Your forecast: Sunny, chance of 100-km-swath of butterflies

Sunday, October 8, 2017, 7:35 PM - Forecasters at the Boulder, Colo., office of the U.S. National Weather Service caught an unusual gathering on radar last week.

Spreading east to west across the Denver area, a blob appeared on radar Tuesday morning, at one point spanning much of the radar's ring of coverage with a mass of purple echoes as it pushed to the northwest.

The meteorologists' initial suspects were birds, as they reported via Twitter after spotting the mysterious splotch.

Seeing birds, bats, and some insects on radar is not that uncommon. “We detect insects and birds all the time, that’s nothing new,” NWS meteorologist Paul Schlatter told the Denver Post.

But this particular image seemed out of the ordinary to the forecasters. For one thing, birds are generally caught on radar in the morning, when large flocks take flight. And the radar signature showed the mass moving north - with the wind - rather than south, where you'd expect North American migratory birds to be heading in early autumn.

After putting out the call for help from birdwatchers on Twitter, the curious forecasters were able to determine the cause of the signal - a swath of Painted Lady butterflies some 110 km wide, spanning the entire width of Arapahoe County, east of Denver.

2017 has seen a bumper crop of the Painted Lady, according to Rick Levy of the Denver Botanic Gardens. Speaking to local news agency 9 News, Levy said that, while the species always comes through Colorado on its way south to Mexico, there were many more of them than usual this year. "[Never] in this number has it happened like this before," Levy said. "As far as I know, there has never been a migration event like this before."

Levy added that they don't know the specific cause of this year's massive migration, but says it's likely related to the availability of food for the butterfly. "They are coming through the Front Range right now and the rabbit brush are in prime bloom and they’ve got lots of food to be able to eat when they’re coming through," Levy told 9 News.

Social media users flooded the Internet with pictures of the winged visitors over the past week as they inundated the area.

While the warm temperatures and favourable winds may have helped spur the mass migration, the weather is about to take a turn for the worse in Colorado, especially if you're a butterfly.

With a major winter storm expected in the area on Monday, Levy cautioned that many of the insects may not survive the coming snow. "Freezing temperatures and insects typically don't go well together," he told 9 News. "That’s why they migrate, to escape cold temperatures."

But there's still a bright side to the story. The tremendous number of Painted Ladies this year suggests that the population, overall, is doing quite well. "It’s not sad because clearly there was a huge boom in their population, and we have this unprecedented amount of butterflies coming through," Levy said. "A lot of them are making it out. I don’t think it’s sad, I just think it’s how biology works."

Sources: 9 News | National Weather Service | Denver Post |

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