Florence had maximum sustained winds of 128km/h, making it a Category 1 storm on the five-level Saffir-Simpson hurricane scale.
"The bottom line is that there is high confidence that Florence will be a large and extremely risky hurricane, regardless of its exact intensity", the NHC said.
The latest forecast from the National Hurricane Centre said the storm winds have reached 75mph (120kph), which means it is now at hurricane strength.
Gov. Northam declared a State of Emergency for the Commonwealth Saturday, one day after Gov. Roy Cooper declared a State of Emergency for North Carolina. The move will allow the state to use the National Guard for preparations and recovery if necessary.
"There is still some uncertainty about the exact location (of landfall). but the confidence is still increasing that Eastern North Carolina will see some pretty significant impacts", Simms said.
"We have already began mobilizing our assets to respond accordingly, and we are here for you!" he tweeted.
A Life-threatening storm surge is becoming more likely along the coastline of South Carolina, North Carolina, and Virginia, and a Storm Surge Watch will likely be issued for portions of these areas by Tuesday morning.More news: Portions Of Bob Woodward's Book On Trump White House Are Leaked
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The storm "is too powerful and its path is too uncertain to take any chances", South Carolina Governor Henry McMaster said in issuing his state's emergency declaration.
"Based on that information, I have chose to order a mandatory evacuation of Zone A in Hampton Roads and the Eastern Shore", Northam said Monday.
Florence is now a category 4 hurricane.
"These three storms are going to be taking diverting paths, one affecting Puerto Rico, but the one we're watching right now closely is Hurricane Florence, which right now is a Category 2 storm". The United States was hit with a series of high-powered hurricanes past year, including Hurricane Maria, which killed about 3,000 people in Puerto Rico, and Hurricane Harvey, which killed 68 and caused an estimated $1.25 billion in damage from catastrophic flooding in Houston.
A warm ocean is the fuel that powers hurricanes, and Florence will be moving over waters where temperatures are peaking near 85 degrees, hurricane specialist Eric Blake wrote.
Olivia could weaken into a strong tropical storm within 48 hours, according to the Central Pacific Hurricane Center in Honolulu.
On Sunday, Florence reached sustained winds of at least 74 miles per hour to be classified a hurricane.
In South Carolina, where 1 million residents have been ordered to evacuate, distributors were sending out gasoline supplies to prevent stations running dry. Dare County Emergency Management officials said they'll extend the order to all residents and visitors in other areas beginning at 7 a.m. Tuesday. By Monday, the storm should turn west-northwest and accelerate towards the coast.