CHICAGO (WGN) — Hundreds of Southwest Airlines customers were left stranded due to widespread cancellations across the country.

According to FlightAware, a website that provides real-time flight insights, Southwest has canceled over 1,000 flights on Sunday. That’s 27% of the airline’s scheduled flights on the nation’s largest domestic airline. 

Southwest said an air traffic management program put in place due to weather has caused significant delays.

The airline released the following statement: “We experienced significant impact in the Florida airports yesterday evening after an FAA-imposed air traffic management program was implemented due to weather and resulted in a large number of cancellations. We are working hard behind the scenes to minimize challenges and fully recover the operation as we take care of displaced Crews and Customers as quickly as possible.”

Passengers have said they have been given very little information from the airline, and were wondering if there was also an issue affecting Southwest’s staffing – not just air traffic controllers.

“It’s only two employees, they only have two people checking you in for luggage and tickets,” customer Kasena Jackson said.

“When I got to the line they rescheduled me on a flight tonight at 9, but that one was just canceled too,” flyer Wendie Kaminski said Saturday.

Southwest Airlines became the latest U.S. airline Monday to require its employees to be vaccinated against COVID-19.

The Dallas-based company said its workers must be fully vaccinated by Dec. 8 in order to remain at the airline. Employees can seek approval to skip the shots due to medical or religious reasons.

Southwest said it has to mandate vaccines because of new rules from the Biden administration requiring companies with federal contracts to have vaccinated staffs. Southwest’s work for the government includes flying the military in emergencies and carrying mail for the U.S. Postal Service.

Rivals American Airlines, Alaska Airlines, United Airlines and JetBlue have already told their staff they needed to be vaccinated

The Associated Press and NewsNation’s Jasmine Cooper contributed to this report.