Ernesto, packing 50 mph winds, was projected to reach hurricane strength by Tuesday but it was too soon to predict whether it would hit the United States, said Michael Brennan, a meteorologist with the U.S. National Hurricane Center in Miami.
"People should pay attention, especially people on the Gulf Coast," Brennan said. "It's a good time for people to update their hurricane plans."
Jamaica and the Cayman Islands, which both stood in the storm's path, issued hurricane watches, meaning severe conditions including winds of at least 74 mph were possible over the next 48 hours. Tropical storm warnings also were in effect for Jamaica and Haiti's southern coast.
Ernesto was on a course that would bring it over Jamaica by Sunday afternoon, dumping 4 to 8 inches of rain on the island with up to a foot possible in some areas, the hurricane center said. Fisherman were warned to return to shore — with tides up to 3 feet higher than normal expected. Prime Minister Portia Simpson Miller met with disaster agencies to prepare.
Jamaica issued advisories by radio and television for residents in low-lying areas across the island to be prepared to evacuate if necessary. Ernesto could be near hurricane strength as it passes close to Jamaica, the hurricane center said.
At 2 p.m. Saturday, Ernesto had maximum sustained winds near 50 mph, with higher gusts. The fifth named storm of the Atlantic hurricane season was centered 245 miles southwest of Santo Domingo, Dominican Republic, and 370 miles east-southeast of Kingston, Jamaica.
The storm was moving west-northwest near 15 mph. It was expected to bring 3 to 6 inches of rain to Haiti and the Dominican Republic as it passed south of Hispaniola.
As heavy showers hit Kingston on Saturday afternoon, traffic jams formed as motorists tried to reach stores and people waited in long lines at supermarkets, filling grocery cars with canned goods, batteries and candles.
"We have been doing brisk business since late afternoon" Friday, said Cynthia Martin, a supermarket supervisor.
Taxi driver Patrick Wallace, 55, said he was hoping for the best as he left a supermarket laden with canned goods.
"It's nature and we can't stop it from taking its course," he said. "I'm hoping if it hits, it will be in the morning so we can see what's going on."
People in the Caymans were advised to complete their weather preparations on Saturday.
Jacky Kennett, who moved to the Cayman Islands with her family from Britain a year ago, was preparing for what could be her first hurricane.
"I went and put fuel in the car, got some money out of the bank, and stopped at the grocery store yesterday to stock up," said Kennett, 47. "I don't want to overreact or anything, but I am worried."
Cuban Civil Defense has placed the western and central areas of the island on hurricane watch, reports CBS News producer Portia Siegelbaum.
Ernesto is expected to lash Cuba's Isle of Youth, south of Havana, starting Monday afternoon while the brunt of the storm won't be felt until late Monday night. Evacuations along the southern coast won't begin until late Sunday.
In Haiti, emergency officials went on local radio to warn people living in flimsy shantytowns on the southern coast to seek shelter in schools and churches.
"These people could be in great danger," said Adel Nazaire, a coordinator with Haiti's civil protection agency. "Flooding is the biggest concern because a lot of residents live along the rivers and the sea."
Elisabeth Verluyten, a disaster management official with the Pan-American Health Organization in Port-au-Prince, said raising awareness is vital as many people won't leave their homes "because they're afraid of losing the little they have."
The impoverished Caribbean nation is 90 percent deforested, increasing vulnerability to deadly flooding and mudslides.
Fears that the storm could damage offshore energy facilities in the Gulf of Mexico had oil and natural-gas prices higher. Oil producers operating in the Gulf said they were prepared to evacuate nonessential personnel if needed.
Max Mayfield, the National Hurricane Center director, said it was too early to say whether the storm would hit the Gulf Coast, which is still recovering from last year's Hurricane Katrina.
"We've got some time. We don't want people to get too excited about this, but they certainly need to be watching it," Mayfield told CNN's Wolf Blitzer.
Meanwhile, former Tropical Storm Debby, now a depression, was expected to stay over the open Atlantic, posing only a threat to ships.