Storm surge of up to 13 feet is also likely across portions of NC.
Its maximum sustained winds have dropped slightly to 110 miles per hour.
Washington, DC, Mayor Muriel Bowser and the governors of North Carolina, South Carolina, Virginia, Georgia and Maryland have declared states of emergency.
Amanda Kinseth, reporter for WPDE ABC 15 News in Myrtle Beach, South Carolina.
As of 2 p.m. Wednesday, the Category 4 storm was about 435 miles southeast of Wilmington, N.C., and moving northwest at about 16 mph.
People fleeing coastal North and SC clogged highways early Wednesday as Florence bore down on the coast for a direct hit in a low-lying region dense with beachfront vacation homes.
Forecasters are predicting that Florence will make landfall in the Carolinas late Thursday or early Friday as a slightly weakened but still unsafe Category 3 hurricane.
"The time to prepare is nearly over", North Carolina Governor Roy Cooper told a morning news conference.
While many coastal residents heeded mandatory evacuation orders, others boarded up homes and businesses and chose to courageous the storm, which is forecast to trigger severe flash flooding as it dumps as many as three feet (almost a meter) of rain in some areas.
Abigail Darlington, covers the city of Charleston for the South Carolina Post and Courier.
Michelle Stober loaded up valuables at her home on Wrightsville Beach to drive back to her primary residence in Cary, North Carolina.
For anyone needing a place to stay to escape the hurricane, both North Carolina and SC are updating their lists of emergency shelters.
In Wilmington, the fire department is urging people to prepare for at least seven days of hardship, collecting water and nonperishable foods. Florence is now heading for ocean water with surface temperatures of around 85 degrees, meaning it will likely strengthen on its way to the East Coast.
Florence could strengthen some over open water and then weaken as it nears land, but the difference won't make it any less risky, forecaster Stacy Stewart wrote in a National Hurricane Center discussion.
Forecasters predict the storm will make landfall late Thursday night or sometime Friday.
Tropical storm-force winds extend outward up to 175 miles (280 kilometers) from its center.
Some areas are expected to see between 20 and 30 inches of rain through early next week in the area shaded in purple in the second image above.
"This rainfall would produce catastrophic flash flooding and significant river flooding", the hurricane center says.
The shift in the projected track spread concern to areas that once thought they were relatively safe.
South Carolina Governor Henry McMaster ordered the mandatory evacuation of one million coastal residents.
The impact of storm surge on the coast will depend on whether the storm's arrival coincides with high tide.
"We are already experiencing heavily impacted traffic on some of the evacuation routes", said Jeff Byard, the associate administrator for response and recovery at the Federal Emergency Management Agency (FEMA).