Brock Long, the administrator of the Federal Emergency Management Agency, underscored the danger of the storm surge in an interview with "Good Morning America".
"Hurricane Florence is looking even bigger than anticipated". Will the winds be that strong or they'll be stronger when they get here than that?
The hurricane center is forecasting the storm to hover near the coast Saturday with winds of around 80 miles per hour (130 kph) before landfall, but with rainfall in the 20 to 30 inches (50 to 75 centimeter) range and up to 13 feet (nearly 4 meters) of storm surge.
Florence's intensity was forecast to fluctuate into Thursday and then slowly decrease. But authorities warn Florence has an enormous wind field as it zeroes in on the Southeast U.S. coast, raising the risk of the ocean surging on to land and making Florence extremely unsafe.
- Hurricane Florence continues to gain strength while steadily making its way toward the East Coast.
With the change in the forecast, Georgia Gov.
It's unclear exactly how many people fled, but more than 1.7 million people in the Carolinas and Virginia were warned to clear out.
Georgia Governor Nathan Deal issued an emergency declaration for all 159 counties in his state. In Myrtle Beach, S.C., and nearby areas, the surge could hit 6-9 feet. The Navy, Air Force and Army were moving ships and aircraft out of harm's way.
Florence's projected path includes half a dozen nuclear power plants, pits holding coal-ash and other industrial waste, and numerous hog farms that store animal waste in huge lagoons.
In addition to inundating the coast with wind-driven storm surges of seawater as high as 4 metres along the Carolina coast, Florence could dump 51-76 cm of rain, with up to 102 cm in parts of North Carolina, the NHC said.
"We are uniquely positioned to establish a quick ad-hoc supply chain to support relief efforts", said Bettina Stix, an Amazon veteran credited with developing DRbA.
Baltimore Sun reporter Sarah Meehan and The Associated Press contributed to this article.