Enough rain could fall to break North Carolina's record for a tropical storm - 24 inches - set near Wilmington during Hurricane Floyd in 1999, said Greg Carbin, chief of forecast operations at the Weather Service's national prediction center.
The NHC said the first tropical storm-force winds of at least 39 miles per hour (63 kph) would hit the region early on Thursday with the storm's centre reaching the coast Friday.
Hurricanes are massive storm systems that form over warm ocean waters and move toward land.
The hurricane is expected to weaken into a tropical depression as it continues westward into SC and Georgia.
The videos reiterate how powerful and unsafe these storms can be. They warned that the storm could unleash catastrophic flooding from southern Virginia to southern Georgia and stretch hundreds of miles inland. But the storm, almost 300 miles wide, is expected to pick up strength over the warm waters of the Atlantic before making landfall.
The new trajectory means the storm will idle at sea for longer, creating even heavier and prolonged rains and storm surges for the Carolinas and possibly northern parts of Georgia.
"I'm not approaching Florence from fear or panic", he said.
A satellite image shows Hurricane Florence looming in the Atlantic Ocean as the sun rises over the U.S. Florence is expected to bring tropical storm conditions to North Carolina and SC on Thursday and hurricane conditions on Friday.
Hurricane warnings continue in parts of eastern North Carolina and northern coastal SC.
Duke Energy Corp expected between 25 percent and 75 percent of its 4 million customers would lose power in the Carolinas.
Its unclear exactly how many people fled, but more than 1.7 million people in the Carolinas and Virginia were warned to clear out.
"We could maybe get some outer bands or very, very outer bands of the hurricane", said Jeremy Geiger, a weather service meteorologist.
"The state is mobilizing all available resources to ensure public safety", Deal said.
The first tropical storm-force winds of at least 63 kph were forecast to hit the coast on Thursday.
Florence is bearing down with such strength that forecasters say she may literally reshape the Carolina coastline.
Officials in the potential path of Florence urged people to evacuate their coastal homes and directed drivers away from the coast.
Saturday's scheduled game between SC and Marshall has been canceled.
Scientists have filmed an astonishing video of the interior of Hurricane Florence, as the storm bears down on North and SC.