The expert's remarks come amid a series of reports of people who refused to evacuate.
A mother and baby were killed after a tree fell on the family's Wilmington, NC house during the storm.
Rescue helicopters and trucks that can navigate floodwaters are also standing by.
The National Hurricane Center warned the threat of tornadoes was increasing as Florence neared shore and South Carolina Governor Henry McMaster said the heavy rains could trigger landslides in the western part of his state.
Coastal streets flowed with frothy ocean water and tens of thousands lost electricity. When Florence started battering eastern North Carolina with record rainfall, the Neuse and Trent rivers began to swell - and combined with high tide, made for risky flooding. The forward speed of Florence has dropped from 17 miles per hour on Wednesday to 10 miles per hour during Thursday afternoon.
A very slow or meandering movement of Florence will translate to long-duration high winds, storm surge and demolishing wave action. The huge storm weakened to a Category 2 hurricane overnight, but forecasters warned that it still packed a unsafe punch, 110 mile-an-hour (175 kph) winds and torrential rains.
"It's an uninvited brute who doesn't want to leave", said North Carolina Gov. Roy Cooper. "Surviving this storm will be a test of endurance, teamwork, common sense and patience". Utility companies said millions were expected to lose power and restoration could take weeks.
"It's not going to take much in a lot of these areas to saturate the soil, so trees are going to come down really easily" and knock down power lines, said Ken Graham, director of the National Hurricane Center.
Forecasters said conditions will only get more lethal as the storm pushes ashore early Friday near the North Carolina-South Carolina border and makes its way slowly inland.More news: WE’RE NEXT! Storm Helene BATTERS Azores as Britain braces for impact
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Prisoners were affected, too. "While we expect an AccuWeather Local StormMax™ of 40 inches of rain, extensive inland flooding and storm surge flooding from Florence, Hurricane Harvey unleashed more than 60 inches of rain locally centered around the United States' fourth largest city, Houston, which has a population of 2.3 million", AccuWeather Founder and President Dr. Joel N. Myers said.
Rivers are expected to burst their banks.Some 1.7 million people have been ordered to evacuate from North Carolina, South Carolina and Virginia. The homes of about 10 million were under watches or warnings for the hurricane or tropical storm conditions.
At 7 a.m., the center of the eye was located about 5 miles (10 kilometers) east of Wilmington, moving west at 6 mph. Hurricane-force winds extended 80 miles (130 kilometers) from its center, and tropical-storm-force winds up to 195 miles (315 kilometers).
North Carolinians made last-minute preparations and hunkered down to await Florence's arrival. As The Weather Channel noted, storm surges of nine feet or higher could be catastrophic, powerful enough to move cars and cover the first floor of buildings.
It remains a Category 1 hurricane with top sustained winds of 90 miles per hour (150 kph), but a gust of 112 miles per hour (180 kph) was reported just offshore.
"Please keep that in mind", and consider leaving soon, Brock Long, administrator of the Federal Emergency Management Agency, said midmorning Thursday.
As Florence drew near, President Donald Trump tweeted that FEMA and first responders are "supplied and ready", and he disputed the official conclusion that almost 3,000 people died in Puerto Rico, claiming the figure was a Democratic plot to make him look bad.
As the soil becomes saturated, gusty winds will topple trees and lead to widespread power outages. "And you don't need power to sling booze", said owner Eli Ellsworth.