It is a rather odd effect, but I’ve seen them before on local radar images. They are most likely low level temperature inversions. There has been a lot of rain and flooding in this area and this being spring the air temperature is getting warmer while the ground is still relatively much colder.
The reason they look like circles is that as the radar makes its circular sweep, it’s picking up the denser moister air closest to the radar station.
Think about standing in the middle of a meadow with a low hanging ground fog. Now imagine holding a flashlight at a 90 degree angle and turning in a circle. The flashlight beam will reflect the fog at the lower level but the beam will also extend past the fog to where the air is clearer and not bounce back any reflection.
But HAL9000 and endthematrix did a much better job of explaining it.
BTW - I’ve also seen local radar images picking up very large flocks of migratory birds flying close to the radar stations with a similar sort of circular effect. So maybe it’s geese?
Well, whatever the explanation, it’s totally gone this morning.
I’m glad you’ve seen this effect before, it’s the first time I’ve seen anything like it, and it was definitely strange, being all over the entire middle west.