"Forecasts generally project the storm to make landfall between northern SC and North Carolina's Outer Banks as a strong Category 3 on Thursday, although shifts in the track are possible and storm impacts will expand great distances beyond where landfall occurs".
"Bad things can happen when you're talking about a storm this size", Trump said Wednesday morning.
The result could be catastrophic inland flooding that could swamp homes, businesses, fields and industrial sites. North Carolina corrections officials said more than 3,000 people were relocated from adult prisons and juvenile centres in the path of Florence, and more than 300 county prisoners were transferred to state facilities.
At 8 a.m. on Wednesday the storm was located about 855 km southeast of Cape Fear, North Carolina.
The hurricane was seen as a major test for the Federal Emergency Management Agency (Fema), which was heavily criticised as slow and unprepared for Hurricane Maria in Puerto Rico past year.
The eastbound lanes of several major highways have been shut down to allow traffic to flow inland, but the exodus was slow along roads jammed with outward-bound vehicles.
For a swath of the North Carolina shore from Cape Fear to Cape Lookout, the storm surge could range from 9-13 feet, the NHC said.
With South Carolina's beach towns more in the bull's-eye because of the shifting forecast, OH tourists Chris and Nicole Roland put off their departure from North Myrtle Beach to get the maximum amount of time on the sand.
"I do worry though about people here because many didn't prepare, you know, because originally they didn't think it was going to be so bad here", Sparks said.More news: Confusion over Donald Trump’s autumn visit to Ireland
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Homeless after losing her job three months ago, 25-year-old Brittany Jones went to a storm shelter at a high school near Raleigh.
"He was thinking of coming here until this morning, and now he asked me if I wanted to come up there", he said.
"I rode out Hugo", Miller said of a 1989 hurricane that caused widespread damage. Home Depot and Lowe's activated emergency response centres to get generators, rubbish bags and bottled water to stores before and after the storm.
"Hugo was a direct hit", he said. "We have everything. We're ready".
Lt. Col. Lindy White, an Oklahoma National Guard spokeswoman, said officials from East Coast states are expected to request additional assistance in the days to come.
"We've seen nor'easters and we've seen hurricanes before", Mr Cooper said, "but this one is different".
A hurricane warning - meaning hurricane conditions are expected within 36 hours - is in effect for a long stretch of the coast, from the Santee River in SC to Duck, N.C., which is part of the Outer Banks. All three states ordered mass evacuations along the coast.
Axios: "The ties between Hurricane Florence and climate change" - "Hurricane Florence is a unique Atlantic hurricane, projected to stall out after hitting land and forecast to dump upwards of 2 feet of rain on several states, much like Hurricane Harvey did in Texas past year".
"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. Tropical Storm Isaac is heading westward toward the central Lesser Antilles, but is a bit weaker, with Guadeloupe, Martinique and Dominica now under tropical storm, not hurricane, warnings.