Hurricane Dorian is strengthening and forecast to hit Florida as a Category 4 storm, threatening to derail travel during the busy Labor Day weekend.

Hurricane Dorian is projected to make landfall in Florida with winds reaching 130 miles per hour, according to the National Hurricane Center. The air traffic control tower will close if wind speeds surpass 55 miles per hour, said Miami International Airport spokesman Greg Chin.

American Airlines on Thursday said travelers booked Sept. 2-3 to or from 13 Florida airports from Key West to Gainesville and including its Miami hub can change their tickets without paying a fee or difference in fare if they can travel as late as Sept. 10.

Southwest Airlines doesn’t charge a flat fee for flight changes, but it won’t make passengers who want to change their flights pay for the fare difference if they can fly within two weeks of their original travel date. The policy applies to passengers with tickets to or from six Florida airports, including Fort Lauderdale and Orlando, from Aug. 31 to Sept. 4.

The airlines weren’t canceling Florida flights yet because the path of the storm is still unclear.

“The risk of devastating hurricane-force winds along the Florida east coast and peninsula late this weekend and early next week continues to increase, although it is too soon to determine where the strongest winds will occur,” the National Hurricane Center said in a forecast on Thursday.

Airlines generally relocate as many of their aircraft from airports in the path of storms as possible and cancel large parts of their schedules so that crews and aircraft are in place to resume operations after the storm has passed.

Hurricane Dorian’s potential to disrupt travel during the busy Labor Day travel period is a challenge for airlines that have weathered a difficult summer, despite strong demand. American and Southwest have faced capacity constraints as the Boeing 737 Max grounding, now in its sixth month, have forced them to cancel thousands of flights during the busiest travel period of the year, driving up costs and denting profits.

Between Aug. 28 and Sept. 3, U.S. airlines will carry a record 17.5 million passengers, a 4 percent increase from last year, according to industry group Airlines for America.