Most tornadoes in Iowa occur in May and June, as you can imagine. However, we have had tornadoes in every month, including December.
One such December tornado developed near Maquoketa on December 14, 1975. It was determined to be an F-1 causing damage in rural areas just south of Maquoketa. Thunderstorms also produced straightline wind damage on this day.
You can find more information about tornadoes in Iowa throughout the year by clicking here for National Weather Service Des Moines, here for National Weather Service Quad Cities, and here for National Weather Service La Crosse.
You can look up Iowa Weather History on the National Weather Service Des Moines page by clicking here.