Tens of millions of people were expected to head out for the holiday weekend, mostly on the roads. But storms slamming parts of the South could make return travel treacherous.

Thousands of flights across the U.S. were delayed Monday, as severe thunderstorms in the South, Plains and Mississippi Valley disrupted traveling Americans during the Memorial Day weekend. As of 4:30 p.m. EDT on Monday, there were over 4,300 flight delays and more than 125 cancellations in and out of U.S. airports, according to FlightAware.

Southwest will continue to offer two free checked bags to Rapid Rewards A-List preferred members and customers traveling on Business Select fares, and one free checked bag to A-List members and other select customers. Passengers with Rapid Rewards credit cards will receive a credit for one checked bag.