Officials said some 1.7 million people in the Carolinas and Virginia were warned to evacuate, but it's unclear how many did.
Weather Underground meteorology director Jeff Masters said Florence eventually could strike as merely a Category 1 hurricane with winds less than 100 miles per hour (160 kph), but that's still enough to cause at least $1 billion in damage.
Jamie Thompson walks through flooded sections of East Front Street near Union Point Park in New Bern, N.C. Forecasters said conditions will deteriorate throughout Friday. As the storm moves inland, Georgia, Virginia and Maryland will also be in peril.
What also makes Florence extremely risky are the deadly storm surges, mammoth coastal flooding and historic rainfall expected far inland. "For these airports along the coast, like Charleston and Myrtle Beach, the wind and the tidal surge will be the biggest issues". "Surge-related flooding can vary greatly over short distances", the advisory said.
He added that people living near rivers, streams and lowland areas in the region were most at risk.
"It truly is really about the whole size of this storm", National Hurricane Center Director Ken Graham said.
Florence's top winds were clocked on Thursday evening at 90 miles per hour (150 km/h) as it churned in the Atlantic Ocean, down from a peak of 140 miles per hour (224 km/h) earlier this week when it was classified a Category 4 storm.
"A slow westward to west-southwestward motion is expected Friday night and Saturday".
The outer bands of scattered rain reached the Triangle early Thursday afternoon, but the center of Hurricane Florence was still 100 miles off shore, southeast of Wilmington, at 5 p.m., according to the National Hurricane Center. Categories only represent the speed of sustained winds, and these are still destructive. It's expected to move slowly over eastern SC on Friday night through Saturday night.
USA carriers were also waiting to see what impact Florence could have on airports near Washington and in northern Virginia. Its surge of ocean water could cover all but a sliver of the Carolina coast under as much as 13 feet, and days of downpours could unload more than 3 feet of rain, touching off severe flooding.
The Hurricane Center's Dennis Feltgen reiterated that warning, saying 90 percent of hurricane-related deaths are from problems that arise with the massive amounts of water.
DEVASTATION: Marine One carrying US President Bill Clinton along with North Carolina Governor James Hunt 20 September, 1999, survey the flood damage over Tarboro, North Carolina, which was damaged by high winds and rain from Hurricane Floyd. The hurricane, which has been downgraded from a category four storm to a category two, is not expected to weaken any further before it hits the coast. Workers are being brought in from the Midwest and Florida to help in the storm's aftermath, it said.
The National Hurricane Center said the storm's strength should continue through Thursday. The number is expected to rise, the flight-tracking service said. Hurricane-force winds extended 130 kilometres from its centre, and tropical-storm-force winds up to 315 kilometres.
The storm is also predicted to bring historic rainfall of up to 35 inches to the Wilmington region.