Swarm of butterflies picked up on Denver weather radar

A painted lady butterfly flies near daisies in a garden in downtown Denver Wednesday, Oct. 4, 2017. Weather forecasters say that a lacy, cloud-like pattern drifting across a Denver-area radar screen turns out to be a 70-mile-wide wave of butterflies such as the painted ladies migrating from their summer home to their winter haunts. (AP Photo/David Zalubowski)

DENVER -- It was a clear night around Denver Tuesday, but the National Weather Service Doppler radar was picking up something heading into town.

A wayward thunderstorm?

Nope! Just a swarm of butterflies!

The forecasters picked up on the radar that it might be birds -- which can happen during migration periods. When there is no precipitation around, the radar goes into a more sensitive "Clear Air Mode" that can pick up movements of small objects, such as a swarm of birds. And right now, birds are flying south for the winter.

(The colors on the radar are denoting direction objects are moving, not their actual color.)

It's a frequent appearance on radars in the Pacific Northwest this time of year as well. Tuesday night saw the Seattle radar light up under clear skies:

But in Denver's case, it turns out it wasn't just migratory birds, it was butterflies!

NWS forecasters said insects rarely produce such a coherent radar signature, but this time there were enough to do so!

Midwestern radar stations occasionally pick up butterflies, but Paul Schlatter with the National Weather Service believes it's a first for Denver.

An unusually large number of painted ladies, which are sometimes mistaken for monarch butterflies, has descended on Colorado's Front Range in recent weeks, feeding on flowers and sometimes flying together in what seem like clouds.

So next time you see a butterfly, it's not just their wings that they can sometimes fill with color!

The Associated Press contributed to this report.

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