The National Hurricane Center's best guess was that Florence would blow ashore as early as Friday afternoon around the North Carolina-South Carolina line, then push its rainy way westward with a potential for catastrophic inland flooding.
Georgia's governor Nathan Deal declared a state of emergency for all 159 counties and said the state "is mobilising all available resources to ensure public safety ahead of Hurricane Florence".
The hurricane was about 470 miles (755 km) east-southeast of Myrtle beach SC, with maximum sustained winds of 125 miles per hour (205 km/h), the Miami, Florida-based weather forecaster said. North Carolina Governor Roy Cooper warned that staying put would be a grave mistake and said people in evacuation zones "need to get out now".
Forecasters predict the storm will make landfall late Thursday night or sometime Friday.
"For a meandering storm, the biggest concern - as we saw with Harvey - is the huge amount rainfall", said Chris Landsea, chief of tropical analysis and forecast branch at the National Hurricane Center.
South Carolina, western and northern North Carolina are expected to receive five to 10 inches and in isolated areas up to 20 inches. Parts of the Carolinas could see 50 cm to 76 cm, with isolated areas getting 101 cm, over seven days along the coast. The center said the waves were measured by satellite.
Emergency preparations in SC now include more than 2,100 of the state's National Guard soldiers. But North Carolina's code lagged the rest of the country by one or two cycles and didn't meet even the 2009 standards for anchoring windows and doors.
Abigail Darlington, covers the city of Charleston for the South Carolina Post and Courier.
Duke Energy, the nation's No. 2 power company, said Florence could knock out electricity to three-quarters of its 4 million customers in the Carolinas, and outages could last for weeks.
For anyone needing a place to stay to escape the hurricane, both North Carolina and SC are updating their lists of emergency shelters.
There has been a dramatic change in the projected path of the first hurricane of the season expected to make landfall on the United States.
"Rather than a very narrow and intense band of winds, the winds are slightly weaker - but [they] cover a much larger area", NHC senior hurricane specialist Stacy Stewart said on Wednesday.
NOAA Hurricane Hunters have been flying constantly from Lakeland Linder International Airport, right into Hurricane Florence.
As it nears the coast, the storm's forward motion will slow to a crawl, but the winds and rain will continue full-strength.
"The Charleston area now is expected to see at least 1-2 inches of rain over the next several days".
The National Hurricane Center projects rainfall anywhere from 5 to 20 inches across SC.
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