SAN JUAN, Puerto Rico (AP) — Puerto Rico and the US Virgin Islands braced for torrential rains today as Tropical Storm Isaac whipped up waves as high as 10 feet (three metres) in the Caribbean and threatened to become a hurricane that could take a shot at Florida.

Some flooding was reported in eastern and southern regions of Puerto Rico as the storm approached.

US 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 those islands and Cuba then possibly move on toward Florida as a hurricane by Monday.

Isaac was centred 200 miles (kilometres) south-southeast of Puerto Rico late Thursday morning, with maximum sustained winds of 40 mph (65kph). It was moving west at 15 mph (24kph) according to the US National Hurricane Centre.

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.

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 second-floor balcony while filling a drum with water in preparation for the storm.

Meanwhile, another tropical storm, Joyce, formed over the open water of the eastern Atlantic, The Hurricane Centre in Miami said. Forecasters said it does not pose an immediate threat to land.

Tropical storm Isaac is expected to be northeast of Jamaica's mainland by Saturday morning while approaching eastern Cuba.