A mayfly emergence from the Mississippi River near and around La Crosse, Wisconsin on the evening of July 20th produced a massive result on radar. Reports came in of swarms and piles — yes, piles — of mayflies gathering throughout the area. The National Weather Service’s La Crosse, Wisconsin Weather Forecast Office keeps track of such events that occur from Davenport, Iowa through St. Paul, Minnesota.
The radar detected the flies about 845 pm, emanating from the river (the source) with echo values similar to that of light-moderate rain (35-40 dBZ). With a general south-to-north wind flow above the surface, the mayflies quickly moved north once in the air. As the flies dispersed moving north-northeast, they also gained altitude with some of the echo being detected as far north as Black River Falls and as high as 2500 feet above ground.
images via National Weather Service