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 slog its rainy way westward with a potential for catastrophic inland flooding that could swamp homes, businesses and farm fields.
At 2300 EDT (0300 GMT), the NHC said Florence had maximum sustained winds of 40 miles per hour (65 km), and was slowly drifting westward over SC.
Now, while the US East Coast gets prepared for the Category 3 storm (earlier downgraded from four), Hurricane Florence satellite images revealed that two more storms are swirling above the Atlantic Ocean, forming a rare phenomenon.
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Numerous roads were closed, and authorities warned of the risk of landslides, tornadoes and flash floods, with dams and bridges in peril as rivers and creeks swelled.
Tropical Storm Florence is becoming a "better organised" weather system that is likely to strengthen into a hurricane late Saturday, weather forecasters said.More news: North Carolina coast feels Florence's first blast of wind, rain
About 5.25 million people live in areas under hurricane warnings or watches, and 4.9 million live in places covered by tropical storm warnings or watches, the National Weather Service said.
Florence has already set a North Carolina record for rainfall totals, exceeding that of Hurricane Floyd, which struck in 1999 and caused 56 deaths.
Port of Wilmington, North Carolina, extended operating hours for trains and trucks to clear cargo containers from the complex.
"We are. coordinating closely with state and local officials to help ensure the safety of our employees and our property prior to the storm's landfall", CSX spokesman Christopher Smith said.
Residents in the path of these devastating storms should comply with all evacuation orders and other emergency instructions.
Masters said there's a tug-of-war between two clear skies high pressure systems - one off the coast and one over MI - and the more the Great Lakes one wins, the more southerly Florence will be.More news: Two Russians accused by United Kingdom in spy poisoning to give interview
Jeff Byard, who is the FEMA associate administrator for the Office of Response and Recovery, said that Florence is "a very unsafe storm", Fox News reported.
With South Carolina's beach towns more in the bull's-eye because of the shifting forecast, OH vacationers Chris and Nicole Roland put off their departure from North Myrtle Beach to get the maximum amount of time on the sand.
Forecasters expected some strengthening today, as Florence marches west-north-west at around 13kmh.
People fleeing coastal North and SC clogged highways Wednesday as Florence bore down for a direct hit in the low-lying region dense with beachfront vacation homes.More news: Weather Channel Reporter Mocked for Appearing to Exaggerate Florence