That comes as an unspecified computer glitch has forced Southwest to manually check-in many passengers, requiring agents to write out boarding passes by hand. Online and mobile check-in appeared to be mostly unaffected, and many customers were able to check in via Southwest's airport kiosks. The biggest hold-up came for fliers trying to check bags for their flights.
The glitch began early Sunday, forcing the carrier to delay about 450 flights out of its daily schedule of about 3,600 flights. It was unclear how many flights might be impacted Monday.
Southwest warned customers to brace for more problems Monday. In a statement posted to the carrier's website, Southwest says:
"We're continuing to use back-up systems around the country to check-in Customers arriving at our airports without printed or mobile boarding passes. Intermittent performance issues continue impacting our in-airport Customer Service technology systems and across our online platforms (Southwest.com, Southwest Mobile App and site)."
On Sunday, the glitch led to long lines -- "miles long," according to some social media accounts -- as the carrier struggled to keep ahead of check-in queues.
Images of long lines surfaced from busy Southwest airports like Los Angeles, Las Vegas, Denver and Baltimore.
Passenger Ida Perez tweeted to NBC News that Sunday's check-in line at Las Vegas' McCarran International Airport wrapped around the building and down the street. Perez said she waited in line for over two hours before finally making her flight.
Emily Mitnick told The Associated Press she missed her 10 a.m. Sunday flight from Detroit to Denver even after arriving to the airport around 8 a.m. She estimated to the news agency that about 1,000 people were in the check-in line awaiting boarding passes. At curbside check-in, she guessed there were a couple hundred people in line there as well.
Mitnick says she did not clear security until about 10:15 a.m.
"The clock was ticking and the flight took off," Mitnick tells AP.