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 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 200 miles (320) kilometers south-southeast of Puerto Rico late Thursday morning, with maximum sustained winds of 40 mph (65 kph). It was moving west at 15 mph (24 kph) according to the Hurricane Center.

The system has slowed down, and as a result, the island will see more rain, said Ernesto Morales, forecaster with the National Weather Service in San Juan, Puerto Rico.

“This is not the moment to become complacent,” he said.

The U.S. territory has opened 428 shelters, and 50 people have taken refuge, said Gov. Luis Fortuno. Some 4,000 people were without power and more than 3,000 without water.

Schools and government offices remained closed Thursday, 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.

While Isaac itself has caused on 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 second-floor 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 all their flights Thursday, Fortuno said. Ferry service to the tourist islands of Vieques and Culebra also was temporarily 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.

Two shelters were open on the island, and 10 people were housed overnight, according to Elton Lewis, director of the Virgin Islands Territorial Emergency Management Agency.

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 mid-morning, the sandbags sat in a pile in the corner.

“We didn’t see this as a big deal,” Bob Scribner said. “Moderate rain and wind.”

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 planned to evacuate about 200 people, including legal teams and relatives of Sept. 11 victims.

Isaac also posed a threat to next week’s Republican National Convention in Tampa, where officials said they were ready to take emergency measures even as 70,000 delegates, journalists and protesters descend on the city.

“Public safety will always trump politics,” Tampa Mayor Bob Buckhorn said. “And so my job, and our job, if we move into that mode, is to make sure we get people out of harm’s way.”

In St. Kitts, the government said all businesses and schools were open after Isaac passed through late Wednesday.

In Dominica, the government said no damage or injuries were reported. The airport and seaports have reopened and things are back to normal, said Benoit Bardouille, CEO of the island’s Air & Seaport Authority.

However, Disaster Coordinator Don Coriette warned that heavy rains would persist through Friday.

“Dominica has been spared the full brunt of Tropical Storm Isaac,” he said. “We want to thank the almighty God for that.”

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.