The US-based National Hurricane Centre said: Florence is forecast to rapidly strengthen to a major hurricane by Monday, and is expected to remain an extremely unsafe major hurricane through Thursday. The storm is expected to slow down and possibly stall near or over the coast on Friday.
North and SC and Virginia ordered mass evacuations along the coast.
Immediately to the north of Virginia, Maryland Governor Larry Hogan said his state was also was "preparing for the potential of historic, catastrophic and life-threatening flooding".
Hurricane Isaac - which late Sunday became the fifth hurricane of the season - is heading west towards the Caribbean.
It added: "The largest threat to life from hurricanes is not the high winds".More news: Florence Now A Major Category 4 Hurricane
As of Monday night, there are three hurricanes in the Atlantic Ocean. Helene will slowly weaken and could impact the Azores as a tropical storm late this weekend. While it's expected to remain a tropical storm while it moves across the islands, National Hurricane Center forecasters caution it may pack more of a punch than the recent Hurricane Lane for some areas.
A Category 4 hurricane at its strongest, Olivia spent about a week swishing around in the Pacific Ocean without making landfall before it weakened into a tropical storm. The video also includes still images from NASA astronaut Ricky Arnold, who photographed the storm from the ISS. Its center will move between Bermuda and the Bahamas on Tuesday and Wednesday and approach the coast of SC or North Carolina on Thursday, the National Hurricane Center said.
Hurricane Helene is moving west-northwest over the eastern Atlantic and is strengthening quickly over the tropical Atlantic.
The states of North and SC and Virginia - all potentially in Florence's crosshairs - have already issued emergency declarations to help speed preparations.More news: Aaron Rodgers Leaves Game Injured, Then Returns To Lead Stunning Comeback
Hurricane Helene is in the far eastern Atlantic Ocean and not expected to be near any land.
A tropical storm watch means that tropical storm conditions are possible within the watch area. generally within 48 hours. It took Matheson and her husband almost the whole day Monday to drive the 60 miles (100 kilometers) off the barrier island.
You can see why there's so much fear spreading now that Florence is being compared to the past hurricane.
"I'm telling them to go inland, but I'm anxious about the rain and tornadoes too", Roberts said. She said it was built in 2009 in Buxton, North Carolina, up on a ridge, and made to withstand a hurricane.More news: Trump Ready to Tax an Additional $267B in Chinese Imports