Forecasters said Florence's surge could cover all but a sliver of the Carolina coast under as much as 3.4 metres of ocean water, and days of downpours could unload more than 0.9 metres of rain, touching off severe flooding.
Strom surge that was projected between nine and twelve feet in Pamlico Sound and the Pamlico and Neuse rivers was expected to cause a "tremendous amount of inland flooding", said Neil Jacobs, assistant secretary at the Commerce Department over the National Oceanic and Atmospheric Administration.
Rescue workers in North Carolina meanwhile were scrambling to save people stranded in their homes.
About 1.7 million people in North Carolina, South Carolina and Virginia have been put under voluntary or mandatory evacuation orders to avoid what emergency officials called a "once in a lifetime" storm.
The downtown area of the city of 30,000 people was underwater and around 150 people were waiting to be rescued, city authorities said on Twitter.
In April, The State newspaper in Columbia reported that an Army investigation had faulted Fort Jackson officials' handling of a 2015 storm that caused an 80-year-old earthen dam to crumble and release 100 million gallons of water.
Florence made landfall as a Category 1 hurricane at 7:15 am at Wrightsville Beach, a few miles east of Wilmington and not far from the SC line, coming ashore along a mostly boarded-up, emptied-out stretch of coastline.
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Meteorologist Ryan Maue of weathermodels.com said Florence could dump a staggering 68 trillion litres of rain over a week on North Carolina, South Carolinas, Virginia, Georgia, Tennessee, Kentucky and Maryland.
Gov. Roy Cooper said Friday that Florence is "wreaking havoc" and he's concerned "whole communities" could be wiped away.
Nearly 20,000 people had taken refuge in 157 emergency shelters, Cooper said.
Myrtle Beach, a SC beach resort, was deserted with empty streets, boarded up storefronts and very little traffic.
According to poweroutage.us, more than 500,000 customers in North Carolina and 50,000 in SC were without power on Friday morning, though the Weather Channel estimates up to 3 million customers could lose power in the Carolinas due to Florence.
Volunteers from the Civilian Crisis Response Team help rescue three children from their flooded home September 14, 2018 in James City, United States.
The ferocious storm is now battering North Carolina in the US.
"The shelters are not taking dogs", Ramirez said. "I charged the batteries of my electronic devices, I have beers and video games".