Hurricane Idalia has ploughed into Florida's Gulf Coast with howling winds, torrential rains and pounding surf, then weakened as it turned its fury on southeastern Georgia.
Hours after Idalia slammed ashore on Wednesday as a powerful Category 3 hurricane at Keaton Beach in Florida's Big Bend region, packing winds of about 200km/h, authorities were still trying to assess the full extent of damage in the hardest-hit areas.
Video footage and photographs from the region around Idalia's landfall showed ocean waters washing over highways and neighbourhoods swamped by extensive flooding at midday. Power outages were widespread.
Florida Governor Ron DeSantis said no fatalities had been confirmed and that it seemed most residents in vulnerable, low-lying areas had heeded evacuation orders and warnings to move to higher ground.
But the Florida Highway Patrol reported earlier in the day that two motorists died in separate rain-related crashes on Wednesday morning. DeSantis later said state authorities were investigating one unconfirmed storm-related traffic death.
Insured property losses in Florida were projected to run $US9.36 billion ($A14.43 billion), investment bank UBS said in a research note based on preliminary estimates.
Still, Idalia appeared from early reports to have been far less destructive than Hurricane Ian, a Category 5 storm that struck Florida last September, killing 150 people and causing $US112 billion ($A173 billion) in damage.
The governor said that as many as 565,000 utility customers had lost electricity at some point during and after the storm.
About 320km to the south, at least 75 people were rescued from floodwaters in St. Petersburg, Florida, municipal officials said on social media, with video showing two emergency workers in a small boat plying submerged streets through heavy rains.
As predicted, Idalia crossed Florida's shoreline in the heart of its largely rural Big Bend region, where the state's northern Gulf Coast panhandle curves into the western side of the Florida Peninsula. The area is roughly bounded by the cities of Gainesville and Tallahassee, the state capital.
The same region, featuring a marshy coast and threaded with freshwater springs and rivers, was devastated by a major hurricane in 1896.
Feeding on the warm, open waters of the Gulf Mexico as it churned toward Florida, Idalia gained strength after skirting western Cuba on Monday as a tropical storm.
The hurricane unleashed destructive winds and torrential downpours that were forecast to cause flooding up to five metres deep along Florida's Gulf Coast. Some 12 hours after landfall, the governor said no drowning victims had been found caught in the storm surge.
Florida's Gulf Coast, southeastern Georgia and eastern parts of North and South Carolina were forecast to receive 10-20cm of rain through Thursday, the National Hurricane Center (NHC) warned.
By early Wednesday afternoon, Idalia's centre had left Florida and moved into Georgia. The storm's most dangerous feature, officials warned, was a powerful surge of wind-driven seawater that flooded low-lying areas.
By midmorning, a storm monitoring station in Steinhatchee, 30km south of Keaton Beach, showed waters reaching 2.4m, well above the 1.8m flood stage.
Idalia attained Category 4 intensity on the five-step Saffir-Simpson wind scale early Wednesday before landfall, but by 7am had weakened into Category 3, the NHC said.
As it entered southeastern Georgia, Idalia's wind speeds ebbed to 145km/h, reducing the tempest to a Category 1 storm. By 5pm, it weakened further into a tropical storm.
