(CBS/AP) SAN JUAN, Puerto Rico - Puerto Rico and the U.S. Virgin Islands braced for torrential rains on Thursday as Tropical Storm Isaac whipped up waves as high as 10 feet (3 meters) in the Caribbean and threatened to become a hurricane that could take a shot at Florida just as Republicans gather for their national convention.
Some flooding was reported in eastern and southern regions of Puerto Rico as the storm approached.
U.S. forecasters said Isaac will likely turn into a Category 1 hurricane by Friday as it nears the Dominican Republic and Haiti. It was expected to weaken a little while heading over their island and the eastern two-thirds of Cuba.
The storm was projected to head toward Florida as a hurricane by Monday, but the U.S. National Hurricane Center in Miami said some forecast models show it could go further west into the Gulf of Mexico, so "significant uncertainty remains about the threat Isaac poses to Florida."
Isaac was centered 165 miles (255) kilometers south of Puerto Rico early Thursday afternoon, with maximum sustained winds of 40 mph (65 kph). It was moving west at 15 mph (24 kph) according to the Hurricane Center.
Meanwhile, another tropical storm, Joyce, formed over the open water of the eastern Atlantic. Forecasters said it does not pose an immediate threat to land. The Hurricane Center in Miami said Thursday the storm has maximum sustained winds of 40 miles per hour (65 kph). Little change in strength was expected in the next 48 hours.
Puerto Rico opened 428 shelters, and 50 people had taken refuge, said Gov. Luis Fortuno. Some 7,800 people were without power and more than 3,000 without water.
Schools and government offices remained closed Thursday on the U.S. territory, but the governor said it was safe for people to go to work if they needed to. However, he warned everyone to stay away from beaches and swollen rivers.
"It's not the day to participate in recreational activities in these areas," Fortuno said.
Jose Alberto Melendez, 51, disregarded that advice, coming to a beach near Old San Juan.
"It's my birthday," he said. "I had already planned to come to the beach."
He unfolded his chair and turned on the radio just as a squall approached, sending him running for shelter.
While Isaac itself has caused no reported injuries or deaths, police in Puerto Rico say a 75-year-old woman died near the capital of San Juan on Wednesday when she fell off a balcony while filling a drum with water in preparation for the storm.
Puerto Rico's main international airport remained open, but Cape Air and American Eagle cancelled their flights Thursday, Fortuno said. Ferry service to the tourist islands of Vieques and Culebra also was suspended.
In Vieques, one of the owners of Bananas Guesthouse said his brother had called from Florida and suggested he tell reporters "there are mudslides and cows flying through the air. But in fact, there's a breeze going by," Glenn Curry said. "We've had a little bit of rain. Nothing much has happened so far ... Overnight it didn't even blow enough to wake me up."
In the U.S. Virgin Islands town of Christiansted, streets lined with historic buildings of Danish architecture, were largely deserted. All but a small handful of businesses and government offices were closed. Hurricane shutters covered the entrances to most buildings and sandbags were stacked in anticipation of potential floods and storm surge.
In St. Croix, the owners of Turtle's, a seaside restaurant, were baking bread for sandwiches, selling coffee and snacks to the few passersby and fielding calls from people about the weather.
"Yes, we're open," Mary Scribner said cheerily. "No, it's not raining!"
The Scribners pulled out sandbags in case the predicted storm surge or flooding impacted their business, but by midmorning, the sandbags still sat in a pile in the corner.
The storm already forced military authorities at the U.S. base in Guantanamo Bay, Cuba, to cancel pretrial hearings for five prisoners charged in the Sept. 11 attacks. They also were evacuating about 200 people, including legal teams and relatives of Sept. 11 victims.
, where 70,000 delegates, journalists and protesters are expected to descend on the city.
Convention CEO William Harris said Thursday he was working with Mitt Romney's presidential campaign and the National Weather Service to track the storm, and he said Florida officials have assured planners they have enough resources to respond to the storm should it make landfall.
But Hillsborough County Sheriff David Gee said some outside agencies that had planned to send officers to help with convention security might be forced to keep them home to deal with a storm.
"My primary concern right now is that we will lose resources," he said
In the Dominican Republic, meanwhile authorities began to evacuate people living in low-lying areas but encountered some resistance.
"Nobody wants to leave their homes for fear they'll get robbed," said Francisco Mateo, community leader of the impoverished La Cienaga neighborhood in Santo Domingo.
Businesses and schools were reopening in islands such as St. Kitts and Dominica following the storm's passage late Wednesday.
"Dominica has been spared the full brunt of Tropical Storm Isaac," said Disaster Coordinator Don Coriette. "We want to thank the almighty God for that."