Fierce winds and rain from Hurricane Hermine have slammed Florida's northern Gulf Coast and blackouts left tens of thousands of households in the dark.
Florida Governor Rick Scott said the storm could lead to deaths and told residents to stay indoors until it had passed.
Hermine, a Category 1 hurricane, made landfall shortly after midnight about 10km southeast of St Marks, bringing heavy rains and packing winds of 130 km/h. It left 70,000 households in the state capital Tallahassee and thousands more elsewhere without power.
"It is a mess ... we have high water in numerous places," Virgil Sandlin, the police chief in Cedar Key, Florida, told the Weather Channel.
Strong gusts downed power lines and trees as widespread flooding inundated communities in northern Florida befroe the hurricane reached into Georgia, where conditions were expected to deteriorate throughout the early morning on Friday.
"The combination of a dangerous storm surge and the tide will continue to cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline," the National Hurricane Centre said in an advisory.
The centre warned that some areas along Florida's northern Gulf Coast may experience 3 m of flooding.
Pasco County reported crews rescued 18 people and brought them to shelters after their homes were flooded in Green Key and Hudson Beach early on Friday.
"Stay indoors even if it's calm outside. The eye of Hermine may be passing through. Let it pass completely before surveying any damage," Governor Scott advised residents in a Twitter post.
Mandatory evacuations were ordered in parts of five counties in northwestern Florida, with voluntary evacuations in at least three more counties. Twenty emergency shelters were opened across the state for those displaced by the storm.
"This is life-threatening," Scott told reporters on Thursday.
