In a weird twist, the swarm itself resembles a huge butterfly.

(Michael Warwick/Shutterstock.com)

Late last week, meteorologists in St. Louis noticed a cloud acting peculiarly: It was beating a path toward Mexico while changing into a variety of odd shapes. Was it a radar glitch? The debris signature of a south-moving tornado?

The answer was more heartening—and bizarre. After analyzing the reflections, the National Weather Service concluded they showed an immense swarm of Monarch butterflies migrating to their winter home in the Mexican mountains:

(NWS St. Louis)

Here's how it technically arrived at that conclusion, for the weather geeks out there:

Keen observers of our radar data probably noticed some fairly high returns moving south over southern Illinois and central Missouri. High differential reflectivity values as well as low correlation coefficient values indicate these are most likely biological targets. 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! NWS St. Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!

North America's Monarch population has been in decline, reaching record-low numbers in the past couple of years due to habitat loss and perhaps extreme weather. These radar shots provide a spot of good news in that, while struggling, the Monarchs aren't extinct quite yet. Indeed, people on the weather service's Facebook page have reported seeing them fluttering around the region. "They are flying over my Missouri home today," says one. Adds another: "I have been seeing some coming thru OKC in the last week or so. They are beautiful."

Also beautiful—and strange—is that the shape of the swarm itself resembles a giant butterfly. The last time that sort of radar coincidence happened may have been in 2011, when thousands of birds formed into a bird shape above Beebe, Arkansas, right before falling out of the sky, dead.

(NWS St. Louis)

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