Weather system Hermine has weakened to a tropical storm as it moves inland after making landfall in Florida's Big Bend area as a hurricane.
The US National Hurricane Centre said the storm's maximum sustained winds have decreased to near 70mph with additional weakening forecast.
The storm is centred about 20 miles west of Valdosta, Georgia, and is moving north-north-east at nearly 14mph.
Hermine was the first hurricane to hit Florida in more than a decade when it came ashore early on Friday.
After pushing into Georgia, Hermine is expected to move into the Carolinas and up the East Coast with the potential for drenching rain and deadly flooding.
The Category 1 storm had hit just east of St Marks, Florida, with winds of around 80mph, according to the National Hurricane Centre.
Projected storm surges of up to 12ft threatened a wide stretch of the coast and an expected drenching of up to 10in of rain carried the danger of flooding along the storm's path over land, including the state capital Tallahassee, which had not been hit by a hurricane since Kate in 1985.
The last hurricane to strike Florida was Wilma, a powerful Category 3 storm that arrived on October 24 2005. It swept across the Everglades and struck heavily populated south Florida, causing five deaths in the state and an estimated 23 billion dollars in damage.
Residents on some islands and other low-lying, flood-prone areas in Florida had been urged to clear out. Flooding was expected across a wide area of the marshy coastline of the Big Bend - the mostly rural and lightly populated corner where the Florida peninsula meets the Panhandle.
Florida governor Rick Scott warned of the danger of strong storm surges, high winds, downed trees and power outages, and urged people to move to inland shelters if necessary and make sure they have enough food, water and medicine.
"You can rebuild a home, you can rebuild property, you cannot rebuild a life," he said at a news conference, adding: "We are going to see a lot of flooding."
Mr Scott, who declared an emergency in 51 counties, said 6,000 National Guardsmen were poised to mobilise for the storm's aftermath. The governors of Georgia and North Carolina also declared states of emergency.
Rains of 4in to 10in are possible along the coasts of Georgia and the Carolinas by Sunday. Lesser amounts are forecast further up the Atlantic Coast, because the storm is expected to veer out to sea.