The National Weather Service said Wednesday that 2 to 4 inches of rain will be common as strong storms fire up through Friday; some areas could see up to 6 inches.
How bad things get will depend on how much rain falls and where.
The weather service says a worst-case scenario would be widespread heavy rain along the Mississippi River north of St. Louis, and along the Missouri River. The Mississippi and many of its tributaries are already above flood stage, and the Missouri is getting close.
Forecasters say the Mississippi could reach its highest level at St. Louis in nearly two decades.