Water kills more people in hurricanes than wind, and the rain and storm surge will make Florence extremely unsafe.
Steve Goldstein of the National Oceanic and Atmospheric Administration said Florence's forward motion had slowed overnight and it was not expected to make landfall in the Carolinas until "some time Friday afternoon, Friday evening or Saturday morning".
Faced with new forecasts that showed a more southerly threat, Georgia's governor joined his counterparts in Virginia and North and SC in declaring a state of emergency, and some residents who had thought they were safely out of range boarded up their homes.
More than 100,000 homes are already without power as weather conditions begin to worsen.
"Do you want to get hit with a train or do you want to get hit with a cement truck?" said Jeff Byard, an administrator with the Federal Emergency Management Agency. "Inland flooding kills a lot of people, unfortunately, and that's what we're about to see", he said.
As of Tuesday, more than 1.7 million people in the Carolinas and Virginia were warned to clear out.
More than 1,200 US airline flights scheduled for Thursday and Friday have been canceled, according to the flight-tracking service FlightAware.
They are heeding the warnings to get out of the Carolinas before the storm hits. Hurricane-force winds extended 80 miles (130 kilometres) from its centre, and tropical-storm-force winds up to 195 miles (315 kilometres). For comparison, Hurricane Irene caused a 7-foot surge in 2011, and Hurricane Hazel caused an 18-foot surge in Calabash and Carolina Shores in 1954.
Florence is forecast to dump up to one metre (40 ins) of rain in some areas after it makes landfall.
"This rainfall will produce catastrophic flash flooding and prolonged significant river flooding", the hurricane center said. "But they may not be that much different in terms of wind speed", said Luettich.
Hurricane Florence's outer edges have started to batter the coast of North Carolina - with winds of 100mph bending trees and shooting frothy seawater onto the streets. Florence is unlikely to disrupt operations at the American Airlines hub in Charlotte or the Delta Air Lines hub in Atlanta.
Warm air rises into the storm. "If we lose the house, oh well, we can get housing".
Steering currents - around clear-weather high-pressure systems and stormy low-pressure systems - redirect hurricanes, with the clear-weather systems acting as walls that storms have to go around. This eyewall has the fastest winds below it and violent currents of air rising through it. "The county is larger than Rhode Island; it is not coastal or subject to any potential storm surge where the jail is at".
"Urgent warning about the rapid rise of water on the SW FL coast with the passage of #Irma's eye".
"A weak Category 3 and a strong Category 2 are pretty close to each other, if you just think about category".