River flooding form Tropical Depression Debby -- downgraded from a tropical storm late Tuesday -- forced up to 20,000 people out of their homes in one Florida county alone, while another area had already seen more than 26 inches of rain, topping the official forecast calling for up to 25 inches in a few areas by the time Debby moves out.
In Pasco County near Tampa Bay, a mandatory evacuation was ordered between the Anclote and Pithlachascotee rivers, Reuters reported. The Anclote rose from 9 feet before Debby's approach to more than 27 feet on Tuesday, flooding areas with water head-high in places.
Boats were used to reach stranded residents, and 106 homes had been damaged.
Wakulla County, meanwhile, has seen more than 26 inches of rain, weather.com said in a Twitter alert Tuesday morning. Authorities there advised people to stay in their homes due to washed out and flooded roads.
Flash flood warnings were issued for parts of northern Florida and southern Georgia as Debby moved eastward. By midday, Debby picked up speed to 6 mph and winds had dropped to 40 mph, but that didn't stop the rain.
Parts of Interstate 10 in north Florida were closed due to flooding on a 50-mile stretch between Jacksonville and Tallahassee. The Florida Highway Patrol warned motorists to use extreme caution on other parts of the highway.
Hundreds of thousands of people have been impacted, many having to leave flooded homes in Florida's Panhandle on Monday and others losing power or having property hit by twisters.
In Tampa Bay, a mother manatee was found dead in the water Tuesday off Bayshore Boulevard. Her two calves, which were still alive and later rescued, washed ashore along the flooded road.
Earlier in the day, tropical-storm warnings were in effect for 450 miles of coastline -- from the Panhandle's Mexico Beach to Englewood, south of Sarasota.
Debby made landfall near Steinhatchee on Tuesday afternoon, earlier than expected. Despite the downgrade to a tropical depression, Debby was still dumping rain and could return as a tropical storm when it exits Florida's Atlantic coast.
On Monday, Florida Gov. Rick Scott declared a statewide emergency as five inches of rain in the course of an hour fell on some areas.
President Barack Obama called Scott on Tuesday and the federal government stands "ready to provide additional assistance if necessary," the White House said.
St. Marks, Fla., saw 21 inches in a two-day period while other areas got around 20 inches, weather.com noted.
Parts of Live Oak, Fla., were evacuated Tuesday due to flooding, it added.
Some areas of northern Florida and southeast Georgia could see up to 15 inches of rain Tuesday through Thursday, weather.com stated.
The National Hurricane Center predicted parts of northern Florida could see 25 inches of rain by the time Debby crosses Florida and exits into the Atlantic.
Weather.com noted that 2012 broke the record for the most named storms so early in the Atlantic season. Debby makes four so far, "leapfrogging Dennis from July 5, 2005.
"In an average year, the fourth named storm would have occurred by August 23," it added. "In terms of named storm counts, we're roughly two months ahead of the pace. That said, there is no correlation between a fast start to the season and the degree of activity of the rest of the season."
Debby was also the first tropical storm of the season to enter the Gulf of Mexico.
Debby earlier left tens of thousands of people without power and forced the closure of key highways and bridges in the Tampa Bay area.
Debby has also spawned some 20 twisters, including one on Sunday that killed a woman, injured her daughter and tore through homes in central Florida's rural Highlands County.
WFLA-TV reported Heather Town died when her home was lifted off its foundation and she and her 3-year-old were thrown into nearby woods. The mother was found clutching the child, who survived.
In Starke, Fla., five dogs drowned early Monday when a swollen creek flooded an animal shelter, The Florida Times-Union reported.
Pinellas County saw flooding and at least 20 homes damaged during a tornado-like storm on Sunday.
The Associated Press and Reuters contributed to this report.