Aug. 25 (Bloomberg) -- A depression over the Caribbean Sea may strengthen later today into the fifth tropical storm of the Atlantic hurricane season, U.S. forecasters said.

Tropical Depression 5 was about 345 miles (560 kilometers) south of San Juan, Puerto Rico, shortly before 5 a.m. local time, according to an online advisory from the National Hurricane Center in Miami. The system's maximum sustained winds of 35 miles per hour were just 4 mph short of tropical-storm strength.

"Some strengthening is forecast during the next 24 hours and the depression could become a tropical storm later today or tonight,'' the center said. "The depression is moving toward the west near 20 mph and a motion generally toward the west-northwest with some decrease in forward speed is expected for the next day or so.'' If it becomes a storm, it will be called Ernesto.