Areas in the northeast of Arkansas state and south-east Missouri received more than 25 cm of rain in only 24 hours, said David Blanchard, a National Weather Service forecaster in Paducah, Kentucky.
More storms and possibly tornadoes were forecast for Saturday evening throughout the two states and and also Kansas, meteorologists said.
Two tornadoes swept through south-central Missouri on Friday afternoon, damaging more than 100 homes and tearing off part of a roof at school moments after a tornado drill.
Officials in Sharp County, Arkansas, have struggled to rescue people 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 one-year-old daughter who died when their vehicle slid off a road into flood waters near Elizabethtown.
Two women died trying to cross a flooded roadway early on Saturday morning local time. Witnesses told rescue officials the women were swept away in a flooded creek.
Others killed included a woman who drove her car into deep water and a woman in the south-western part of the state whose car struck a guard rail, officials said.
One woman was also killed in north-west Arkansas when her boat was struck by lightning as she tried to make it to shore. Another person in the boat was injured and treated at a hospital.
In the southern state of Kentucky, flooding forced more than 100 people out of a Louisville apartment complex, the local mayor said, while roads were closed and dozens of cars remained stranded in heavy rains, officials said.
At least 40 people were killed in April this year when tornadoes swept across the southern states, with 28 people dying in the state of Tennessee alone.