Airlines canceled hundreds of weekend flights and thousands more were delayed as thunderstorms in Florida slowed traffic in one of the country’s top travel destinations during spring break.

More than 5,900 U.S. flights were delayed and 1,930 were canceled on Saturday, according to flight-tracking site FlightAware.

Thunderstorms are especially challenging for airlines because they are harder to predict and plan for compared with other systems like winter storms and hurricanes, during which airlines often cancel flights hours if not days in advance.