Thunderstorms and staffing issues have forced American Airlines and Spirit Airlines to cancel and delay hundreds of flights this week.

According to flight-tracking site FlightAware, Americans Airlines on Tuesday saw 288 cancellations, which accounted for 9% of its flights. American has also had 445 delays, or 15% of its flights.

On Monday, American had 563 canceled flights, or 18%, with 1,048 delayed flights, or 34%.

Dallas/Fort Worth International Hub reported it was due to nine hours of severe high winds, hail, and lightning. Lightning becomes a safety issue for crews who work on the ground.

“Mother Nature isn’t playing nicely and many flights in and out DFW are delayed or canceled,” American Airlines tweeted to a customer on Sunday.

Spirit Airlines was experiencing similar issues.

Spirit on Tuesday had 352 canceled flights, or 51%, and 77 delayed flights, or 11%. Things were worse on Monday, with 334 canceled flights, or 42%, and 236 delayed flights, or 29%.

“We’re working around the clock to mitigate the travel disruptions caused by overlapping operational challenges including weather, system outages, and staffing shortages in some areas of the operation,” Spirit said in a statement.

“In responding to these challenges, Spirit has implemented some proactive cancellations again today to reset our operations.”

Southwest Airlines also suffered through a high number of cancellations and delays in recent days.

Airlines will go through with canceling flights if they suspect the issue cannot be easily resolved so that passengers will not be trapped in airports.