A day after stalling off Haiti's coast, Gustav was centered about 80 miles (130 kilometers) east of Kingston, Jamaica, and moving toward the west-southwest near 8 mph (13 kph) at 5 a.m. EDT.
The storm was expected to pass very close to Jamaica later in the day, according to the National Hurricane Center in Miami. It's maximum sustained winds were near 50 mph (85 kph). The tropical storm was forecast to regain strength and the hurricane center said it could become a hurricane again by Friday.
The new tropical depression had maximum sustained winds near 35 mph (55 kph). The depression was centered about 355 miles (575 kilometers) east-northeast of the northern Leeward Islands and moving toward the west-northwest near 5 mph (8 kph).
The depression had the potential to become a tropical storm later Thursday or on Friday, the hurricane center said.
Gustav poured more misery Wednesday onto Haiti after landslides and flooding killed 23 people. Elsewhere, oil workers began leaving their rigs and New Orleans drew up evacuation plans as forecasters warned the storm could plow into the U.S. Gulf coast as a major hurricane.
Gustav killed 15 people on Haiti's deforested southern peninsula, where it dumped 12 inches (30 centimeters) or more of rain. A landslide buried eight people, including a mother and six of her children, in the neighboring Dominican Republic.
Cuba replaced its hurricane warning for Granma province with a tropical storm warning Thursday. All other hurricane watches and warnings were discontinued.
A tropical storm warning and hurricane watch were in effect for Jamaica and a hurricane watch was in effect for the Cayman Islands.
Gustav's expected track pointed directly at the Cayman Islands, an offshore banking center where residents boarded up homes and stocked up on emergency supplies.
By Labor Day, Gustav could make landfall anywhere from south Texas to the Florida panhandle, and hurricane experts said everyone in between should be concerned.
"We know it's going to head into the Gulf. After that, we're not sure," said meteorologist Rebecca Waddington at the National Hurricane Center. "For that reason, everyone in the Gulf needs to be monitoring the storm."
New Orleans began planning a possible mandatory evacuation, hoping to prevent the chaos it saw after Hurricane Katrina struck three years ago Friday. Mayor Ray Nagin left the Democratic National Convention in Denver to help the city prepare.
Oil prices spiked more than US$2 to close above US$118 a barrel, rising for a third day on fears that Gustav - like Katrina and Rita - could damage the Gulf Coast energy infrastructure, home to 15 percent of the nation's natural gas output, a quarter of its oil production and nearly half its refining capacity.
Royal Dutch Shell PLC said it was evacuating 300 people from rigs Wednesday, and other producers were doing the same. Transocean Inc., the world's largest offshore drilling contractor, said all 11 of its Gulf rigs were pulling up and securing drill pipe and other underwater equipment as a precaution.
Any damage to the oil infrastructure could send U.S. pump prices spiking, possibly before the busy Labor Day weekend.
"A bad storm churning in the Gulf could be a nightmare scenario," said Phil Flynn, an analyst at Alaron Trading Corp. in Chicago. "We might see oil prices spike US$5 to US$8 if it really rips into platforms."
Gustav is particularly worrisome because there are few surrounding wind currents capable of shearing off the top of the storm and diminishing its power, the hurricane center said. "Combined with the deep warm waters, rapid intensification could occur in a couple of days."
Nearly 30,000 people were evacuated from low-lying areas in eastern Cuba, and state television showed muddy, waist-high water damaging homes. The Cayman Islands ordered citizens to secure loose materials in their yards to prevent them from becoming missiles in high winds, and told them to stock up on food, medicine and fuel for generators.
In the Dominican Republic, a mother's screams and the roar of falling earth jolted a Santo Domingo shantytown from its sleep Tuesday. Marcelina Feliz and six of her seven children - ranging in age from 11 months to 15 years - were killed when a landslide crushed their tin-roofed house.
Feliz, 32, was found hugging the body of her smallest child, rescue officials said. A neighbor was also killed.
"I don't know how I can live now, because none of my family is left," said Marino Borges, Feliz's husband and father of several of her children.