Its storm surge and the prospect of 1 to 3½ feet of rain were considered a bigger threat than its winds, which had dropped off from an alarming 140 miles per hour - Category 4 - earlier in the week.
Rescue workers in North Carolina meanwhile were scrambling to save people stranded in their homes.
"Life-threatening, catastrophic flash flooding and prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians through early next week, as Florence is expected to slow down while it moves inland", the NHC discussion for Florence reads.
A tornado watch was also in effect for parts of North Carolina.
Evacuation warnings are in place for 1.7 million people.
These types of slow-moving storms - like Hurricane Harvey - can be particularly unsafe because of the rain and flooding they can bring.
In the town of Hampstead, emergency responders going to a call for cardiac arrest Friday morning found their path blocked by downed trees.
But forecasters said its extreme size meant it could batter the US East Coast with hurricane-force winds for almost a full day.
North Carolina Governor Roy Cooper told a news conference that whole communities "could be wiped away".
Forecasters say Florence is now a tropical storm but will continue to threaten North and SC with powerful winds and catastrophic freshwater flooding. The record was set by Hurricane Helene in 1958 with 135 miles per hour.
The state's transportation secretary, James Trogdon, said the state may see "flood events" that normally only occur once every 1,000 years.
The Carolinas are no stranger to hurricanes or tropical storms.
More than 12,000 people were in shelters in North Carolina and 400 in Virginia, where the forecast was less dire.
Duke said it had more than 20,000 personnel ready to start fixing outages as soon as conditions allowed, including over 8,000 from Duke's Carolinas utilities, 1,700 from the Midwest, 1,200 from Florida and 9,400 from other utilities.
Officials have warned people against entering attics, so as to avoid drowning, unless they have a means to cut through to the roof.
The port city of Wilmington woke Friday to the sound of exploding electrical transformers with strong gusts throwing street signs and other debris as well as water in all directions, according to an AFP reporter at the scene.
"We're looking at the same amount of rainfall in three days".
Meteorologist Ryan Maue of weathermodels.com said Florence could dump a staggering 18 trillion gallons of rain over a week on North Carolina, South Carolinas, Virginia, Georgia, Tennessee, Kentucky and Maryland.
Florence was one of two major storms on Friday. McMaster's remarks are set to begin at 2:30 p.m.
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