,300 flights, about a third of its schedule, while 2,000 others, half of its schedule, were delayed.

Southwest Airlines Pilot Association, the airlines' pilot union, said 52% of pilots were rerouted on Thursday.

Seattle-based Alaska Airlines canceled more than 500 flights, or 65% of its schedule on Friday, and said planes and airport ramps were covered in thick sheets of ice, slowing its Pacific Northwest operations.

"While it's difficult, especially at this time of year, we strongly encourage guests to reassess their need to travel due to continued icy weather and limited availability," it said on Friday. "Due to very full flights over the next several days, it is likely to take several days to accommodate guests who need new flights."