Fierce winds and massive waves are expected to lash the coasts of North and SC and Virginia even before Florence makes landfall by early Friday, bringing a storm surge as much as 13 feet (4 meters).
It diminished from hurricane force as it came ashore, but forecasters said the 350-mile-wide storm's slow progress across North and SC could leave much of the region under water in the coming days.
Expected to make landfall by Friday, the impact of the storm will be widespread, with destructive winds, life-threatening storm surge, unsafe surf, torrential rainfall, flooding and the potential for tornadoes.
The News & Observer reports that the storm's path shifted early Wednesday and it is now bearing down on southern North Carolina and northern SC, where it could dump up to 40 inches of rain in places.
Officials said some 1.7 million people in the Carolinas and Virginia were warned to evacuate, but it is unclear how many did.
In all, an estimated 10 million people live in areas expected to be placed under a hurricane or storm advisory, according to the U.S. Weather Prediction Center.
At this height of the Atlantic hurricane season, Florence was being trailed on east-to-west paths by two other storms, Hurricane Helene and Tropical Storm Isaac. All three states ordered mass evacuations along the coast.
Axios: "The ties between Hurricane Florence and climate change" - "Hurricane Florence is a unique Atlantic hurricane, projected to stall out after hitting land and forecast to dump upwards of 2 feet of rain on several states, much like Hurricane Harvey did in Texas previous year".
Boarding up his home in Myrtle Beach, South Carolina, Chris Pennington watched the forecasts and tried to decide when to leave. In South Carolina, close to the Georgia line, Beaufort County emergency chief Neil Baxley told residents they need to prepare again for the worst just in case.
Florence is the most unsafe of three tropical systems in the Atlantic.
Industrial waste, including toxic ash from power plants, could also be spread by flooding from the massive storm, which has caused mandatory evacuations of coastal areas in both Carolinas and Virginia, the AP reports.
The storm surge, or wind-driven seawater, poses a huge danger, FEMA Administrator Brock Long warned on ABC's "Good Morning America".
"This rainfall would produce catastrophic flash flooding and significant river flooding", the hurricane center says.
Some areas are expected to see between 20 and 30 inches of rain through early next week in the area shaded in purple in the second image above.
The NHC said the first tropical storm-force winds of at least 39mph (63 kph) would hit the region early on Thursday with the storm's centre reaching the coast Friday.
Authorities, including President Trump and the Federal Emergency Management Agency (FEMA), are urging residents in the Carolinas as well as parts of Virginia and Georgia to evacuate as the storm continues its path.
"It truly is really about the whole size of this storm", National Hurricane Center director Ken Graham said.