These could be as high as 13ft (4m) along parts of the North Carolina coast. "We're still going to have a Category 4 storm surge".
As of 8:00 am (1200 GMT), the eye of the storm was 170 miles (275 km) east-southeast of Wilmington, North Carolina, moving northwest at 12 mph.
The storm's winds may have weakened in recent days - they are now at 110mph (175km/h) - but there are fears Florence's slow-moving nature could bring different problems.
The storm was heading for the coast of the two states but heavy rain was also expected in Virginia to the north and Georgia to the south. This storm is a slow-moving mammoth and will linger for days on the coast, heavily affecting not only North and SC but also Georgia and parts of Virginia before moving further inland, causing devastation to entire states throughout the weekend.
That said, the storm is still expected to stall once it hits the Wilmington coast, with weather officials early Thursday raising the expected total rainfall for Wilmington to 35 inches - weather officials had predicted 30 inches for Wilmington in briefings released Wednesday.
Florence's maximum sustained winds were clocked on Thursday at 165 kph after it was downgraded to a Category 2 on the five-step Saffir-Simpson scale, according to the NHC. President Trump has declared an emergency in the three states and says the government is "as ready as anybody has ever been".
"Don't play games with it. That's the second story of a house", North Carolina Gov. Roy Cooper said Thursday morning. Now, there are signs that Hurricane Florence could deliver a similar blow to the East Coast beginning on Thursday, making it even more important to evacuate before the storm arrives. Home Depot and Lowe's activated emergency response centres to get generators, trash bags and bottled water to stores before and after the storm.
Body surfer Andrew Vanotteren, of Savannah, Georgia, crashes into waves in the surf bolstered by the incoming Hurricane Florence on the south beach of Tybee Island, Georgia.
A power outage model run at the University of MI projects that 3.2 million customers will be without electricity because of the storm, mostly in the eastern half of North Carolina. Workers are being brought in from the Midwest and Florida to help in the storm's aftermath, it said.
Isaac, along with Florence, Helene and two other systems increasingly likely to form, have set a hectic pace for the Atlantic - even busier than past year when Harvey, Isaac and Maria made landfall within three weeks of each other.
The NHC said: "Little change in strength is expected over the next several hours as Isaac moves through the Leeward Islands".
On Instagram, parenting blogger Chrissy Marie, based in North Carolina, posted a photo of her kids' hurricane party, complete with bright pink balloons.
Adding to the storm stress is uncertainty about where exactly Florence will make landfall, after a shift in its track put more of the Southeast in danger.
The forecasters say the slow speed will give Florence time to pound the Carolinas with bands of heavy rain.
Officials in the potential path of Florence urged people to evacuate their coastal homes and directed drivers away from the coast.