The colourful specks swirled one way and then another, mystifying weather forecasters as they pored over radar images trying to decode the patterns.
But, after some detective work, they finally figured out that pictures showed not a weather front moving through the Denver area but a 70-mile cloud of butterflies.
Paul Schlatter, of the National Weather Service, said he first thought it might be migrating birds.
But the wave was moving north-west with the wind at a time when birds are heading south for the winter.
He appealed to social media users for help in identifying the phenomenon.
He had his answer when members of the public said they had seen Painted Lady butterflies floating in the wind.
1/4: We believe migrating butterflies are the cause of yesterdays radar signature. Thanks for all the reports and sightings! #cowx— NWS Boulder (@NWSBoulder) October 4, 2017
2/4: Insects rarely produce such a coherent radar signature. Migrating birds do all the time. #cowx— NWS Boulder (@NWSBoulder) October 4, 2017
3/4 Things with big wings need to fly together in the same direction with the wind to generate that signature in ZDR (purple image). #cowx— NWS Boulder (@NWSBoulder) October 4, 2017
Unusually large numbers had descended on Colorado’s Front Range in response to plentiful flowers.
The species is found across the continental US and they migrate to the south-western US and north-western Mexico in the autumn.