Category 4 storms, with winds of at least 130 miles per hour, can cause "catastrophic" damage, the hurricane center said, snapping trees, and toppling power polls.
The National Hurricane Center said the storm is about 1,640 miles east of the Windward Islands, which are in the West Indies.
The governors office they will mobilize resources, including the Virginia National Guard and pre-position people to assist in storm response.
Florence's precise path remained uncertain on Saturday, but the NHC said the "risk of direct impact" somewhere between Florida and North Carolina was increasing. Cooper urged residents to "review your emergency plans and gather your supplies now".
Hurricane Florence regained Category 1 strength on September 9 as it continued to track to the U.S. East Coast, according to U.S weather officials.
In its 11 a.m. Sunday update, the NHC designated Florence as a hurricane.
Florence had been affected by wind shear but the conditions in front of Florence are now favorable for development and likely even quicker development. Swells generated by Florence are affecting Bermuda and are beginning to reach portions of the U.S. East Coast.
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A current projection still shows a possible southeast landfall late Thursday or early Friday as a powerful hurricane. The NWS is expecting to issue strong rip current advisories for all area beaches Sunday, and Loewenthal said that is expected to continue through the week.
With an eye on Florence, governors in Virginia, North Carolina and SC have declared states of emergency, warning residents to prepare for a unsafe storm.
South Carolina Gov. Henry McMaster emphasized Saturday that there's no way to know yet when and where the storm will hit land, or when evacuations might be called. OPCON4 means that the counties are on "alert" status and "county officials have begun discussions with South Carolina Emergency Management, coastal communities including local municipalities, and will continue to monitor the situation closely", according to a release from Horry County Director of Public Information Kelly Moore.
The National Hurricane center said Saturday that Florence will start to rapidly intensify Sunday, and be a major hurricane by Tuesday. The further south Florence moves, the more likely it is that we'll benefit from the rain bands that bring intermittent heavy showers and storms to the region.
Forecasters say two low pressure systems off the coast of Africa behind Florence also have high chances of developing into tropical storms.
The storm was slowly moving west toward the East Coast at 6 miles per hour on Sunday, but forecasters expect it to strengthen into a major hurricane by the time it reaches the coast.
Meanwhile, over the Pacific Ocean, Hurricane Olivia is approaching Hawaii with maximum winds of 85 miles per hour.
A third storm, Helene, off the Cabo Verde Islands was expected to become a hurricane later on Sunday.