Southwest Airlines is grappling with a wave of cancellations and delays that are stranding holiday travelers, a situation the CEO says highlights its need to modernize its operation.

While most carriers were plagued by winter storms over the holiday weekend, only Southwest was forced to cancel a majority of its trips, pointing to systemic scheduling issues that have plagued the airline for years.

The airline canceled more than 2,500 flights on Tuesday, according to data from flight tracking website FlightAware, accounting for most of the 2,900 cancellations in the U.S. that day.

That comes after Southwest canceled more than 2,900 flights on Monday — a whopping 71 percent of its flights — while other airlines emerged relatively unscathed. 

Things may only get worse from here for Southwest customers. The airline said Monday that it would operate just one-third of planned flights “for the next several days” in order to salvage its schedule.

What happened at Southwest? 

Southwest says it was uniquely affected by winter storms that slammed airports in Chicago and Denver, two of Southwest’s largest hubs. The airline did not have the capacity to reschedule thousands of flights that were derailed. 

“We were fully staffed and prepared for the approaching holiday weekend when the severe weather swept across the continent. … These operational conditions forced daily changes to our flight schedule at a volume and magnitude that still has the tools our teams use to recover the airline operating at capacity,” Southwest said in a statement.

Others, meanwhile, said that the airline overbooked its holiday schedule and didn’t have the workforce available to rebook flights in the event of weather disruptions.

Southwest workers have long complained that the airline has failed to modernize its decades-old communications systems, raising the likelihood of meltdowns when things go wrong.

Southwest CEO Bob Jordan acknowledged those issues in an internal message to employees Sunday.

“Part of what we’re suffering [from] is a lack of tools. We’ve talked an awful lot about modernizing the operation, and the need to do that. And crew scheduling is one of the places that we need to invest in. We need to be able to produce solutions faster,” he said.

TWU Local 550, which represents flight dispatchers and meteorologists, wrote on Twitter Monday evening that the situation is “untenable and is a direct result of the lack of investment and foresight by Southwest Airlines leadership.”

“The lack of vision for the future, long-needed overhauls of technology, and limited changes to the network are all compounding each other,” the union added. “‘Sorry’ is not enough this time. Action must be taken and Southwest Airlines must be held accountable.”

Southwest uses a point-to-point system that relies less on central hubs and allows for more direct flights between faraway locations. The downside is that when flights are canceled, pilots and other workers struggle to reach their next flight, causing further disruptions.

Helane Becker, an airline analyst at Cowen, said in a Tuesday note that Southwest “had an extraordinary number of employees calling in sick,” which contributed to staffing issues, particularly at its Denver hub. The U.S. is struggling with a surge of viral illnesses, including COVID-19, influenza and respiratory syncytial virus.

The Department of Transportation (DOT) tweeted Monday night that it is monitoring widespread complaints from customers. Many have reported being unable to rebook flights or find their luggage.

“USDOT is concerned by Southwest’s unacceptable rate of cancellations and delays & reports of lack of prompt customer service. The Department will examine whether cancellations were controllable and if Southwest is complying with its customer service plan,” the department wrote on Twitter.