Residents compared the storm to a freight train that slammed into their homes.
"The state emergency operations centre obviously is in full mode... Our priority today is search and rescue"
Charlie Crist, Florida governor
"It woke me out of a dead sleep," David Wholly, a resident, told the Orlando Sentinel newspaper.
"I heard the noise, and it sounded like a train coming, and I ran to the bathroom. The tree went right through the bedroom window where my head was."
Charlie Crist, Florida governor, declared a state of emergency in Lake County and three other counties to free up emergency supplies and funding.
"The state emergency operations centre obviously is in full mode... Our priority today is search and rescue," Crist said.
He added that the White House had pledged its "full support" after the storm.
About 20,000 people were reportedly left without electricity after the storm. Authorities opened shelters for those residents affected.
Ben Nelson, state weather forecaster, said the thunderstorm system "was in the Gulf of Mexico [Thursday]," and it began "to produce tornadoes in the morning".
Such conditions are not uncommon in the region. Some 42 people were killed in the winter of 1998, when Florida had seven tornadoes and fierce storms sparked by El Nino warming conditions.