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.
In the 11 p.m. ET advisory, Florence was 280 miles southeast of Wilmington, NC, packing maximum-sustained winds of 110 mph and moving to the northwest at 17 mph.
Tropical storm winds are between 39 and 73 miles per hour, while hurricanes are 74 miles per hour or higher.
"It looks like the hurricane should approach the Wilmington, N.C., area and then potentially stall before shifting south and perhaps paralleling along the coastline towards the Savannah area", Speciale said.
This storm is heading toward North and SC late Thursday into Friday. It had been projected to shift north after making landfall. After Florence's rain starts to fall this week, he said, the rainfall could continue through Monday.
North Carolina Department of Transportation officials said maintenance crews are prepping their vehicles and will be holding swift water rescue training Wednesday.
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 week. There are 5.25 million people under hurricane warnings and watches and another 4.89 million under tropical storm watches and warnings.
Right now the storm does not have a lot of speed behind it, so the rainfall will likely be extensive along the coastal areas, with historic flooding predicted there.
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The storm, which fell from category four to category three on the five-step Saffir-Simpson scale, is expected to strengthen despite it weakening on Wednesday.
And it led to mixed signals from officials in SC, whose governor had canceled mandatory evacuation for several coastal counties.
Florence could strengthen some over open water and then weaken as it nears land, but the difference won't make it any less unsafe, forecaster Stacy Stewart wrote in a National Hurricane Center discussion.
Meteorologists are warning of potentially catastrophic flooding in the area, with isolated amounts of up to 40 inches of rain expected in parts of SC.
"It doesn't look like it's going to weaken at this point", Flaherty said. "We are ready for the big one that is coming!"
The impact of storm surge on the coast will depend on whether the storm's arrival coincides with high tide.
President Trump says the Federal Emergency Management Agency and first responders are poised to help - but he also warned people to get out of the storm's path.
"But despite that, bad things can happen when you are talking about a storm this size", he added.