Officials were trying to "find and rescue anyone else we might have missed throughout the night," Tamara Roberts of the Sharp County, Ark., Sheriff's Office said.
Stormy weather buffeted the region Friday. Areas in northeast Arkansas and southeast Missouri received more than 10 inches of rain in 24 hours, said David Blanchard, a National Weather Service forecaster in Paducah, Ky.
More storms and possibly tornadoes were forecast for Saturday, and Louisville mayor Jerry Abramson rescue boats and National Guard helicopters are on stand-by, CBS News Louisville, Ky., affiliate WLKY reports.
"There's so much moisture in the atmosphere, you could get a lot of rain in no time flat," said Brian Smith, a meteorologist with the National Weather Service.
Two tornadoes swept through south-central Missouri on Friday afternoon, damaging more than 100 homes and tearing off part of a roof at a middle school moments after a tornado drill.
A firefighter who videotaped the twisters moving through St. James estimated they were on the ground for 10 minutes.
Students at St. James Middle School said they had just completed a tornado drill when they were forced to rush back into the hallway for the real thing. No teachers, children or staff members were injured.
Officials in Sharp County, Ark., worked Saturday to rescue people who were stranded after heavy rains flooded much of the county, including one person stuck in a tree, sheriff's officials said.
Six people were killed in Kentucky, including a father and his 1-year-old daughter who died when their truck slid off an interstate into flood waters near Elizabethtown. Christopher Richardson, 31, was pronounced dead at the scene, Kentucky state police said. His daughter, Hannah, was flown to Kosair Children's Hospital in Louisville, where she died later Saturday, Jefferson County Deputy Coroner Gayle Norris said.
Two women died trying to cross a flooded roadway early Saturday. Witnesses told rescue officials the women were swept away in a flooded creek, Fire Battalion Chief Mat Ragland said.
Others killed included a Jessamine County woman who ran her pickup truck into high water and a woman in the southwestern part of the state whose car struck a guard rail.
In northwest Arkansas, Deborah Massey, 51, died when her boat was struck by lightning as tried to make it to shore, Washington County Sheriff Tim Helder said. Another person in the boat was injured and treated at a hospital.
In Kentucky, flooding forced more than 100 people out of a Louisville apartment complex, Mayor Jerry Abramson said.
Portions of Interstate 64 just east of Louisville were closed in both directions due to standing water. Dozens of cars were stranded, Abramson said.
"At one point, just about every road in the county was flooded," said Michael Key, a Hardin County 911 dispatcher, after 5 inches of rain fell.
Maggie DiPietro, 58, was among about two dozen people who sought shelter at an Elizabethtown community center. She said she woke up shortly after 2 a.m. Saturday and found about 2 inches of water in her home.
"By the time the police came and rescued me, it was almost up to my calves," she said.
Thousands across the region were without power Saturday, including more than 5,000 Louisville Gas & Electric customers.