I have seen clouds like that only once. I think I have the pictures, but I probably couldn't produce them on a dare. "My" clouds weren't as big or as well formed as that, but they were a sickly green color. Shortly after we noticed them, we had to take shelter because of the hail.

I think the clouds I linked to were the ones that produced softball sized hail.

As I said, if you are under clouds like that, seek shelter. (in her best motherly voice)

BTW, I posted that, went out for a quick ride, and got soaked in a spring shower. My weather luck is not good this year.