Forecasters at the NHC said Florence would continue to strengthen and "rapid intensification is likely to begin tonight".
"Isaac is expected to become a hurricane later today or tonight", the NHC said, noting however that weakening is anticipated to begin by the middle of the week.
The hurricane center warned that "swells generated by Florence are affecting Bermuda and will reach portions of the U.S. East Coast over the weekend".
Authorities in the Carolinas are warning residents to prepare for a potential disaster as Hurricane Florence gains strength ahead a potential landfall later in the week.
At 11 a.m. Hurricane Florence was located about 750 miles southeast or Bermuda and about 610 miles northeast of the Northern Leeward Islands.
"I encourage Virginians to monitor forecasts and make their own preparations now", Northam said. North Carolina Highway Patrol Sgt. Chris Knox told NPR that North Carolina residents needed to prepare: "When we put those orders out, when we tell people 'this is a flood-prone area, ' that you need to find somewhere else to go, don't roll the dice".
Gov. Roy Cooper announced the declaration in a news release Friday evening as the storm neared Bermuda.
Florence had been the first Category 4 hurricane of the Atlantic season, but weakened as it passed through cooler waters.
Obama Slams Donald Trump Publicly For First Time
Obama's remarks will be on "the state of our democracy", his communications director Katie Hill said in a statement. Aides close to Obama have said he's bit his lip, wanting to avoid an ugly tit-for-tat and to stay above the fray.
At its present course and speed, the center of Florence will likely approach the SC or Georgia coastline on Thursday or Friday.
"All indications are that Florence will be an extremely risky Category 4 hurricane while it moves over the western Atlantic toward the southeastern United States", the Miami-based hurricane centre said.
Even if Florence remains entirely at sea, the storm is likely to generate heavy seas and hazardous rip currents along the US this weekend, the hurricane center warned.
SC coastal areas can expect tropical storm force winds by Thursday morning on the storm's latest track, according to John Quagliariello from the National Weather Service in Columbia.
Isaac is moving toward the west near 9 miles per hour (15 km/h).
The latest forecast shows Florence nearing the U.S. East Coast as a Category 4 hurricane with 140 miles per hour winds on Thursday.
Another storm, Helene, is tracking off the Cabo Verde Islands.
Isaac will be near the Lesser Antilles later this week and could bring impacts to parts of the Caribbean Islands during this time frame.