The sixth tropical storm of the Atlantic hurricane season, named Fabian, was expected to strike the Florida Keys early today but wasn't expected to develop hurricane force. Tropical-storm warnings went up for parts of Cuba as heavy rain and winds gusting to 45 m.p.h. swept central Cuba and showers extended north over southern Florida and the Bahamas. The Keys were under a tropical-storm watch, and the National Hurricane Center at Coral Gables, Fla., urged that residents secure their boats and property. Hurricane center director Bob Sheets said: "The storm center's going to be fairly close to us, but we're going to be on the weak side, and we think rain is going to be the main thing we're going to have a problem with."