UPDATE 1-Tropical Storm Sandy forms in the Caribbean

(Updates storm strength, location)

MIAMI Oct 22 (Reuters) - Tropical Storm Sandy formed in the western Caribbean Sea on Monday and is expected to become a hurricane on Wednesday as it approaches Jamaica, the U.S. National Hurricane Center said.

The storm was centered about 395 miles (640 kilometers) south-southwest of Kingston, Jamaica, and had top sustained winds of 45 miles per hour (75 kph).

The storm did not pose a threat to the Gulf of Mexico, where U.S. oil and gas operations are clustered.

A tropical storm watch was in effect for both Jamaica and Haiti, meaning residents should expect heavy rains and strong winds within 48 hours.

Computer models showed Sandy was on a projected path that would carry it over Jamaica on Wednesday and then eastern Cuba and the Bahamas.

Sandy is expected to dump as much as 5 to 10 inches (12.5-25 cm) of rain across parts of Jamaica, Haiti, the Dominican Republic and eastern Cuba, forecasters said. (Reporting by Kevin Gray; Editing by Peter Cooney)