"Bad things can happen when you're talking about a storm this size", Trump said Wednesday morning.
Forecasters with The Washington Post's Capital Weather Gang are now saying Florence is a Category 4 storm that could bring very unsafe winds, flash flooding and power outages.
The trend is "exceptionally bad news", said University of Miami hurricane researcher Brian McNoldy, since it "smears a landfall out over hundreds of miles of coastline, most notably the storm surge".
Some 38cm (15in) to 64cm (25in) of rain is forecast in some areas - with up to one metre (40in) at the centre of the storm.
Areas along the coast from Cape Fear to Cape Lookout, NC, including the Neuse, Pamlico, Pungo and Bay rivers may experience storm surges from 9 to 13 feet.
Enough rain could fall to break North Carolina's record for a tropical storm - 24 inches - set near Wilmington during Hurricane Floyd in 1999, said Greg Carbin, chief of forecast operations at the Weather Service's national prediction center.
The South Carolina Department of Corrections' Palmer Pre Release Center a Level 1 Minimum Custody Prison in Florence, South Carolina, was evacuated in advance of Hurricane Florence due to security reasons.
The National Weather Service, Federal Emergency Management Agency (FEMA), and both the governors of North and SC have warned that the storm could be extremely unsafe and mandatory evacuations are underway in many parts of the Carolinas.
Homes along the coast in Topsail Beach, North Carolina, are evacuated before Hurricane Florence on September 11, 2018.More news: Hurricane Florence forces grocery store closings
Another FEMA spokesman said: "I need to re-emphasise this is a unsafe storm". Some of these reactors - including Duke Energy's Brunswick and Harris nuclear plants in North Carolina and Dominion Energy's Surry plant in Virginia - may be directly in the hurricane's path, Reuters reported.
"He was thinking of coming here until this morning, and now he asked me if I wanted to come up there", he said.
"Stocks tend to fall in front of a storm because of uncertainty", said Sandler O'Neill analyst Paul Newsome.
"The big picture: There are several characteristics of the changing climate that are helping to increase the risks of damage from Hurricane Florence, even though global warming is not directly causing such a storm to spin up".
"Usually the north and east side [of the storm] is where we see the highest rainfall amounts and the greatest tornado threat".
"Do you want to get hit with a train or do you want to get hit with a cement truck?" he said. "We have everything. We're ready".
He says the time to flee Hurricane Florence is now. If you have any preparations to make, make sure you get them done today.
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. Residents in some parts of the Carolinas have been ordered to evacuate.
President Donald Trump is urging those in the path of Hurricane Florence to act now to "get out of its way". Isaac could bring heavy rainfall to Martinique, Dominica and Guadeloupe.