A second wave of migrating Painted Ladies arrived in central Iowa and eastern Nebraska on September 28 when a cool front passed through our area. It had been looking as if the first wave that had arrived on September 5 was about finished, and then all of a sudden there were many more again. These butterflies were generally smaller than those in the first wave, and unlike them, were a mix of different sizes and degrees of wing wear and wing damage. Many of them seem to have ridden the wind toward the west-southwest, reaching the Denver/Boulder, Colorado area on October 3, when the National Weather Service in Boulder spotted radar echoes that they first attributed to migrating birds. They soon realized that the echoes were due to large numbers of Painted Ladies flying northwest with the local winds occurring at that time. Their observation came at the same time that large numbers of Painted Ladies suddenly appeared in Denver for the second time this fall:
From USA Today: Massive Wave of Butterflies Lights Up Denver Weather Radar
From BBC News: Butterfly swarm shows up on Denver radar system
Wind flow patterns from October 1 to today show a persistent low-pressure system in the Denver area with winds spiraling in towards it, sometimes from nearly the entire range of compass directions. From what I’ve seen so far, I think that many of the Denver butterflies sailed from the upper Midwest on west-southwest winds, reaching Colorado in about five days and then dropping downward into the vortex of the low-pressure center.
--Royce Bitzer, October 6, 2017