"When you stall a system like this and it moves real slow, some of that rainfall can extend well away from the center", Mr Graham said, according to the Associated Press.
The NHC said: "Isaac is a small hurricane and uncertainty in the forecast is higher than normal".
One model the weather service uses shows Florence making landfall along the southern Outer Banks, then interacting with a low-pressure trough moving across the Great Lakes that steers the storm back out to sea early next week, Wimberley said. The storm was moving west at 13 miles per hour, and forecast paths show it will likely made landfall near Wilmington on Thursday.More news: Joseph Randle, Former Dallas Cowboy, Arrested For Rape
South Carolina Gov. Henry McMaster (R) on Monday ordered the evacuation of every resident who lives on the state's coastline.
The NHC also was tracking two other hurricanes farther out in the Atlantic. Tropical Storm Isaac is expected to strengthen to a hurricane soon, and will likely impact the Lesser Antilles by the middle of this week.
The governors of North and SC and Virginia have all declared states of emergency ahead of the approaching storm.
A slew of unsafe storms - hurricanes, tropical storms and a typhoon - are on the move and threatening life and property in both the Atlantic and Pacific oceans. The NHC has also issued an advisory for the Eastern Pacific on Tropical Storm Paul, and the Central Pacific Hurricane Center has issued advisories for Hurricane Olivia, which is moving quickly westward toward Hawaii. A high-pressure ridge building to the north and northwest of Florence is expected to steer the hurricane west-north-westward to northwestward toward the southeastern US coast. Hurricane Florence has been upgraded to a major category 4 hurricane as of noon on Monday.More news: Rapper Mac Miller dead at 26
Forecasters said the storm will rapidly intensify beginning Sunday evening and become a major hurricane by Monday a status it is expected to maintain through Thursday, the same day it should approach the southeastern United States coast. The state's emergency management agency said it is "preparing for the possibility of a large-scale disaster".
No coastal watches or warnings were in effect as of Monday morning. Olivia should weaken through Tuesday but remain a strong tropical storm when it reaches the Hawaiian Islands.
In Guam, Typhoon Mangkhut - a Category 4 storm - narrowly missed dealing a direct blow to the island.More news: Here’s What Shahid Kapoor Has Named His Son