Florence crashed into the Carolinas on Friday with 90-mph (144 kph) winds, torrential rains and a powerful storm surge before slowing to a pace that meant it would plague the area with days of flooding.
The storm is a Category 1 with 75 miles per hour winds and higher gusts.
Parts of New Bern, North Carolina, which is home to 30,000 people, are 10ft (3m) underwater.
North Carolina alone is forecast to get 9.6 trillion gallons, enough to cover the Tar Heel state to a depth of about 10 inches (25 centimeters).
Duke Energy Corp, the biggest utility in the area with over 4 million customers, estimated the storm could cause between 1 million and 3 million outages.
The Loudoun County Department of Emergency Management has activated its Emergency Operations Center as Hurricane Florence batters the Atlantic Coast.
The White House said on Friday President Donald Trump had spoken with state and local officials, assuring them the federal government was prepared to help.
"This storm will bring destruction", North Carolina Governor Roy Cooper said. According to the South Carolina Department of Public Safety, a pick-up truck drove into standing water on the road and lost control, overturning into a ditch.
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A crowd protests in Dallas on Friday night. "He loved everybody", said minister Michael Griffin of Singing Hills Church of Christ. In a written statement to The Dallas Morning News , Guyger's sister, Alana Guyger, called her "caring, loving and compassionate".
Tom Balance, owner of a seafood restaurant in New Bern, had decided against evacuating his home and was soon alarmed to see waves coming off the Neuse and the water getting higher and higher.
The state's transportation secretary, James Trogdon, said the state may see "flood events" that normally only occur once every 1,000 years.
Storm surges of up to six feet were forecast for Belhaven, North Carolina, where this video was recorded on the night of September 13.
The storm was expected to move across parts of southeastern North Carolina and eastern SC on Friday and Saturday, then head north over the western Carolinas and central Appalachian Mountains early next week, the NHC said.
The National Weather Service's Advanced Hydrologic Prediction Service is predicting "Major Flooding", its most severe classification, from south of the Pamlico Sound in North Carolina, to as as far south as the Waccamaw River near Myrtle Beach, SC.
Sustained wind speeds were clocked at 50 miles per hour with higher gusts.
He said 20,000 people were being housed in shelters across the state.
"We're looking at the same amount of rainfall in three days".
Winds of up to 110km/h are also expected for several days as the weather system slowly grinds over.
This landfall has been a long time coming: The hurricane arrived more than two weeks after the National Hurricane Center issued its first advisory for the storm.