Forecasters said rapid intensification is likely to begin by tonight and Florence is expected to become a major hurricane by Monday.
In the Pacific, Hurricane Olivia is now a Category 1 storm with winds of 75 mph, and is approximately 595 miles east-northeast of Hilo, Hawaii, as of 5 p.m. local time Sunday.
Florence was expected to approach the southeastern coast of the United States on Thursday, possibly near the North Carolina-South Carolina border, according to maps of the storm's trajectory predicted by the NHC. Inland flooding from Hurricane Florence could be significant, the NHC said since Florence is expected to become a very powerful storm in the coming days.
And the impact won't just be along the coasts of SC and North Carolina, where the storm is expected to make landfall.
Located 765 miles southeast of Bermuda, Florence is moving west at around six miles per hour.
Hurricane-force winds could buffet the Carolinas by Wednesday night with landfall likely in SC and North Carolina on Thursday, followed by heavy rains that could cause flooding in much of the U.S. Southeast, the NHC said. Still, the storm would produce flooding rains across a large part of the U.S. South, expanding the damage, according to Enki Research's Watson.
"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 NHC said Sunday.
Lining up behind Florence, Isaac was about 1,150 miles (1,855 kilometers) east of the Windward Islands with top winds of 75 mph (120 kph) early Monday, accelerating on a path to cross into the lower Caribbean Thursday as a weak hurricane.
The Atlantic hurricane season officially began June 1, but cooler water and higher wind shear - winds moving at different speeds and directions - early in the season are less than ideal for tropical systems to gain and maintain strength.
Hurricane Isaac was expected to lose strength as it reaches the Caribbean, and Helene, much farther out to sea, may veer northward into the open ocean as the 2018 hurricane season reaches its peak. Those on the coast and inland from SC into the mid-Atlantic region should be on alert and have a hurricane plan in place, according to the agency.
South Carolina Governor Henry McMaster on Saturday declared a state of emergency in anticipation of the storm.
Hurricanes impacting the Carolinas isn't rare, but it has been some time since a major hurricane has made landfall in North Carolina. It's expected to strengthen through Tuesday before beginning to weaken. Hurricane Floyd was a Category 4 storm before it hit Florida, traveled up the coast and struck Cape Fear as a strong Category 2 storm.
As of 5 p.m. EDT, Florence was centred about 720 miles (1,160 kilometres) southeast of Bermuda, moving west at 7 mph (11 kmh).
Florence had maximum sustained winds of 90 miles per hour, making it a Category 1 storm on the five-level Saffir-Simpson hurricane scale. As NPR's Bill Chappell reported, the town of Mountain View on the Big Island recorded 51.53 inches of rain as a result of Lane, the third-highest total ever measured from a US storm.
Florencewill probably create more wind than Hurricane Hugo and more water than Hurricane Matthew, McMaster warned.
That's because Florence will reach areas that have already experienced a lot of rain in recent weeks - and it won't take much for flash floods to start in these regions. If it comes well inland then a vast majority of the state will be impacted.
Florence is moving toward the west-northwest near 15 miles per hour. NOAA NWS National Hurricane Center/Handout via REUTERSA photo taken from the International Space Station by astronaut Ricky Arnold shows Hurricane Florence over the Atlantic Ocean in the early morning hours of September 6, 2018.
Even in the unlikely event that the storm center remains just offshore, it will nearly certainly come close enough to bring risky wind and flooding to coastal areas. Weakening is forecast to begin by the middle of the week as Isaac approaches the Lesser Antilles.