Hurricane Florence, which is nearing the US East Coast, could kill "a lot of people", officials warn.
The Federal Emergency Management Agency (Fema) says storm surges could bring catastrophic flooding to inland areas.
Nearly 1.7m people along the coastlines of North Carolina, South Carolina and Virginia have been ordered to evacuate.
Strong winds and heavy rains have already begun lashing North Carolina's coastline, leading to some early flooding.
Some 11,000 power outages had already been reported in the state. Reuters news agency reports.
Florence was 155 miles (250km) east of Myrtle Beach in South Carolina at 17:00 EDT (21:00 GMT), and is projected to make landfall on Friday at 08:00 local time.
It was downgraded to a category two storm with 105mph (165km/h) winds, and the US National Hurricane Center (NHC) said it does not expect much change in the strength before the eye of Florence makes landfall.
Why isn't the downgraded storm less of a threat?
Fema administrator Brock Long told Thursday morning's news conference that while Florence's wind speed had dipped, its wind field had expanded and total rainfall predictions were unchanged.
Floodwaters may rise up to 13ft (4m) as rivers see their flows "reversed", meteorologists have warned.
"So this is a very dangerous storm," said Mr Long. "Inland flooding kills a lot of people unfortunately and that's what we're about to see."
"Your time is running out," he warned those who had not yet heeded the warning to evacuate. "The ocean is going to start rising."
"Your time to get out of those areas in storm surge inundation is coming to a close. I cannot emphasise that enough."
He said that people living near rivers, streams and lowland areas in the region were most at risk.
What's the situation on the ground?
The latest weather predictions show the storm slowing to a near standstill as it pummels the coast with "copious amounts of rain" from Thursday night to Saturday, said Mr Long.
Parts of the Carolina coast are expecting 20-30 (50-75cm) inches of rain, with isolated regions seeing up to 40in of downpour.
Myrtle Beach, South Carolina, is imposing a 12-hour curfew from 19:00 local time on Thursday.
Petrol stations in the area are reporting shortages and energy companies predict that one to three million homes and businesses may lose power.
Over 1,400 flights have been cancelled, according to FlightAware.com, as most of the coastal region's airports are closed to ride out the storm.
North Carolina Governor Roy Cooper warned people: "Today the threat becomes a reality."
Emergency workers are arriving from other parts of the US to aid in rescues.
The Coast Guard has shallow-water response boats ready to help trapped residents.
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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