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.
"Rapid strengthening is forecast, and Florence is forecast to become a major hurricane this morning, and is expected to remain an extremely unsafe major hurricane through Thursday", it said.
At around 12 p.m. EDT (1600 GMT), the National Hurricane Center (NHC) announced that Hurricane Florence had strengthened to a Category 4 storm. Forecasters said it now has maximum sustained winds of about 130 miles per hour and even stronger gusts.
"Equally, if not more disastrous potential exists with the slow movement that we expect to see Florence have after reaching the coast", Knabb says.
Feasting off the energy from unusually strong sea surface temperatures, Hurricane Florence is intensifying quickly and tracking inexorably toward landfall along the U.S. Southeast or Mid-Atlantic coast on Thursday.
South Carolina Governor Henry McMaster said Sunday he was asking President Donald Trump for a federal emergency declaration in anticipation of "significant impacts not only on our state but the entire east coast".
"We do not want to risk one SC life in this hurricane", Governor Henry McMaster said at a news conference.
Since reliable record-keeping began more than 150 years ago, North Carolina has only been hit by one Category 4 hurricane: Hazel, with 130 miles per hour winds in 1954.
Meanwhile, the two other tropical storms in the Atlantic - Florence and Helen - are expected to become hurricanes today.
A Category 3 or higher is considered a major hurricane and has winds between 111 and 129 miles per hour.
The hurricane was gaining strength as it traveled over warm Atlantic waters, about 1,240 miles (2,000 km) east-southwest of Cape Fear, North Carolina, the National Hurricane Center said in an advisory.
Follow Mike Masco on social media for the latest updates on Hurricane Florence.
"That scenario has a high probability of occurring in North Carolina and Virginia and possibly portions of neighboring states in the Southeast, Appalachians and mid-Atlantic late this week and this coming weekend", Sosnowski added.
Experts have forecast the upgraded tropical storm will gather force - going from a category 1 hurricane to a category 3 on Monday morning.
A tropical storm warning is in effect for Maui County and Hawaii County and a tropical storm watch is in effect for Oahu. As a major storm, the hurricane is expected to produce life-threatening conditions.
AccuWeather meteorologist Brett Rossi said the ground in North and SC and Virginia is already saturated from recent rains.