Updated at 3:22 p.m. ET
(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 in the Caribbean and threatened to become a hurricane that could take a shot at Florida just as Republicans gather for their national convention in Tampa, Fla.
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.
David Bernard, chief meteorologist of CBS Miami station WFOR-TV, reports the first disruption to the convention could be this weekend, when airlines may start limiting flights into the city to lessen the chance of damaging planes stuck on the ground in the storm.
The storm itself isn't projected to be near Tampa until Monday night or Tuesday morning, Bernard reports. While Isaac may weaken while going over Haiti and eastern Cuba, it may intensify again Sunday night or Monday morning when it hits the Atlantic and Florida Straits, possibly reaching hurricane-force winds near the Florida Keys.
There's still a chance Isaac's path could take it east up Florida's coast to Miami or west of Tampa into the Gulf of Mexico, Bernard reports.
Tampa 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."
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.
Isaac was centered 165 miles south of Puerto Rico early Thursday afternoon, with maximum sustained winds of 40 mph. It was moving west at 15 mph according to the Hurricane Center.
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.
"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."
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. Little change in strength was expected in the next 48 hours.