Helene hit Florida's Big Bend region as a powerful category four hurricane on Thursday at 11.10pm and left a chaotic landscape of overturned boats in harbours, felled trees, submerged cars and flooded streets.
Police and firefighters carried out thousands of water rescues throughout the affected states, including in Atlanta, where an apartment complex had to be evacuated due to flooding.
Helene came ashore in Florida with 225km/h winds, weakening to a tropical storm as it moved into Georgia early on Friday.
As of early afternoon, the storm had been downgraded to a tropical depression and was packing maximum sustained winds of 55km/h as it slowed over Tennessee and Kentucky, the National Hurricane Center said.
Helene's heavy rains were still producing catastrophic flooding in the southern Appalachians, the NHC said.
More than 50 people were trapped on the roof of a hospital at midday on Friday in Unicoi County, Tennessee, local media reported, as floodwaters swamped the rural community.
The National Weather Service issued flash flood warnings for several counties in Georgia, South Carolina and North Carolina on Friday morning.
"This is a PARTICULARLY DANGEROUS SITUATION. SEEK HIGHER GROUND NOW!" the service said.
In western North Carolina, Rutherford County emergency officials warned residents near the Lake Lure Dam just before noon to immediately leave for higher ground, saying "Dam failure imminent".
In nearby Buncombe County, landslides forced interstates 40 and 26 to close, the county said on X.
The extent of the damage in Florida began emerging after daybreak.
In coastal Steinhatchee, a storm surge - the wall of seawater pushed ashore by winds - of 2.4-3 metres moved mobile homes, the NWS said on X.
In Treasure Island, a barrier island community in Pinellas County, boats were grounded in front yards.
The city of Tampa posted on X that emergency personnel had completed 78 water rescues of residents and that many roads were impassable because of flooding.
The Pasco County sheriff's office rescued more than 65 people overnight.
The US Coast Guard said it had saved nine people from storm waters.
Video posted online showed a Coast Guard crew pulling a man and his dog wearing life vests from the ocean on Thursday after his sailboat became disabled off Sanibel Island.
