Most hardcore fly anglers have heard of the famous Mother’s Day caddis fly hatch that takes place across the upper Rocky Mountain region each spring.

But how about the Memorial Day mayfly hatch on the upper Mississippi River?

Haven’t heard of it?

Well, the mayflies were so thick along the Mississippi in portions of Wisconsin on Saturday, May 29, that their swarms looked like a summer thunderstorm on National Weather Service radar.

According to information posted by the NWS on its Web site, about 9:13 p.m. the newly airborne bugs appeared as pink, purple and white bands on the radar image along the Mississippi near La Crosse, Wisconsin. The weather service said the radar phenomenon lasted for about 20 minutes as the insects flew from the river and nearby water areas and were carried along on south-to-southeast winds.

It’s not the first time the flying insects have shown up on radar in parts of Wisconsin, Minnesota and Iowa, though in the past it has occurred later in the summer.

Dan Baumgardt, a meteorologist with the weather service in La Crosse, told the St. Paul Pioneer Press that he sees about three to four “mayfly emergence events” on and near the Mississippi each year, though an unusually warm spring contributed to the earliest one he’s seen.

“Usually, we see these more in mid-June to late July,” he said, adding that warming water temperature likely contributed to the early hatch.

In addition to showing up on radar, during their short lifespan that includes little more than hatching, reproducing and dying, mayflies are naturally attracted to light. As a result, the millions of the dead insects that accumulate in lighted areas can pose a major headache for city crews in the region.

In fact, it’s reported that Red Wing, Minn. has utilized city snowplows to clear several inches of dead mayflies from its roadways.