Florence has 130 miles per hour winds, enough to make it a risky Category 4 hurricane.
Maximum sustained winds have decreased to near 40 miles per hour (65 km/h) with higher gusts.
The tropical storm remnants are passing south of the island and moving west across the Caribbean region.
Puerto Rico, which is still recovering from a lethal blow by Maria last year, could see some rain and gusty wind, but nothing close to last year.
"It's not a zero event".
Forecasters are predicting it will feel "very warm and humid" when the storm arrives.
Tropical Storm Issac was one of four weather systems rolling across the Atlantic Thursday, including the risky category 2 Hurricane Florence, which was packing winds of nearly 100mph as it headed towards the Carolinas on the east coast of the United States.
"But it could have a positive effect on eastern parts because it could also bring tropical air". "It's a nasty interplay that makes it very hard". The forecast was complicated because the storm was still small, allowing it to quickly change. The storm has nothing to weaken it in it's path until it reaches landfall. This could lead to historic flooding along the coast. Some fluctuation in wind speeds are expected, and weakening forecast Thursday, but it's still expected to be a fierce storm triggering unsafe flooding.
As the storm continues toward Southeastern North Carolina, it will bring life-threatening storm surges, flooding and risky waves to the coast, the briefing said.
The NHC is estimating that as many as 20 inches of rain could fall in some places along the southern North Carolina coast while bringing in excess of 10 inches of rain in parts of SC.
Tropical storm warnings were issued for Martinique, Dominica, and Guadeloupe.
"Just because we have a landfall to the south doesn't mean your out of the woods because the wind around this storm is huge", National Hurricane Center Director Ken Graham said Wednesday morning.
The storm's eye was about 145 miles southeast of Wilmington as of 11 a.m., the briefing said.
According to the NHC 11 p.m. Tuesday update, Tropical Storm Isaac's center is "outrunning" the deep convection by about a degree due to strengthening westerly shear. "Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other risky conditions", the hurricane center briefing said. Flash flooding is going to be a big issue soon - certainly catastrophic. "Prepare for long power outages".
SAN JUAN, Puerto Rico - Power outages were reported across Puerto Rico Friday morning as Tropical Depression Isaac dropped rain across the USA territory.
A system in the central Gulf of Mexico was a little less certain.
The agency is also monitoring a tropical disturbance that has a 60 percent chance of forming a tropical storm. A hurricane hunter plane is scheduled to investigate the system Thursday.