Hurricane Florence, which is nearing the US East Coast, could “kill a lot of people”, officials warn.
Federal Emergency Management Agency (Fema) administrator Brock Long said storm surges may bring catastrophic flooding to inland areas.
Though Florence has been downgraded to a category two storm with 105mph (165km/h) winds, Mr Long said it remained “very dangerous“.
He said “feet of rain not inches” would drench the Carolinas and Virginia.
Mr Long 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 Long. “Inland flooding kills a lot of people unfortunately and that’s what we’re about to see.”
He warned that the deluge 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 storm’s centre is still 145 miles from Wilmington, North Carolina, but tropical-storm strength winds and heavy rains have already begun hitting the state’s Outer Banks barrier islands.
Florence is projected to make landfall at about 08:00 local time on Friday (12:00 GMT).