NHC said on the forecast track, Isaac should move across the central Lesser Antilles and into the eastern Caribbean Sea later today, and then move across the eastern and central Caribbean Sea through the weekend.
"On the forecast track, the center of Florence will move over the southwestern Atlantic Ocean between Bermuda and the Bahamas Tuesday and Wednesday, and approach the coast of SC or North Carolina on Thursday", the NHC said.
"There are no coastal watches or warnings in effect", said the NHC in its 11 a.m. September 12 update. Maximum sustained winds are near 60 miles per hour or 95 km/h.
The poorly organized, 45-mph storm is not expected to strengthen but should maintain its tropical storm strength for the next few days. The center of Helene was about 95 miles west of Flores in the Azores Islands, the NHC said in its latest advisory.
In May, the Canadian Hurricane Centre said its forecasters were expecting a "near-normal to above-normal" number of storms this season, a prediction that has proven to be correct - so far.More news: Naomi Osaka "Not Sad" About Serena Controversy Overshadowing US Open Win
Some deterioration in sea conditions are expected tonight and will continue into tomorrow with swells expected to peak near 3.5 metres in open water.
It's not clear if the storm is expected to strengthen back into a hurricane after it hits the eastern Caribbean.
Duke Energy North Carolina President David Fountain said Florence is so massive and its potential for damage so extensive that people could be without power for a very long time.
Sustained winds on Tuesday afternoon approached 105 miles per hour, making it a very strong Category 2 storm, though it was expected to gradually weaken over cooler waters. It should dissipate by early next week, the hurricane center said. Seven-day rainfall totals are forecast to reach 500 millimetres over much of North Carolina and Virginia, and even 760 mm in some places.
Some coastal flooding is possible in areas of onshore winds.