"We may be facing a situation where hurricane-force winds are weaker than anticipated but perhaps impacting a slightly broader region than once thought", he said.
But the storm still carried "very unsafe winds", the Center added.
Florence's winds dropped from a peak of 225km/h to 165km/h overnight, reducing the hurricane from a terrifying Category 4 to a 2.
"Just because the wind speed came down, the intensity of this storm came down to a Cat 2, please do not let your guard down", said Brock Long, the administrator of the Federal Emergency Management Agency (FEMA), speaking earlier Thursday. Though forecasters later downgraded Florence to a tropical storm, the monster system is barely moving over the Carolinas and could dump drenching rains of up to 3½ feet (1 meter).
Despite pleas from officials, some residents rejected calls to evacuate.
"This storm will bring destruction", North Carolina Governor Roy Cooper said. "Catastrophic effects will be felt".
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Tropical Storm Florence trudged inland on Saturday, flooding rivers and towns, toppling trees and cutting power to almost a million homes and businesses as it dumped huge amounts of rain on North and SC, where five people have died.
Winds and rain were arriving later in SC, and a few people were still walking on the sand at Myrtle Beach while North Carolina was getting pounded.
Long-duration, damaging wind gusts of 55 miles per hour or greater and saturated ground may cause a tremendous amount of trees to fall and lead to widespread power outages from the southern tip of Delmarva to Charleston, South Carolina and as far inland as Raleigh and Charlotte, North Carolina, to Columbia, South Carolina.
"Now's the time when we stop projecting and we just sit back and watch", says Young. "We got thrown into mailboxes, houses, trees", said Holt, who had stayed at home because of a doctor's appointment that was later canceled. "We have a safe basement and generator that comes on automatically".
By mid-afternoon the winds had dropped to 75 miles per hour (120 kph) and the centre was moving west at 6 miles per hour (10 kph), the NHC said, and parts of North and SC would get as much as 40 inches of rain (1 metre).
"Gradual weakening is forecast while Florence moves farther inland during the next couple of days, and it is expected to weaken to a tropical depression" by Saturday night, the center said in a bulletin.
"So this is a very unsafe storm", said Mr Long.
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The National Weather Service said as much as 18cm of rain had fallen overnight in some coastal areas. The rainfall will cause prolonged significant river flooding.
As the Carolinas prepare for Hurricane Florence's wrath, Duke Energy is preparing for anywhere between 1 and 3 million people to lose power during the storm.
Washington, DC, Mayor Muriel Bowser and the governors of North Carolina, South Carolina, Virginia, Georgia and Maryland have declared states of emergency.
Duke Energy, the major power supplier for North and SC, told the New York Times on Wednesday that the storm could knock out power for up to 3 million customers across the two states.
Roughly 1 million people are under evacuation orders.
The catastrophe modeling firm said 250,000 homes in North Carolina alone are projected to be affected by the storm.
"Either no (hotel) rooms are available, or we are denied because the breed or size of dogs", she said.
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East Kingston Fire Chief Ed Warren joined his crew on their way to a station in North Andover where they would be assigned. In nearby Methuen, police chief Joseph Solomon said 20 to 25 homes were on fire in Lawrence when he responded to help.