US East Coast residents are running out of time to flee before Hurricane Florence batters the region, officials have warned.
Up to 1.7 million people have been ordered to evacuate across South Carolina, North Carolina and Virginia.
South Carolina authorities have turned four motorways into one-way routes away from the coast to speed the exodus.
The category four storm with 130mph (215km/h) winds is forecast to make landfall early on Friday.
A National Weather Service forecaster in Wilmington, North Carolina, said: “This will likely be the storm of a lifetime for portions of the Carolina coast.
“And that’s saying a lot given the impacts we’ve seen from Hurricanes Diana, Hugo, Fran, Bonnie, Floyd and Matthew.
“I can’t emphasise enough the potential for unbelievable damage from wind, storm surge and inland flooding with this storm.”
On Wednesday, Georgia declared a state of emergency, following the Carolinas, Virginia, Maryland and Washington DC.
Jeff Byard, of the Federal Emergency Management Agency, said: “This is going to be be a Mike Tyson punch to the Carolina coast.”
North Carolina Governor Roy Cooper warned citizens that a “disaster is at the doorstep”, and that “tens of thousands” of buildings are likely to be flooded.
A National Hurricane Center (NHC) plane flying inside the storm found that pressure is continuing to drop, indicating its strength may still be growing.
Waves measuring 83ft (25m) tall were recorded at sea on Wednesday morning.
But while many coastal residents have complied with mandatory evacuation orders, others are boarding up their homes and vowing to ride out the storm.
In other developments:
- Hurricane Florence could wreak more than $170bn (£130bn) of havoc and damage nearly 759,000 homes and businesses, says analytics firm CoreLogic
- North Carolina Farms low-lying parts of North Carolina are moving livestock and their waste to save the animals and prevent drinking water supplies being contaminated. Hurricane Floyd left hundreds of thousands of dead hogs and chickens floating in floodwaters in the state in 1999
- While evacuations of prisoners in North Carolina are underway, nearly 1,000 prisoners in South Carolina will not be moved from their cells, despite a mandatory evacuation order affecting their area. “In the past, it’s been safer to leave them there,” said a state department of corrections spokesman
- Animal-lovers are worried about herds of wild horses that roam North Carolina’s Outer Banks, but experts say these equines can sense bad weather coming and head to higher ground
In a video posted to his Twitter account on Wednesday, US President Donald Trump warned residents in Florence’s bullseye to heed official warnings.
“Get out of its way, don’t play games with it. It’s a big one, maybe as big as they’ve seen, and tremendous amounts of water,” said Mr Trump.
“Bad things can happen when you are talking about a storm this size. It’s called Mother Nature. You never know, but we know. We love you all, we want you safe.”
What makes Florence so dangerous?
Forecasters say the storm poses such a threat because it is expected to slow down and hover for nearly three days over the Carolina coast, before dipping south towards Georgia.
It is forecast to bring 20-40in (50-100cm) of rain and life-threatening storm surges of up to 13ft.
Hurricane force winds will emanate up to 70 miles from the centre of the storm, say meteorologists, meaning the impact may be felt on shore well before Florence makes landfall early on Friday.
National Hurricane Center Director Ken Graham warned that rivers up to 40 miles inland may flood.
Mr Graham said on Wednesday morning the Pamlico and Neuse rivers in North Carolina will see their flows “reversed” as storm surges push water back inland.
He added that half of fatalities during hurricanes are caused by storm surges, and another quarter of deaths are due to inland rains and flooding.
Is global warming to blame?
The relationship between climate change and hurricanes is a complex one.
Warmer seas power hurricanes. So as the temperature of ocean water goes up, we might expect the intensity of hurricanes to increase in future.
A hotter atmosphere can also hold more water, so this should allow hurricanes to dump more water on affected areas.
But there are so many factors that contribute to these rare events, it has been difficult to tease out clear trends from the data.
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