The National Hurricane Center says Tropical Storm Omar has formed as the 15th named storm of the Atlantic season.

Omar formed Tuesday in the eastern Caribbean to the south of Puerto Rico and is expected to gradually strengthen over the next 48 hours.

A tropical storm watch was in effect for Puerto Rico, the U.S. Virgin Islands, the British Virgin Islands and the far eastern portion of the Dominican Republic.

At 11 a.m. EDT, the storm had maximum sustained winds near 40 mph. Its center was about 355 miles southwest of San Juan, Puerto Rico, and moving east-southeast near 2 mph. It is expected to turn gradually to the east and then northeast later Tuesday.

(Copyright 2008 by The Associated Press. All Rights Reserved.)