Fierce winds and massive waves are expected to lash the coasts of North and SC and Virginia even before Florence makes landfall by early Friday, bringing a storm surge as much as 4m, the National Hurricane Center in Miami warned.
The state of North Carolina is preparing for a major storm.
Florence is about 1,000km southeast of Cape Fear, North Carolina, and is packing maximum sustained winds 220km/h, the NHC said.
Currently, Category 4 storm Hurricane Florence is barreling its way across the Atlantic Ocean toward the Carolinas and Virginia, prompting large-scale evacuations and threatening to cause enormous amounts of damage wherever it makes landfall.
The president also said the states affected could get "more water than we have ever seen in a storm or a hurricane".
"Follow the advice of local authorities in North Carolina, South Carolina, Virginia, Maryland the District of Columbia and Georgia including any evacuation orders, and monitor news and social media for more information".
"The closer to the centre, the closer to feeling Category 4, which is catastrophic", Wagstaffe said.
Florence is now around 800 miles from Cape Fear, North Carolina, moving at 16mph with winds of 130mph.
While one million people are evacuating three U.S. states ahead of Hurricane Florence making landfall, one group of scientists has flown right into harm's way.
Florence could hit the Carolinas harder than any hurricane since Hazel packed 130mph (209kph) winds in 1954 and killed 19 people in North Carolina.
Liz Browning Fox was planning to ride out the storm on the Outer Banks, defying evacuation orders.
Both of the Carolinas and Virginia ordered mandatory evacuations for some of their residents starting at noon Tuesday and over a million people are expected to flee. In the six decades since then, many thousands of people have moved to the coast. Storm surge may be more than 10 feet in these areas, according to ADCIRC, a computer model developed over 30 years by the University of Notre Dame and University of Texas.
Walmart stores in Charleston, S.C. shut down indefinitely at midnight last night.
At 2:00 pm (1800 GMT), Florence was a Category 4 hurricane packing winds of 130 miles per hour (215 kilometers per hour), the Miami-based National Hurricane Center (NHC) said.
North Carolina is also evacuating prisoners and staff from low- and medium-security prisons that lie in the path of Hurricane Florence over concerns they won't be able to withstand the storm.
Meanwhile, there are two other storms in the vicinity - Isaac and Olivia have become tropical storms, with the former heading for the Caribbean and the latter set to hit Hawaii early on Wednesday.
"They told me to bring a pillow and blanket", Whisler said. The watch "means there is a possibility of life-threatening inundation, from rising water moving inland from the coastline, in the indicated locations during the next 48 hours", the Hurricane Center said.