Tropical Storm Caribbean-Bound

As forecasters watch Tropical Storm Helene, another possible hurricane is brewing off Africa, according to the National Hurricane Center in Miami.

Tropical Storm Isaac is south of the Cape Verde Islands moving west-north-west at 18 miles an hour. Its top winds are near 50 miles an-hour with higher gusts.

Hurricane specialist Jack Beven at the National Hurricane Center says Isaac is strengthening and "could turn into a hurricane in 24 to 48 hours." but forecasters don't know yet if it will threaten the U.S. mainland.

At 5 p.m. EDT, Isaac was about 1,915 miles east of the Caribbean islands, at latitude 13.8 north and longitude 31.0 west, the U.S. hurricane center said.

It was moving west-northwest at 18 mph, packing maximum sustained winds of 50 mph and was expected to strengthen over the coming day.

The storm strengthened from a tropical depression to a tropical storm on Thursday.

©2000, CBS Worldwide Inc., All Rights Reserved