By 3 p.m. E.T. on Friday, 1,500 flights in and out of the U.S. had been canceled and more than 2,600 had been delayed – the highest of any day this week, according to FlightAware.com, which tracks global flight operations. 

Flight statuses are “at the mercy of the Omicron variant and the weather right now,” said Charlie Leocha, president of Travelers United, a nonprofit that focuses on consumer issues with travel.

No airline was spared from cancellations Friday, though some were hit harder than others. 17% of Allegiant Air flights were canceled, and another 9% were delayed, according to FlightAware. 11% of United Airlines flights were canceled, with 5% delayed. 

American Airlines, meanwhile, only canceled 1% of its flights Friday and delayed just 4%.

To put that in perspective, prior to the pandemic, 2.15% of U.S. flights were canceled in 2019, according to the Bureau of Transportation Statistics.

The increased cancellations Friday come after millions of Americans are returning from their first holiday traveling since before the pandemic, just as the Omicron variant went from accounting for 12.6% of all reported U.S. cases of COVID-19 to 73.2% as of Dec. 18, according to the Centers for Disease Control and Prevention.

That, combined with an increase in airline staffers contracting the virus, contributed to a week in which airlines around the world were forced to cancel thousands of flights each day. In some parts of the country, severe weather has also impacted flights.

“We continue to have an unusual number of cancellations this week—some have been due to severe weather, and some due to other factors—including the type of unanticipated staff impacts from COVID experienced by other airlines and partners in places where we fly,” an Allegiant Air spokesperson said in a statement to Fortune. 

Skywest Airlines, which canceled 14% of its flights and delayed another 8% Friday, said their teams are working “nonstop” to minimize the impact of Omicron on its service. “We continue to experience the impact of weather and the omicron variant across several SkyWest hubs,” the airline said in a statement to Fortune.

Others, like Southwest Airlines, have managed to operate at normal capacity throughout much of the week despite the Omicron surge. On Monday, the airline told Fortune that the company has not experienced operational issues this holiday period due to COVID outbreaks among its personnel. That was the case Friday as well.

“We’re set up for a great day today with minimal cancellations…due to weather conditions in various parts of the country,” a spokesperson for Southwest said.

And as Omicron continues to work its way through the general population, including airline crews, an end to the mass flight cancellations is within sight, said Helane Becker, an airline analyst at securities firm Cowen.

Becker says she expects the number of cancellations to decrease by mid-January—once Omicron cases are predicted to have peaked. 

“We expect [cancellations] to last into next week and then once we are past peak the issues should abate,” Becker said.

Experts also say that the recently updated guidance on isolation restrictions from U.S. health officials, which reduced quarantine for Americans who catch the coronavirus from 10 to five days, should help airline staff return to work faster and help reduce cancellations.

The new CDC guidelines “should help quite a bit to alleviate some of the issue,” Becker said.

But until flights return to normal, experts say the best way to prepare for upcoming travel is to check your flight status online before you leave for the airport. 

“For the foreseeable future, go online after you’ve checked in for a flight and check…Airlines will always have details on the status of your flight,” Leocha said. “Do this before you go to the airport.”