Hurricane Florence, which is nearing the US East Coast, could “kill a lot of people”, officials warn.
Federal Emergency Management Agency (Fema) spokesman Jeff Byard said storm surges may bring catastrophic flooding to inland areas.
Though Florence has been downgraded to a category two storm with 110mph (175km/h) winds, Mr Byard said it remained “very dangerous”.
He warned of “feet of rain not inches” in the Carolinas and Virginia.
Mr Byard told Thursday morning’s news conference that while Florence’s wind speed had dipped, its hurricane-force 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 Byard. “Inland flooding kills a lot of people unfortunately and that’s what we’re about to see.”
He warned that flooding would begin within a matter of hours.
“Your time is running out,” he said. “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.
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 Fema administrator Brock Long.
Nearly two million people have been ordered to evacuate the coast lines in North Carolina, South Carolina and Virginia, and several highways have been reversed to aid the exodus.
Over 1,400 flights have been cancelled, according to FlightAware.com, as most of the coastal region’s airports closed to ride out the storm.
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.