Shares of Southwest closed down 4.2% to $51.67 in the regular session, but changed direction after hours rising by 0.5%.
The Dallas-based carrier company cancelled at least 30% of its scheduled flights on Sunday, a second straight day of heavy cancellations, although Southwest declined to confirm exactly how many of its flights were ultimately grounded.
"With fewer frequencies between cities in our current schedule, recovering during operational challenges is more difficult and prolonged," the airline told Reuters in a statement.
However, flight tracking website flightaware.com says that Southwest cancelled 1,103 flights on Sunday in addition to another 808 cancelled flights on Saturday.
"ATC issues and disruptive weather have resulted in a high volume of cancellations throughout the weekend while we work to recover our operation," Southwest said. "We appreciate your patience as we accommodate affected Customers, and Customer Service wait times are longer than usual."
SWAPA , the Southwest Airlines Pilots Association, has previously said that it plans to take union action and file a temporary restraining order to stop Southwest complying with a Covid-19 mandate for federal contractors that was set by President Joe Biden.
However, SWAPA put out a statement saying that it is "aware of operational difficulties," affecting the airline but denied being the reason for the cancellations as its pilots were "not participating in any official or unofficial job actions."
Meanwhile, Alan Kasher, Southwest's executive vice president of daily operations, said Sunday that while the airline was staffed for the weekend it did not anticipate the size of flight disruptions across its network,
"Although we’ve made schedule adjustments leading into the fall, our route system has not fully recovered—that will take time," Kasher said on the company's intranet.