The governors of North and SC and Virginia declared states of emergency far ahead of the approaching storm.
On Friday, North Carolina governor Roy Cooper declared a state of emergency and urged residents to use the weekend to prepare for the possibility of a natural disaster.
Hurricanes impacting the Carolinas isn't rare, but it has been some time since a major hurricane has made landfall in North Carolina.
It was expected to strengthen into a hurricane then weaken by the middle of the week.
The storm is 685 miles southeast of Bermuda, moving westward at 7 mph with maximum-sustained winds of 90 mph - and it's strengthening as it heads toward the East Coast.
Swells generated by Florence could cause unsafe rip currents and coastal flooding Saturday in areas including coastal DE and New Jersey, the National Weather Service said.
Still, the storm would produce flooding rains across a large part of the U.S. South, expanding the damage, according to Enki Research's Watson.
Florence rapidly strengthened into a potentially catastrophic Category 4 hurricane Monday as it closed in on North and SC, carrying winds and water that could wreak havoc over a wide stretch of the eastern United States later this week.
Wherever it lands, it won't move much for three or four days, creating a storm surge and wind event, Osterberg said.
Forecasters urged residents from SC to the mid-Atlantic to get ready - and not just for a direct blow against the coast.
This map shows the projected path of Hurricane Florence on Sunday, Sept. 9, 2018.
At 5 p.m. EDT, the storm's center was located about 525 miles (845 kilometers) south-southeast of Bermuda and about 1,170 miles (1880 kilometers) east-southeast of Cape Fear, North Carolina.
Hurricane Florence was raised to Category 3 Monday as it approaches the Carolinas coast.
A mandatory evacuation was ordered for residents of Hatteras Island and some other areas of Dare County in North Carolina. South Carolina Gov. Henry McMaster also suspended his campaign and asked President Donald Trump for a federal emergency declaration.
Florida-based Carnival Cruise Line was re-routing its cruise ships, but there were other hurricanes to contend with.
Florence is now moving west at around 13 miles per hour (20 kph), and is forecast to drench a large swath of the US East Coast running from northern Florida to New Jersey. It is expected to accelerate during the next 36 hours and head into the eastern Caribbean Sea by Thursday.