Florence is expected to go move slowly as it approaches North and SC, whipping hurricane-force winds and dumping relentless rain at least through Saturday.
In Pender County, North Carolina, a woman suffered a heart attack and died because hurricane debris blocking roads prevented paramedics from reaching her.
The expected high winds and slow movement of the hurricane as it comes ashore are likely to make rescue efforts in flooded areas challenging, senior US Defence Department officials say.
Speaking on Thursday, North Carolina's governor, Roy Cooper, said: "We can not underestimate this storm".
It was causing a life-threatening storm surge in parts of eastern North Carolina.
In its 0300 GMT advisory, the National Hurricane Center said the maximum sustained winds of Florence were 150kph, dropping it to the weakest of five categories on the Saffir-Simpson scale. On North Carolina's Outer Banks, water flowed through the streets of Hatteras Village, and some of the few people still left in Nags Head took photos of angry waves topped with white froth. In the Philippines, evacuations were under way with Super Typhoon Mangkhut expected to hit on Saturday in an area impacting an estimated 5.2 million people.
North Carolina Governor Roy Cooper told a news conference, that a "historic storm" would unleash rains and floods that would inundate nearly the entire state.
"There is going to be a lot of rain".
The catastrophe modeling firm said 250,000 homes in North Carolina alone are projected to be affected by the storm. The forecasters say the slow speed will give Florence time to pound the Carolinas with bands of heavy rain.
"This rainfall will produce catastrophic flash flooding and prolonged significant river flooding", NHC said.
The remainder of SC and North Carolina into southwest Virginia could see 6 to 12 inches, with some isolated areas seeing 15 inches.
In addition, hurricane-strength winds are predicted to batter everything in the storm's path. Airlines canceled more than 2,400 flights through Sunday.
Forecasters said conditions will only get more lethal as the storm smashes ashore early Friday near the North Carolina-South Carolina line and crawls slowly inland. A state of emergency has been declared in five coastal states - North Carolina, South Carolina, Georgia, Maryland and Virginia - as well as the U.S. capital Washington. President Donald Trump declared a state of emergency on the federal level Tuesday for the Carolinas and Virginia.
The hurricane was "wreaking havoc" and could wipe out entire communities as it makes its "violent grind across our state for days", the governor said.
He said hurricane-force winds extend outward 80 miles from the center of the storm and tropical storm-force winds extend almost 200 miles out.
A hurricane warning is in effect for South Santee River, SC, north to Duck, NC, and the Albemarle and Pamlico Sounds.
A Hurricane Watch is in effect for.
In a new demonstration of the network's CGI capabilities, hosts have presented from the centre of a digital storm surge to demonstrate the hurricane's damaging potential. The forward speed of Florence has dropped from 17 miles per hour on Wednesday to 10 miles per hour during Thursday afternoon. The storm, they warned for the final time, was not to be played with.
The East Coast isn't the only area facing the brunt of a storm.
Why isn't the downgraded storm less of a threat?Members of the U.S. National Guard marched in and out of a room nearest the main entrance, their camouflage uniforms standing out in the drab room.
Massive power outages were reported, and downtown streets had fallen trees, broken storefront windows and damaged street signs. The National Weather Center upgraded Subtropical Storm Joyce to Tropical Storm Joyce in its 11 p.m. Thursday update. It's forecast to gradually turn toward the northeast on Friday and then accelerate northeastward over the weekend.