Southwest Airlines Co.'s chief operating officer on Thursday is expected to face questions from lawmakers over the airline's December meltdown, while reiterating confidence in efforts to improve its operations.

After a winter storm threw its operation into disarray, Southwest canceled over 16,700 flights over a roughly 10-day period at the end of last year in one of the worst airline disruptions in recent years, a debacle that the company says could end up costing it over $1 billion.

The meltdown angered thousands of customers, strained the company's relations with employees and has made the airline -- which prides itself on a reputation for customer service -- the butt of late-night television jokes, including a sketch on Saturday Night Live last month. It has also galvanized lawmakers and regulators who have long sought stricter consumer protection measures for air travel.

Southwest Chief Operating Officer Andrew Watterson plans to apologize on Thursday when he addresses the Senate Committee on Commerce, Science, and Transportation, and to try to reassure lawmakers that the airline won't experience another similar incident, according to his prepared remarks.

"Let me be clear: we messed up. In hindsight, we did not have enough winter operational resilience," he plans to say in prepared testimony viewed by The Wall Street Journal. "Please know that with the mitigations we have in place, we are confident in our flight network and the schedules we have published for sale."

Chief Executive Bob Jordan had a conflict and wouldn't be able to attend the hearing, according to Southwest.

Capt. Casey Murray, the president of the Southwest Airlines Pilots Association, said the roots of the disruption go back long before the December storm, and are evidence of a deeper malaise within the company.

In his prepared remarks for Thursday's hearing, he said that the airline has ignored the union's warnings about its outdated scheduling practices and technology, citing what he described as a yearslong pattern of operational failures, poor management decisions, and underinvestment.

In December, "managers' overconfidence in their planning and a systemic failure to provide modern tools to employees doomed SWA's recovery before the first snowflake hit the ground," he plans to tell the committee.

As travel demand has bounced back from the Covid-19 pandemic, airlines' stumbles have frustrated regulators and lawmakers, who have called for new measures that would penalize airlines for schedule disruptions and compensate consumers.

The Transportation Department is investigating whether Southwest set itself up to fail with an unrealistic schedule over the holidays.

Southwest has said it is fully staffed and was prepared for the busy holiday season.

The trigger for the meltdown was a severe winter storm that swept across the country before Christmas. As temperatures plummeted, fuel turned to sludge, jet bridges froze, and workers could only be outside for short periods.

Other airlines contended with the same challenges, but Southwest struggled to right itself while rivals recovered within a few days. By Dec. 26, Southwest's operation was spiraling, and the airline slashed nearly two-thirds of its schedule for three days to reset itself.

In his prepared remarks, Mr. Watterson describes how worse-than-anticipated weather in Denver and Chicago -- two of the airline's largest airports and cities where about a quarter of its flight crews begin their trips -- set off a cascade of unplanned cancellations that Southwest couldn't recover from.

The debacle has raised questions about the pace of Southwest's investment in technology and core elements of its business model, including its point-to-point route network.

Mr. Watterson acknowledged those concerns in his prepared remarks, but said Southwest's operation had been strong for months before the meltdown, and said the airline is confident that it can manage disruptions now.

Southwest has said that the disruption revealed an unforeseen gap in a system it uses to reschedule crews following disruptions -- something the airline is now working to rectify.

Executives, including Mr. Watterson said the software did not break down, but wasn't able to keep up with the volume of last-minute changes as Southwest canceled more and more flights. As a result, it was trying to pair pilots and flight attendants with new flights based on outdated information, exacerbating the quagmire. The airline had to resort to manual processes to repair its schedules, a tedious and time-consuming process.

Southwest is in the process of testing an update to that system with General Electric Co., the provider, that Southwest has said will let it use the software to clean up past problems -- something it wasn't previously designed to do. Mr. Watterson said that will equip Southwest to better handle recovery from mass cancellations. He said the airline is also working to improve the tools it uses to communicate with crews and has implemented procedures that will help it more quickly see when things are going wrong and respond.

The airline is also conducting a review with Oliver Wyman, an outside aviation consulting firm, and its board has formed an operations review committee. The airline said it plans to spend $1.3 billion on investments and upgrades to its IT infrastructure, work that Mr. Watterson said will be informed by the reviews.