A tropical storm warning was in effect Sunday for the Cayman Islands as a tropical depression moved through the Atlantic on a path that could threaten the U.S. Gulf Coast later this week as a hurricane, forecasters said.

The system was expected to become Tropical Storm Wilma by Monday, which would make it the 21st named storm of the season, tying the record for the most storms in an Atlantic season, the National Hurricane Center in Miami said.

The only other time so many storms have formed since record keeping began 154 years ago was in 1933.

At 8 p.m. EDT, the depression was centered about 150 miles southeast of Grand Cayman, forecasters said. It was moving west-northwest near 2 mph and had sustained winds near 35 mph. Depressions become tropical storms when their winds reach 39 mph.

Long-term forecasts show the storm would likely move west and north, putting the storm in the Gulf of Mexico on Thursday or Friday. Forecasters said water temperature and other conditions were favorable for it to become a significant hurricane.

"Once storms get into the Gulf of Mexico, I'm aware of only one storm on record that dissipated. It has almost nowhere to go except land somewhere," hurricane specialist James Franklin said in Miami. "As it enters the Gulf of Mexico, the northwest Caribbean has some of the deepest warm water in the Atlantic basin and that's fuel for tropical cyclones."