"The depression will likely become a tropical storm later (Thursday) and could reach hurricane intensity within three days," the Miami-based hurricane center said. The center's computer models predicted the depression would become an "intense hurricane."
If it becomes a tropical storm, it would be named Hanna.
Forecasters said the depression was moving toward the west-northwest at about 5 mph (8 kph). It was expected to turn toward the northwest later Thursday, which will take it well north of the Leeward Islands during the next 24 hours, the center said.
The depression's maximum sustained winds were near 35 mph (55 kph).