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It was not a Happy New Year for thousands of air travelers in the U.S.

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Airlines canceled more than 2,400 U.S. flights on New Year’s Day, as a combination of severe weather nationwide and a surge in COVID-19 omicron variant infections continued to disrupt air travel over the holidays, The Associated Press reported.

Since Christmas Eve, airlines have canceled more than 12,000 U.S. flights and have been forced to delay thousands more, according to FlightAware, a flight-tracking site.

“We look at flights but see, the thing is they’re connected to people,” Capt. Dennis Tajer, an American Airlines pilot and spokesperson for their pilots’ union, Allied Pilots Association, told USA Today. “For each one of those passengers, there may be five to 10 family members and friends that were counting on them being at the holiday table.”

The travel issues come at a time when airlines braced for one of the busiest days since the pandemic began, CNBC reported. Last year, the Transportation Security Administration screened nearly 580 million people, up 79% from 2020 but still about 30% less than 2019 figures, before the pandemic.

Saturday’s cancellations mark the highest single-day toll since just before the Christmas holiday, the AP reported. Airlines were reporting increased staffing shortages due to the surge in omicron cases.

But the wintry weather also exacted a toll. In Chicago, 800 flights were canceled at O’Hare Airport and another 250 were scrapped at Midway, according to the AP.

Southwest Airlines said it planned to suspend operations at both Chicago airports on Saturday afternoon, The New York Times reported. The airline cut more than 470 flights nationwide, more than any other U.S. carrier. That accounted for 13% percent of its schedule.

A Southwest spokesperson told CNBC that the flights were canceled because planners were “anticipating the gusty winds and blowing snow that decades of our history operating in this airport show us will slow down the airspace and also make deicing and getting aircraft back into the air very challenging.”

>> Coronavirus: Hundreds more flights canceled as airlines grapple with omicron, weather

Denver, Detroit and Newark, New Jersey, also canceled more than 100 flights each. American Airlines, Delta Airlines, United and JetBlue scrubbed more than 100 flights each, according to the AP.

Delta Air Lines canceled 9% percent of its scheduled flights, while American and United Airlines each cut 7% percent, according to the Times. In a statement, United, which has its headquarters in Chicago, said that the nationwide spike in coronavirus cases had also affected its ability to staff flight.

“It’s tough enough to travel in the winter just because of bad weather, but COVID and the omicron variant are introducing new wrinkles that none of us had anticipated,” independent travel industry analyst and Atmosphere Research Group president Henry Harteveldt told USA Today. “Airlines have only so many of those standby crew members available.”

SkyWest, a regional carrier that operates flights as American Eagle, Delta Connection and United Express, grounded more than 400 flights, or 21% of its schedule, according to the AP.

“If you’re traveling within the next week, chances are it’s still going to be a very, very uneven, perhaps even chaotic operating environment for airlines,” Harteveldt told USA Today. “My hope is that within the next week, because of the new guidelines posted by the CDC that say employees who are fully vaccinated and asymptomatic can return to work if they feel well enough to do so, we will start to see the number of cancellations go down.”