"With time, the wind pushes the water into every nook and cranny you can think of", Graham said. "When weather forecasters tell us 'life-threatening, ' we know that it is serious".
The first effects were already being seen on barrier islands as unsafe rip currents hit beaches and seawater flowed over a state highway.
"Please be prepared, be careful and be SAFE", he tweeted.
US President Donald Trump has signed declarations of emergency for both North Carolina and SC, freeing up federal money and resources for the storm response.
Similar evacuations are happening all the way up to Virginia, where the governor has ordered a mandatory evacuation for residents of some low-lying coastal areas.
On Tuesday, Washington D.C.'s mayor, Muriel Bowser, declared a state of emergency in the District ahead of the hurricane, effective immediately.More news: A steady stream of insane from the White House
President Donald Trump was briefed on the hurricane and said the "federal government stands by, ready to assist 24/7".
A hurricane watch has been issued for the East Coast from Edisto Beach, S.C., to the North Carolina-Virginia border, including the Pamlico and Albemarle Sounds.
But the landfall intensity of that storm was 250 km/h when it hit Puerto Rico, it added.
Some forecasts call for Florence to stall out after it strikes the coast. Ryan Moser, a spokesman for the North Carolina-based electricity company Duke Energy, said operators would begin shutting down nuclear plants at least two hours before hurricane-force winds arrive. "This is the craziest one".
The meteorologist added that Hurricane Florence will continue to fluctuate in size, but it is on track to be "on the upper end of size of hurricanes we've seen in the recent past", which has included devastating storms such as Matthew in 2016, and Harvey, Irma and Maria previous year.
"These folks are putting themselves in harm's way to help fellow Americans, and we thank them so much for that", said NYC Emergency Management Commissioner Joseph Esposito.
He said the agency has implemented changes since 2017, improving their partnerships with the private sector, in particular fuel, transportation and electrical companies, to better meet needs on the ground after a disaster.