'Disruption we've never seen': What's causing the Southwest Airlines meltdown?

A massive storm has blanketed much of the U.S. with snow, slowing air travel during a peak season and causing tens of thousands of flight cancellations. The holiday meltdown at Southwest Airlines, however, has far eclipsed its competitors.

By Wednesday afternoon, the company had canceled more than 2,500 flights planned for the day, which amounted to 62% of the day's total, according to the flight tracking website FlightAware.

By contrast, as of Wednesday afternoon, United Airlines had canceled just 11 flights scheduled for the day; while American Airlines had canceled 23, FlightAware said. Each figure accounted for less than 1% of the respective company's total flights.

In an interview with ABC News' Byron Pitts, Secretary of Transportation Pete Buttigieg on Tuesday described the wave of flight cancellations at Southwest Airlines as a "shocking and unacceptable level of disruption." Meanwhile Sen. Maria Cantwell, D-Wash., the chair of the Senate Committee on Commerce, Science and Transportation, said the problems at Southwest Airlines "go beyond weather."

While the snowstorm grounded flights in and out of key airports in Southwest's network, the historic scale of cancellations stems from the company's uniquely complex flight coordination model and its antiquated internal scheduling systems, according to flight experts, Southwest Airlines officials and union leaders.

"This is a level of disruption we've never seen in this country before," Ross Feinstein, industry veteran and former director of operations communications at American Airlines, told ABC News. "​​The numbers are staggering."

In a video posted by Southwest Airlines on Tuesday, CEO Bob Jordan apologized for the delays.

"I'm truly sorry," Jordan said. "Our network is highly complex and the operation of the airline counts on all the pieces, especially aircraft and crews remaining in motion to where they're planned to go."

"After days of trying to operate as much of our full schedule across the busy holiday weekend, we reached a decision point to significantly reduce our flying to catch up," he added.

Here's what's causing the meltdown at Southwest Airlines:

A unique flight-scheduling model

A snowstorm battered much of the U.S. in recent days, leaving 34 dead in Western New York and forcing many indoors nationwide.

The storm also curtailed air travel, including at Southwest Airlines, which saw major airports in Denver and Chicago hammered by harsh conditions.

But the particularly severe disruption at Southwest Airlines is due in large part to the company's approach to flight scheduling, which departs from the typical hub-and-spoke model that most airlines rely on, Henry Harteveldt, a travel industry analyst at Atmosphere Research Group, told ABC News via email.

"At an airline like American or United, for example, a plane may operate on a fairly narrow 'track,' traveling from its base hub to a variety of cities," Harteveldt said. "That helps the airline 'compartmentalize' its operation when bad weather hits."

Southwest Airlines, instead, uses a point-to-point model in which "aircraft may make multiple stops along a route as they ply their way across the country," he added. "So, when an extensive bad weather system hits, Southwest is affected in ways that other airlines are not."

Jordan, the CEO of Southwest Airlines, acknowledged the company's atypical scheduling strategy in a statement on Tuesday.

"Southwest is the largest carrier in the country, not only because of our value and our values, but because we build our flight schedule around communities, not hubs," he said.

In order to return to normal operations, the company said it would operate roughly one-third of its schedule over the next two days, as it resets its complex model of scheduling and coordinating trips. As of Wednesday, Southwest Airlines had canceled 58% of its flights scheduled for Thursday, FlightAware data showed.

An overwhelmed internal management system

In addition to its complicated model for assigning flights, Southwest Airlines also suffers from an antiquated internal system used for managing and staffing those trips, company officials, union leaders and experts said.

"They've had IT-related issues in terms of tracking their crews and scheduling," said Feinstein, formerly of American Airlines. "Issues with what they use to monitor aircraft locations, crew, flight attendants, all of the above."

Casey Murray, the president of the Southwest Airlines Pilots Association, said in a statement on Tuesday that the technology used internally to oversee staffing and scheduling has faced difficulties stretching back at least a decade.

"We all know that the company has had its head buried in the sand when it comes to its operational processes and IT," Murray said. "We aren't undermanned. We're undermanaged."

Southwest Airlines CEO Bob Jordan affirmed such concerns in a message to employees, according to a memo obtained by CNN.

"Part of what we're suffering is a lack of tools," Jordan said. "We've talked an awful lot about modernizing the operation, and the need to do that."

The mismanagement has forced ground workers to complete 16- or 18- hour workdays over this holiday season, exposing them to bitter cold, sickness and even frostbite, Randy Barnes, president of TWU Local 555, which represents ground workers at Southwest Airlines, said in a statement.

"When you're dealing with sub-zero temperatures, driving winds and ice storms you can't expect to schedule planes as if every day is a sunny day with moderate temperatures and a gentle breeze," Barnes said. "The airline needs to do more to protect its ground crews."

The lack of adequate staffing has extended to the company's call centers, where customers face long wait times as they seek an alternative flight or a reimbursement.

In an interview with ABC News, Buttigieg, the Transportation Secretary, lamented the persistence of such challenges at Southwest Airlines, including passengers "unable to get anybody on the phone to help them" and the company's "inability to fully keep track of where their own flight crews are."

What to expect looking ahead

Feinstein, formerly of American Airlines, said he expects the flight cancellations to endure for weeks, since the initial cancellations create a "domino effect" in which the company struggles to rebook delayed passengers, leaving others unable to find flights.

"It has become a nightmare getting passengers to their final destinations on flights that aren't canceled, other airlines, rental cars, buses or trains," he said.

So far, however, the outlook appears relatively positive for flight prospects at the company on Friday. Southwest Airlines has canceled 39 flights scheduled for that day, which makes up less than 1% of the total planned.

Still, on Tuesday, Buttigieg vowed that the Department of Transportation would investigate the causes of the large-scale delays.

With the chaos still ongoing, Southwest Airlines already has reached an unprecedented level of disruption, experts said.

"I don't recall seeing any other individual airline have such extensive problems as Southwest is having now," said Harteveldt, the industry analyst.

ABC News' Sam Sweeney and Tal Axelrod contributed to this report.