Sept 24 (Reuters) - Tropical storm Ian is expected to become a hurricane on Sunday, reaching major hurricane strength by late on Monday before it reaches western Cuba, the U.S. National Hurricane Center (NHC) said.

The center of Ian is forecast to pass well southwest of Jamaica on Sunday and pass near or west of the Cayman Islands early on Monday, the agency said in its latest advisory.

Ian will then move near or over western Cuba on Monday night and early on Tuesday, emerging over the southeastern Gulf of Mexico on Tuesday, it added.

The storm is located about 395 miles (635 km) southeast of Grand Cayman packing maximum sustained winds of 50 miles per hour (80 kph), NHC said (Reporting by Anirudh Saligrama in Bengaluru; Editing by William Mallard)