Evacuations have begun for people who live in the path of the hurricane which is expected to last a few days.
The centre urges people within the affected areas to take all necessary actions to protect life and property from rising water and the potential for other risky conditions.
The storm's maximum sustained winds as of 11 a.m. are 105 miles per hour (165 km/h), showing that the storm has decreased in strength quite a bit, but it is still a unsafe hurricane.
"If you're going to leave - and you should leave - if you have not left these evacuation zones, you should leave now because time is running out", McMaster said.
"This rainfall will produce catastrophic flash flooding and prolonged significant river flooding", the National Hurricane Center (NHC) warned.
More than 1.5 million people have been ordered to evacuate along 300 miles of coastline.
At a press conference Thursday morning, officials stressed that Hurricane Florence's wind speed may have fallen, but the danger has not.
It diminished from hurricane force as it came ashore, but forecasters said the 350-mile-wide storm's slow progress across North and SC could leave much of the region under water in the coming days.
EPL: Cardiff coach, Warnock names Chelsea player he wants to sign January
The gap was 30 points at the beginning of the season.to cover 30 points in only one season is very hard ". So I know that the matches after the break are very, very hard .
At 15:00 GMT (11:00 am), Florence was over the Atlantic Ocean about 230 km (145 miles) east-southeast of Wilmington, North Carolina, and moving northwest at 17 kph (10 mph), the NHC said.
Forecasters at the National Hurricane Center in Miami don't expect it to strengthen from a Category 2 hurricane before it moves ashore, but they say the real problem will be water as Florence lingers along the coast through Saturday.
They say it remains very large and very unsafe, bringing more than 30 inches of rain to the coast and heavy winds that will impact a giant swath of land. "Hurricane-force winds extend outward up to 80 miles (130 km) from the centre and tropical-storm-force winds extend outward up to 195 miles (315 km)".
A hurricane watch is in effect for Edisto Beach, SC, to South Santee River, SC.
With Florence, the surge threats and rain estimates have not changed, Tillis said, and everyone needs to listen to local law enforcement and first responders. A 78-year-old man was electrocuted attempting to connect extension cords while another man died when he was blown down by high winds while checking on his hunting dogs, a county spokesman said.
A state of emergency has been declared in five coastal states: North Carolina, South Carolina, Georgia, Maryland and Virginia and the U.S. capital Washington.
Despite all the warnings from numerous authorities, some residents say are planning to ride out the storm.
Weather Underground meteorology director Jeff Masters said Florence eventually could strike as a Category 1 with winds less than 160km/h, but that's still enough to cause at least $1 billion in damage. It's now predicted to make landfall near Wilmington and then head west across SC.