On Thursday evening, the tropical storm's center was 15 miles west of Kingston, Jamaica's low-lying capital. Forecasters said it could strengthen into a hurricane before slamming into Grand Cayman on Friday night.
Even as tourists searched for flights off the islands, officials urged calm. Theresa Foster, one of the owners of the Grand Caymanian Resort, said Gustav didn't look as threatening as Hurricane Ivan, which destroyed 70 percent of Grand Cayman's buildings four years ago.
"Whatever was going to blow away has already blown away," she said.
Forecasters said parts of Jamaica could get 25 inches (63 centimeters) of rain, which could trigger landslides and cause serious crop damage.
By early Thursday evening, dozens of roofs were ripped from houses, trees were toppled and many roads were left impassable by floodwaters and debris.
Jamaica evacuated low-lying areas including Portmore, a crowded and flood-prone area outside Kingston. Kingston's main airport was closed and buses stopped running even as people streamed into supermarkets for emergency supplies.
Oil prices spiked above $120 a barrel before settling below $116 in a session made volatile by fears that the storm could affect production in the Gulf area, home to 4,000 oil rigs and half of America's refining capacity. Hundreds of offshore workers pulled out, and analysts said the storm could send U.S. gas prices back over $4 a gallon.
"Prices are going to go up pretty soon. You're going to see increases by 5, 10, 15 cents a gallon," said Tom Kloza, publisher of the Oil Price Information Service in Wall, New Jersey. "If we have a Katrina-type event, you're talking about gas prices going up another 30 percent."
Meanwhile, Tropical Storm Hanna formed in the Atlantic, but it was too early to predict whether it could threaten the U.S. east coast. Forecasters cautioned that Gustav's path remained equally uncertain.
"It is simply impossible to determine exactly where and when Gustav will make final landfall," said Richard Knabb of the National Hurricane Center in Miami. "The chances of hurricane-force winds within the next five days are essentially the same at each individual location from the Florida Panhandle coast westward through the entire coastline of Louisiana."
But with Hurricane Katrina's third anniversary falling on Friday, Louisiana wasn't taking any chances. Gov. Bobby Jindal declared a state of emergency to lay the groundwork for federal assistance. Texas Gov. Rick Perry issued a disaster declaration, and together they put 8,000 National Guard troops on standby.
New Orleans Mayor Ray Nagin said he would order a mandatory evacuation of the city if forecasters predict a Category-3 strike - or possibly even a Category-2 - within 72 hours. Both Jindal and Nagin were meeting with U.S. Homeland Security Secretary Michael Chertoff.
"I'm panicking," said Evelyn Fuselier of Chalmette, whose home was submerged in 14 feet (4 meters) of Katrina's floodwaters. "I keep thinking, 'Did the Corps fix the levees?,' 'Is my house going to flood again?' ... 'Am I going to have to go through all this again?"'
Haitian officials said a 7-year-old girl was among the 59 dead, most of whom were killed in the mountainous center of the country's southern peninsula.
"Some of them were killed when trees fell on their houses," said Jean-Michel Sabbat, a senior civil protection official. "Some of them, flood water swept them away."