The clouds covered everywhere and roiled and boiled and moved like ocean waves. They said it looked like
Independence Day, or massive sand dunes in the sky. I say... Iowa, you have been goatse'd by nature!
Local TV
KCRG has a larger gallery of 16 great images of this phenomenon, which they say "looks like severe weather but isn't". Apparently these "wave clouds"
(Wikipedia entry) are more commonly seen over mountainous areas, but somehow wound up getting created over flatlands in this case, which led to people getting all excited and taking pictures of the sky.
At that story link, check out the "Time Lapse of Wave Clouds" video - that's the most mind-boggling vision of what they were seeing, and really shows why they thought it was aliens.