An Extreme Heat Warning covers Macon County until 8 p.m. Saturday after the Weather Service upgraded from a Heat Advisory.
Heat index values up to 105 are expected. The warning also covers Adair, Linn, Chariton, Randolph, and Sullivan counties, among others in northern Missouri.
Hot temperatures and high humidity may cause heat illnesses to increase significantly during extreme heat, the Weather Service said.
The Weather Service said to drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check on relatives and neighbors.
The Missouri State Emergency Management Agency Watch Center said Friday that it is monitoring heat.