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Bug derecho: Swarm of Mayflies caught on radar in Upper Midwest

Mayflies are captured on radar reflectivity after an extremely large hatching event near La Crosse, Wisc. on July 20, 2014. (NWS LaCrosse)

Fair warning to bug-haters: this might be the grossest thing you see today.

On Sunday evening, the La Crosse, Wisc. radar lit up with what, to the untrained eye, looks like a pretty intense rainfall event over the Mississippi River. Reflectivities maxed out at around 40 dbz, and it looked like a strong line of thunderstorms was developing. But not a drop of rain fell.

What the radar “saw” was an epic Mayfly hatching event in which, I can only guess, trillions of huge bugs rose like zombies from the Mississippi River, and then proceeded to cover everything that wasn’t moving in a bug blanket. You think I’m exaggerating, but I encourage you to hold that thought until you see the photos below.

According to the National Weather Service in La Crosse, Wisc., the flies began to catch air around 8:45 p.m., and by later that evening, were swarming cities up and down the river. They also point out that this hatching was of the large, black/brown Bilineata species.

Winds were out of the south that evening, so naturally the bugs were pushed north. You can see on the radar loop that they dispersed to the northeast as they gained altitude. The Weather Service writes that some of the bug echo was “detected as far north as Black River Falls and as high as 2500 feet above ground.”

(Joey Hulett via NWS)

(WKBT News 8 via NWS)

(Tory Stoffregen via NWS)

(@paulmhuddleston on Twitter via NWS)

(Kelly Gardner via NWS)

(Ryan Hollis via NWS)

These hatchings happen a few times every year in the warm season. Fortunately, the insect inconvenience doesn’t last long; the lifespan of a Mayfly is only a few hours to a few days. Just long enough to mate, lay some eggs, and start the process all over again.

Angela Fritz is an atmospheric scientist and The Post's deputy weather editor.
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