Massive waves of up to 83 feet were measured inside the storm on Wednesday, the National Hurricane Center's Tropical Analysis and Forecast Branch said, citing satellite altimeter data.
"These enormous waves are produced by being trapped along with very strong winds moving in the same direction the storm's motion," the agency said.
Emergency preparations in South Carolina now include more than 2,100 of the state's National Guard soldiers. In North Carolina, Cooper says, "2,800 soldiers will be on active duty, joined by even more on standby."
Satellite images show the storm has maintained a distinct eye and is well organized. Its maximum sustained winds are expected to top 145 mph before losing some steam near the coast. Current forecasts call for Florence to be at least a Category 3 storm when it arrives at the Carolinas.
Just because we have a landfall to your south, that doesn't mean you're out of the woods, because the winds are huge around this system," NHC Director Ken Graham said, pointing out the wide risk of flooding.
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. It goes well inland."
Another 25 percent of deaths are related to rain, he said.