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Wintry weather combined with the pandemic frustrated air travelers whose return flights home from the holidays were canceled or delayed in the first days of the new year.

A winter storm in the Mid-Atlantic and the continued spread of the omicron variant have caused cancellations to move into Monday, with nearly 200 flights canceled between Chicago’s two major airports.

More than 2,500 U.S. flights and more than 4,100 worldwide were grounded Sunday, according to tracking service FlightAware.

That followed Saturday’s mass cancellations of more than 2,700 U.S. flights, and more than 4,700 worldwide.

“It was absolute mayhem,” said Natasha Enos, who spent a sleepless Saturday night and Sunday morning at Denver International Airport during what was supposed to be a short layover on a cross-country trip from Washington to San Francisco.

Saturday’s single-day U.S. toll of grounded flights was the highest since just before Christmas when airlines began blaming staffing shortages on increasing COVID-19 infections among crews.

A winter storm that hit the Midwest on Saturday made Chicago the worst place in the country for travelers throughout the weekend. About a quarter of all flights at O’Hare Airport were canceled Sunday.

Denver’s airport also faced significant disruptions. Enos, who was flying on Frontier Airlines, didn’t learn that her connecting flight home to California was canceled until she had already landed in Denver. Then it was a rush to find alternative flights and navigate through baggage claims packed with stranded and confused travelers, amid concerns about the spread of the highly transmissible omicron variant of COVID-19.

“It was a lot of people in a very small space and not everybody was masking,” said the 28-year-old financial analyst. “There were a lot of exhausted kids and some families were so stressed out.”

Southwest Airlines had canceled about 600 flights, or 16% of Monday’s schedule, by midday. Spokesman Brad Hawkins said storms over the weekend and on Monday affected operations at some of its biggest airports, including Chicago, Denver and Baltimore, and left planes and crews out of position.

United Airlines said the nationwide COVID-19 spike caused by the omicron variant has affected its flight crews, resulting in canceled flights. Delta cited winter weather and omicron, but said it expected fewer than half as many cancellations Tuesday and Wednesday. American cited the storm in the Washington area, and said the number of employees calling in sick because of COVID-19 was similar to the past few days, although it declined to give figures.

SkyWest, a regional carrier that operates flights under the names United Express, American Eagle and Delta Connection, grounded more than 350 flights Monday after scrubbing 500 on Sunday.

Thousands of miles from the snow storms, Hawaiian Airlines said it had to cancel several flights between islands and across the Pacific due to staffing shortages.

Airlines are paying temporary bonuses to encourage pilots and flight attendants to pick up flights left empty by co-workers with COVID-19. United will pay pilots triple their usual wages for picking up open flights through most of January. Spirit Airlines reached a deal with the union to pay flight attendants double through Tuesday.