Miami - Tropical Storm Hermine was downgraded from hurricane status as it weakened after hitting Florida's Gulf coast early on Friday, but officials still warned of "life-threatening" conditions.
The storm's centre was around 80km northeast of Florida's capital Tallahassee at 09:00 GMT, the Miami-based National Hurricane Center (NHC) said.
Winds gusted up to around 110km/h and heavy rain caused flooding and power cuts to tens of thousands as the storm moved northeast inland at around 22km/h, said the NHC.
Hermine made landfall as a Category One hurricane around 05:30 GMT near St Marks, just south of Tallahassee, the NHC said.
"There is a danger of life-threatening inundation within the next 12 to 24 hours along the Gulf coast of Florida," it added.
Some 70 000 people in rain-drenched Tallahassee were left without power as local television stations broadcast video footage of buffeting winds, lashing rain and flooded streets.
Authorities in several counties have issued mandatory evacuation orders for residents on the coast and low-lying regions.
"This is life-threatening," Governor Rick Scott told journalists on Thursday, urging residents to take warnings seriously.
"We have a hurricane. You can rebuild a home. You can rebuild property. You cannot rebuild a life."
Hundreds of schools and government offices will be closed on Friday as residents brace for the storm's full impact.
Pentagon spokesperson Jeff Davis said 100 Florida National Guard personnel were activated, with 6 000 more on alert in the state and 34 000 ready to deploy from elsewhere in the United States.
President Barack Obama has asked FEMA administrator Craig Fugate to keep him updated on the situation "and to alert him if there are any significant unmet needs," said White House spokesperson Josh Earnest.
"Local, state and federal officials have been working diligently to prepare for these storms and have resources on hand to respond to them as necessary," he added.
Hermine is expected to dump a total of 12 to 25cm of rain over the southeastern United States, with possible isolated maximum amounts of 38cm.
"These rains may cause life-threatening floods and flash floods," it said.
The storm's centre should continue to move further inland across southeastern Georgia on Friday and into the Carolinas overnight and on Saturday, forecasters said.
As it continues to weaken, heavy rain could reach the mid-Atlantic coast from Virginia to New Jersey beginning early on Saturday, the NHC said.
Some tornadoes are possible across northern Florida and southern Georgia, with the risk spreading across the eastern Carolinas on Friday.
Georgia has declared a state of emergency in 56 counties, and North Carolina in 33 counties.