At 11 a.m. EDT, the depression that formed Thursday had maximum sustained winds near 35 mph, below tropical storm strength of 39 mph, the National Hurricane Center reported. It could become Tropical Storm Ernesto later Friday.
"It's starting to have some of the signs of a tropical storm," said Eric Blake, hurricane specialist. "We have it passing pretty close to Jamaica in a couple of days and slowly intensifying. It could be a hurricane in the northwest Caribbean Sea."
The depression was centered about 340 miles south of San Juan, Puerto Rico. Forecasters said it was moving west at about 15 mph.
Meanwhile, Tropical Storm Debby weakened and was only a threat to ships in the open ocean. The fourth named storm of the 2006 Atlantic hurricane season had top sustained winds near 40 mph, down from 50 mph earlier in the day.
At 11 a.m., the storm was about 1,365 miles west-northwest of the Cape Verde islands, which are about 350 miles off the African coast. It was moving northwest at about 17 mph and was expected to continue on that path for the next day, the National Hurricane Center said.