• Airlines canceled more than 6,000 flights from Wednesday through Friday, days before Christmas.
• High winds, bitter cold and snow were slowing airline operations this week.
• Airlines are expecting a surge in travelers over the year-end holidays.

Airlines canceled hundreds of flights this week as winter storms, bitter cold and high winds snarled U.S. travel ahead of Christmas weekend.

Carriers scrubbed more than 6,000 U.S. flights from Wednesday through Friday, according to tracking site FlightAware. That period includes what airlines expected to be the busiest travel times before Christmas Day. More than 21,000 U.S. flights were delayed from Wednesday through Friday.

Chicago's two main airports — O'Hare and Midway — and Denver International Airport had the biggest share of canceled flights on Thursday. Seattle-Tacoma International Airport, New York's LaGuardia Airport, Detroit and Chicago Midway were set to have the most disruptions Friday.

Airlines warned that the snow, ice, high winds and cold temperatures could affect travel from Seattle to Boston to North Carolina.

Wednesday's cancellations accounted for about 2% of U.S. airlines' schedule, while about 30% of flights were delayed by an average of 47 minutes, FlightAware data showed. Disruptions worsened throughout Thursday.

American, Southwest, United, Delta, Spirit, JetBlue, Alaska and other airlines issued weather waivers for dozens of destinations around the country, allowing travelers to change their departures without paying a change fee or difference in fare.

Airlines routinely will cancel flights ahead of bad weather so travelers, crews and planes aren't stranded at airports at the last minute, a situation that can cause disruptions to snowball.

The weather could hurt what airlines expected to be busy travel days to cap a rocky year. United said it expects year-end holidays to be busier than Thanksgiving with 440,000 passengers a day on average. The carrier projected Jan. 2 will be the busiest day since the Covid pandemic started.

Disruptions over the spring and summer from bad weather and labor shortages sparked an outcry from customers and politicians, and prompted airlines to trim their schedules.

Late last year and in early 2022, the omicron wave of Covid sidelined crews and led to hundreds of flight cancellations.

American Airlines, for its part, has been offering extra pay for crews to work on peak holidays to shore up staffing.

"It's all hands on deck to ensure our customers are cared for during the holiday travel season, including when severe weather hits," American said in a statement. "Critical to our preparations was sizing the airline for the resources we have available and operating conditions we face, as well as being able to react quickly to get our customers on their way once the weather clears."