Wintry weather and the coronavirus pandemic frustrated US travelers as return flights from holidays were canceled or delayed in the first days of the new year. By early afternoon on Sunday, more than 2,300 US flights and more than 3,900 worldwide had been canceled, according to tracking service FlightAware. That followed Saturday cancellations of more than 2,700 US flights and more than 4,700 worldwide. The single-day US toll on Saturday 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 made Chicago the worst place in the US for travelers throughout the weekend, even as airports recovered on Sunday morning. About a quarter of all flights at O’Hare airport were canceled. Southwest Airlines said it was working to help customers affected by about 400 flights canceled around the country on Sunday, about 11% of its schedule. The Dallas-based airline said it anticipated even more operational challenges to come as the storm system pushes into east-coast states. Delta said it was issuing a travel waiver for planned flights this week out of mid-Atlantic airports in Baltimore and Washington, in preparation for forecast winter weather. American Airlines said most canceled flights on Sunday had been cut ahead of time to avoid last-minute airport disruptions. SkyWest, a regional carrier that operates under the names American Eagle, Delta Connection and United Express, grounded nearly 500 flights on Sunday, about 20% of its schedule, according to FlightAware. Airlines have said they are taking steps to reduce cancellations caused by the spread of the Omicron variant. United is offering to pay pilots triple or more of usual wages for picking up open flights through January. Spirit Airlines reached a deal with the Association of Flight Attendants for double pay for cabin crews through Tuesday, said a union spokesperson. Airlines hope extra pay and reduced schedules get them through the holiday crush and into the heart of January, when demand usually drops off. The decline could be sharper than normal this year because most business travelers are still grounded.