When US national weather service meteorologists first picked up this mystery formation on their St Louis office radar no one was quite sure what it was.
But after some detective work the images were revealed to be exactly what they looked like - butterflies. Namely, Monarch butterflies migrating to Mexico for the winter.
Here is the US national weather service's (slightly complex) explanation of how they worked that out, for those interested.
High differential reflectivity indicates these are oblate targets, and low correlation coefficient means the targets are changing shape. We think these targets are Monarch butterflies.
A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape!
- U.S. National Weather Service