A man walks a flooded street in Old Town Alexandria, Virginia.
The weather system is now a category four storm carrying sustained winds of about 215km/h.
A tropical storm watch was issued for north of the North Carolina-Virginia border to Cape Charles Light, Virginia, and the Chesapeake Bay south of New Point Comfort. "It's big and it's vicious".
People along the Carolina coasts are evacuating ahead of Hurricane Florence.
"It is an extremely risky, life-threatening, historic hurricane".
As of 5:00 am (1100 GMT), Florence was a risky Category 4 hurricane packing sustained winds of 130 miles per hour (220 kilometers per hour), the Miami-based National Hurricane Center (NHC) said.
Tropical storm force winds are expected to begin 11 a.m. Thursday and hurricane force winds are anticipated by 10 p.m. Thursday. Combined with high tides, the storm surge could swell as high as 12 feet.
President Donald Trump declared states of emergency for North and SC, opening the way for federal aid. SC has already ordered the evacuation of a million people, and residents in the Outer Banks and elsewhere in North Carolina being told to go inland, ideally out of the river flood zone.
TRT World's Nicole Johnstone reports from Wilmington, North Carolina.
North and SC and Virginia ordered mass evacuations along the coast.
As Southeastern N.C. settled into Hurricane Florence Watch over the weekend, it also kept an eye out for another arrival - Jim Cantore. "It is an extremely, dangerous, life-threatening, historic hurricane", said North Carolina Gov. Roy Cooper, according to The Associated Press. The silver lining is that the winds won't be as strong as they would be if the storm just barreled toward the coastline.
Rainfall could be up to 20in (50cm) in some areas.
The US Navy is sending 30 ships stationed in Virginia out to sea.
Residents prepared by boarding up their homes and stocking up on food, water and other essentials, stripping many grocery store shelves of merchandise.
"But this is a big change in the forecast from what we had last night, especially for people from Savannah to Beaufort to Charleston, Hilton Head Island and all the way to Georgetown and Myrtle Beach - much worse for you and still just as bad for our friends from Jacksonville, North Caroline down to Wilmington".
The difference between the two groups, prior research has found, is often a difficult-to-forecast process in which the hurricane rapidly strengthens, usually in the presence of highly favourable environmental conditions, such as extra warm seas to considerable depths, lots of available humidity in the air and slack winds around the storm.
North Carolina officials evacuated long-term care facilities and hundreds of prisoners in vulnerable areas, and also closed state parks, museums and other attractions.