Southwest Airlines canceled about 600 flights and delayed more than 4,000 between last Friday and Sunday, according to reports.
Those numbers represent about 40% of Southwest's total flights over the weekend, as Fox 11 Dallas first reported.
On Monday and Tuesday, the airline delayed a combined 2,355 flights, according to data from global flight tracker FlightAware.com.
"As the weather radar has been illustrating over the past few days, the cancellations are due to convective thunderstorms across the nation from Denver and Chicago down to Houston, Dallas, the Gulf Coast and Florida – all areas with our busiest flight schedules," a spokesperson for the airline told FOX Business.
Southwest noted that it operates more than 3,400 flights per day and made some "proactive cancellations," which "allow Southwest to alert [c]ustomers sooner to changes in their flights, minimize delays at airports, reduce possible weather-related diversions and react nimbly to the resulting air traffic control initiatives that are often implemented to reduce air traffic/manage flights into airports, or along approach routes, experiencing severe weather."
One Southwest customer, Sara Kyle, spoke to Fox 11, calling the delays "frustrating."
"Had to book an extra night at the hotel, extend our rental car," Kyle said after her fight was delayed with only several hours' notice, according to the outlet. "My husband is missing a few meetings this morning. Minor inconveniences, but it would’ve been nice to have a heads up."
The airline also said it is increasing overtime pay for employees between July 1 and July 7 as travelers prepare to celebrate the 4th of July. Ground operations agents, flight attendants and cargo agents will all be eligible for overtime.
The Transportation Security Administration (TSA) screened more than 2 million travelers on Sunday and Monday compared to just over 600,000 people screened on the same days last year.
American Airlines has faced similar obstacles in recent months as travel demand increases amid a nationwide staffing shortage. The airline canceled hundreds of flights in June and asked Dallas-based employees to volunteer for extra work.
Southwest, American, Delta Air Lines, JetBlue Airways, United Airlines and a number of others secured an initial $25 billion in federal coronavirus relief through the CARES Act in April in an effort to help the airlines keep workers employed through the pandemic and another $25 billion in low-interest loans.
Southwest announced in August that it would not pursue another loan from the Trump administration and instead sought capital from other financing sources. Southwest and Delta encouraged employees to take voluntary time off while the company was overstaffed last year in an effort to avoid furloughs.
Outgoing CEO Gary Kelly then told employees in December after hinting that the company may need to lay off some workers that such action would no longer be necessary after the Trump administration signed a $900 billion relief package, which included $15 billion for airlines to keep all their employees on the payroll through March 31.
American and United Airlines, which together furloughed 32,000 employees in October, said at the time that they would rehire laid-off workers amid the bill's signing.
FOX Business' Lucas Manfredi contributed to this report.