More than 2,500 U.S. flights canceled Sunday citing COVID, inclement weather

More than 2,500 United States flights have been canceled as of Sunday afternoon as airlines face inclement weather and increasing numbers of COVID-19 cases.

Globally, 4,188 flights have been canceled, 2,510 of them within, into, or out of the United States alone, according to FlightAware. More than 13,700 flights globally are facing delays, affecting 6,202 U.S. flights.

Thousands of flights have faced issues during the holiday season in the United States, largely due to COVID-19 cases as the new omicron variant sweeps the nation.

Chicago's O'Hare Airport was ranked highest on FlightAware's Misery Map Sunday. More than five inches of snow hit some parts of the Chicago area in a New Year's winter storm, and 47% of all O'Hare flights have been canceled or delayed. The previous day, 70% of all flights leaving O'Hare were canceled or delayed.

Southwest is one of the worst-hit airlines with more than 420 flights canceled as of Sunday afternoon.

Over the last 10 days, more than 14,000 flights have been canceled according to a CNN tally of FlightAware data.

Allegiant spokeswoman Hilarie Grey told CNN: "As you've noted, 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."

In an attempt to ease the Omicron-induced staffing shortage, United Airlines has begun offering pilots triple pay for January flights, CNBC reports.