Tropical Storm Debby has inundated coastal Georgia and South Carolina with a deluge of rain that could flood Charleston, Savannah and other cities, a day after it slammed into Florida's Gulf Coast as a hurricane.
At least six people have died in Florida and Georgia in the wake of the storm, which is expected to linger over the southeastern and mid-Atlantic coasts for days.
Between 25cm and 51cm of rain was expected to fall along parts of Georgia, South Carolina and North Carolina through Friday and cause catastrophic flooding, the National Hurricane Center said.
The governors of those states have declared states of emergency.
The storm featured 72kmph winds as it moved slowly just south of Savannah, Georgia, early on this morning.
Heavy rainfall could cause flooding in parts of the mid-Atlantic through Sunday, the centre said.
More than 20cm of rain have already fallen on Savannah and Valdosta, Georgia, the National Weather Service said.
Charleston and Hilton Head, South Carolina, have received between 25 and 30cm of rain so far, the weather service said, with more on the way.
In Savannah, it was coming down hard outside the landmark Bellwether House, a 16-room bed-and-breakfast in the city's historic district, but general manager Victoria Hill was not too concerned.
"This place was built in 1876," she said. "It's built rock-solid, and you can imagine how many storms have hit over the last century."
Ms Hill said a lot of tourists canceled their reservations because of the storm, but a handful of guests were staying on.
"We here, we're open and we're holding up. We'll all ride this out together."
Charleston Mayor William Cogswell said more than 61cm of rain is expected in his city before the storm passes. Even at low tide, storm surges of between 1.2 and 1.8m will prevent floodwaters from draining into the sea, he said.
There are "not enough pumps in the world" to handle that much rain, Cogswell said last night.
He has since extended a citywide curfew until Wednesday morning.
"Nobody should be out on the streets in these conditions unless it is an absolute emergency," the mayor said.
About 80km west of Charleston, fears of a breach at the McGrady Dam in Colleton County, part of the state's Lowcountry, prompted the county sheriff's office to warn residents to evacuate immediately this morning.
Debby made landfall as a Category 1 hurricane in the Big Bend region of Florida's Gulf Coast yesterday morning, dumping 20 to 41cm of rain in parts of central Florida, according to local reports.
The storm has been blamed for five deaths in Florida and one near Valdosta, Georgia.
Nearly 110,000 customers were without power in Florida as of this morning, according to poweroutage.us, down from a peak of 350,000 yesterday.
And more than 17,000 were without power in Georgia. Hundreds of flights to and from the state were cancelled.
Weather conditions could spawn tornadoes as well, according to the National Hurricane Center.
A suspected twister flipped over cars and damaged restaurants in Moncks Corner, South Carolina, about 48km north of Charleston, early this morning. Mayor Thomas Hamilton Jr said there were minor injuries.
The hurricane center said Debby would slow down and move east and off Georgia's shore before turning north and drifting inland over South Carolina near Charleston on Thursday.
Vice President Kamala Harris postponed a presidential campaign stop scheduled this week in Savannah, the Savannah Morning News reported.
Savannah Mayor Van Johnson said the city could expect a "once in a thousand year" rain event.
"This will literally create islands in the city," Mr Johnson said.
Cedrick King, a businessman from coastal Brunswick, Georgia, said he and his family had packed up their belongings and were ready to make the five-hour drive to Atlanta.
"We're headed north, far away from this storm," he said.
