A flash flood warning has been issued for Wyandotte County until 7:15 p.m.
Included are Kansas City, Kan., Edwardsville and Bonner Springs, the National Weather Service said.
Also included is Platte County, Mo., and some cities in Johnson County.
An inch of rain has already fallen in the area, the weather service said, and flash flooding is already occurring.
The weather service asked drivers to be careful and not to drive into flooded roadways. Drivers should turn around and go back if they encounter water in the roads.
Drivers also are asked to report flooding to the local law enforcement agency.