Tropical Storm Isaac, with maximum sustained winds of 70 miles per hour (110 kph), was expected to grow into a Category 1 hurricane on Sunday night but weaken again as it approached the Eastern Caribbean by mid-week, on a track taking it well south of Puerto Rico, the NHC said.
Florence strengthened into a hurricane again Sunday as forecasters warned the menacing storm could deliver a brutal hit along the East Coast by the end of the week.
Tropical Storm Helene formed late Friday night off the coast of Africa, one of three storms - two named - now in the Atlantic.
Hurricane Florence on Sunday had winds moving at 75 mph, and was 750 miles southeast of Bermuda, moving west at 6 mph.
Tropical Storm Florence is quickly approaching the eastern United States, and according to the National Hurricane Center, the storm's threat to the East Coast keeps rising.
The third hurricane in the Atlantic, Hurricane Florence, could pose a danger to parts of the U.S.
Tropical Storm Florence turned into a hurricane Sunday morning and swirled toward the US for what forecasters said could be a direct hit on the Southeast toward the end of the week.
Tropical Storm Helene is just off the African coast and tropical storm warnings have been issued for the southern Cabo Verde Islands.
"A big question on this track is how intense the storm remains over the Research Triangle area of North Carolina - Raleigh-Durham-Chapel Hill", Watson said. The storm is traveling over warm water; it is expected to increase its speed, and become a hurricane by Saturday night.
McMaster said he had asked President Donald Trump to declare a federal emergency in SC in anticipation of the storm's arrival.
The storm's maximum sustained winds remain near 85 km/h with higher gusts and it is moving west near 15 km/h.
'There is an increasing risk of two life-threatening impacts from Florence: storm surge at the coast and freshwater flooding from a prolonged heavy rainfall event inland, ' the National Hurricane Center said at 11am. The state Emergency Management Division said residents should begin preparing their homes and property for the storm.
Navy ships off Virginia's coast were preparing to sail out of the path, a North Carolina university has already canceled classes and people have begun stocking up on plywood, bottled water and other supplies.
Regardless of exact forecast tracks, significant impacts for parts of the Carolinas and Virginia are likely.
Monday is the climatological peak date of hurricane season, the height of the eight-week period when the most powerful storms usually form, CNN meteorologist Pedram Javaheri said. NASA's Aqua satellite caught Hurricane Florence developing an eye Sunday as the storm ramped up its intensity.
The low pressure area that has formed into Depression Nine has moved very little during the past several hours.