Florence is moving toward the west near 6 miles per hour, and this general motion is expected to continue today.
Governors of North and SC and Virginia declared states of emergency far ahead of the approaching storm. Rapid intensification is expected through Monday, with Florence approached Category 4 status. The storm's maximum sustained winds are 65 miles per hour (100 kph) and it is moving west at 14 miles per hour (22 kph). The storm was centered about 810 miles (1305 kilometers) southeast of Bermuda and moving west at 5 mph (7 kph).
While the Centre noted it is still too early to predict the "exact timing, location and magnitude of those impact", it also urged officials from northern Florida and North Carolina to monitor the hurricane's progress and have emergency-response plans in place.
"Now is the time for your family also to prepare and stay tuned for more updates", South Carolina Gov. Henry McMaster said.
Hurricane Florence is aiming for a region of the U.S. East Coast that is especially vulnerable to storm surge and flooding from heavy rains.
Tropical Storm Helene formed late Friday night off the coast of Africa, one of three storms - two named - now in the Atlantic.More news: EPL: Gareth Southgate warns Mourinho over Rashford
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Other major hurricanes that have made landfall on North Carolina have come up from the south toward the north and made landfall that way. Some inland flooding is anticipated, but due to the recent dry spell, a repeat of 2016's Hurricane Matthew flooding is unlikely, unless the storm stalls over the coast and continues to feed off the warm ocean waters. Additional strengthening is expected during the next few days and Helene is forecast to become a hurricane on Sunday. The state's emergency management agency said it is "preparing for the possibility of a large-scale disaster".
Foremost among those is Tropical Storm Florence, which is expected to re-strengthen into a hurricane Sunday and looks increasingly likely to hit the East Coast.
Forecasters at the NHC said Florence would continue to strengthen and "is expected to remain an extremely risky major hurricane through Thursday".
In the Pacific, Hurricane Olivia triggered warnings for multiple Hawaiian islands as it blew west toward an arrival over the state as soon as late Tuesday or early Wednesday.
A third storm, Hurricane Helene, was spinning in the Atlantic off West Africa's Cabo Verde Islands with 75-mph (121-kph) winds on Sunday, but did not appear to pose an immediate threat to land.
A westward motion is forecast to continue through the end of the week, with Isaac expected to move across the Lesser Antilles and into the eastern Caribbean Sea Wednesday night or Thursday. No tropical storm or hurricane has ever gotten close to the United States when it was anywhere near Florence's position.