However, it is not expected to bring the severe weather seen during Storm Ophelia last October, which caused widespread damage and left three people dead.
Isaac is forecast to move into the eastern Caribbean Sea on Thursday, and then move across the eastern and central Caribbean Sea through Saturday, the hurricane center said.
Hurricane Helene is forecast to weaken over the eastern Atlantic Ocean, and interests in the Azores islands should monitor the storm's progress.
Florence, however, continues to threaten the east coast as a fierce storm that could trigger flooding as far south as Georgia. Aircraft data indicated maximum sustained winds near 60 miles per hour with higher gusts, according to the National Hurricane Center.
Rainfall of 1 to 2 inches was expected to affect Puerto Rico and the United States Virgin Islands with isolated areas receiving up to 4 inches.
The storm is tracking to the west at 16 miles per hour.
Surf swells generated by Isaac are affecting portions of the Lesser Antilles. While Florence is not expected to pose any danger to Britain, remnants of Hurricane Helene are expected to hit the country on Tuesday.
Isaac was a hurricane Sunday night but downgraded when maximum sustained winds fell below 74 miles per hour.
"A life-threatening storm surge is highly likely along portions of the coastline of SC and North Carolina, and a Storm Surge Warning is in effect for a portion of this area", the National Hurricane Center said Wednesday.
In addition to hurricane-force winds, Florence was forecast to produce large swells and life-threatening rip currents from the U.S. East Coast to Bermuda.
"There are no coastal watches or warnings in effect", said the NHC in its 11 a.m. September 12 update. While early on Wednesday the odds of it forming dropped, by afternoon forecasters had again increased the chance to 70 percent over two days.