At 8 a.m. on Wednesday the storm was located about 855 km southeast of Cape Fear, North Carolina.
Hurricane conditions are likely to hit the area around North Carolina's southern coast on Thursday night and Friday, but tropical storm conditions will arrive on earlier Thursday, according to the hurricane center. "Please heed the warnings", Mr Brock said: "Your time is running out".
Since Tuesday, forecasts have shifted the storm track towards the south and southwest after it reaches the coast, which could increase the storm's severity in coastal SC through Myrtle Beach and Charleston and even into parts of Georgia.
The U.S. Coast Guard closed ports in Wilmington and Morehead City in North Carolina and Hampton Roads, Virginia, to inbound vessels greater than 500 tons and was requiring vessels of that size to leave if they did not have permission to be in the ports.
People in areas vulnerable to a risky hurricane have left or are fleeing ahead of the storm's expected Friday or Saturday landfall.
North Carolina governor Roy Cooper urged residents to remain alert despite changing forecasts.
Florence crashed into the Carolina coast on Friday, felling trees, dumping almost three feet of rain on some spots and leading to the death of four people before it was downgraded to a tropical storm still capable of wreaking havoc.
A view of Hurricane Florence from the International Space Station.
Reacting to the possibility of a more southerly track, Georgia Gov. Nathan Deal declared an emergency but did not immediately order any evacuations.
The shift south and west will encompass more of SC and western North Carolina.
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Florence is now heading for ocean water that has surface temperatures of about 85 degrees, meaning it will most likely strengthen on its way to the East Coast.
Amanda Kinseth, reporter for WPDE ABC 15 News in Myrtle Beach, South Carolina.
The NHC said Helene was weakening, however, and posed no danger to land.
The slow movement, combined with the massive amount of moisture this storm holds, will bring unsafe rains - from 20 to 30 inches in coastal North Carolina and 40 inches possible in isolated areas, the weather service says.
Hurricane-force winds were blowing 80 miles from its centre, and tropical storm force winds reached up to 195 miles from the eye.
To back up that point, Graham cited a sobering statistic: "50 percent of the fatalities in these tropical systems is the storm surge - and that's not just along the coast. We have two boats and all our worldly possessions", said Susan Patchkofsky, who refused her family's pleas to evacuate and stayed at Emerald Isle with her husband.
The forecast suggests as much as 40in of rain over seven days along the coast, with the deluge continuing even as the centre of the storm pushes its way over the Appalachian Mountains.
President Trump chimed in Wednesday, warning residents of afflicted areas to "be ready, be prepared!"
NHC Director Ken Graham warned that rivers up to 60km inland may flood.