I just read about it. There are three or four other pix of the changing scene on a couple of newspapers if you search for asperstus.
It actually is not threatening, but a mixing of an inversion. I read elsewhere that the cloud was not officially 'recognized' for this until not too long ago. Which leads one to think about how clouds are officially named and recognized.
I've tried taking pictures of the related gravity wave clouds and it is really hard to get a good contrast and color, so this photographer was pretty lucky to see it, and then to capture it.
I've been fortunate enough to see a Kelvin-Helmholtz formation on Lake MIlacs in Minnesota last year. DIdn't have a good camera with me but the photos I did get were not at all as impressive and the real phenomenon.
Love clouds. thanks ocala.