Some areas could receive as much as 40 inches (one meter) of rain, forecasters said.
Just as Hurricane Florence closes in on the Southeast, the area covered by hurricane-force winds has doubled - meaning far more people will get blasted with winds topping 73 miles per hour.
Federal, state and local officials are concerned about massive amounts of rain and powerful winds the storm will bring.
The worst may come as the gargantuan storm swirls past the shore and dumps days of rain on mountainous interiors, potentially creating disastrous mudslides. "Surviving this storm will be a test of endurance, teamwork, common sense and patience".
Cooper requested additional federal disaster assistance in anticipation of what his office called "historic major damage" across the state.
The next concern for airlines is Tropical Storm Isaac, which threatens to hammer vacation destinations in the Caribbean, as Hurricanes Irma and Maria did previous year.
Cooper said 108 shelters are open across the state, with more than 7,000 people already staying at them, as of Thursday afternoon. Another 400 people were in shelters in Virginia, where forecasts were less dire.
Prisoners were affected, too. Florence is expected to arrive in southeast North Carolina on Friday morning before drifting south into SC.
"It truly is really about the whole size of this storm", National Hurricane Center Director Ken Graham said.
More than a million people along the coastlines of North Carolina, South Carolina and Virginia have been ordered to evacuate.
Janey Camp, a research associate professor of civil and environmental engineering at Vanderbilt University, said the "biggest concern" for those not along the coast "is the deluge of precipitation that comes with" hurricanes.
Around midday, Spanish moss blew sideways in the trees as the winds increased in Wilmington, and floating docks bounced atop swells at Morehead City.
Roads and intersections on North Carolina's Outer Banks barrier islands were already inundated with water.
Businesses in Wrightsville Beach, N.C., prepare for Hurricane Florence on Wednesday.
More than 1,200 USA airline flights scheduled for Thursday and Friday have been canceled, according to the flight-tracking service FlightAware.
6 a.m.: The eyewall of Hurricane Florence is onshore on North Carolina and landfall is expected soon near Topsail Beach.
Florence's hurricane force winds now extends 80 miles from the center, while the tropical storm force winds reaches almost 200 miles, according to the National Hurricane Center.
A buoy off the North Carolina coast recorded waves almost nine metres high as Florence churned toward shore.
Forecasters said conditions will only get more lethal as the storm pushes ashore early Friday near the North Carolina-South Carolina line and makes its way slowly inland.
And the storm is still expected to bring catastrophic winds, rain, storm surges and flooding, the National Hurricane Center briefing said.
The winds in Charleston, South Carolina, will start to pick up tonight and will remain fairly steady throughout the storm.
And don't be fooled by the fact that Florence has weakened slightly to a strong Category 2 hurricane.