I saw this in Iowa about 20 years ago. The cloud appeared to be rolling along in an otherwise empty sky. When it passed overhead, there was a gust of wind - about 15mph. No apparent change in temperature - it was a warm day.
Half an hour later another roll cloud followed. Then after another half hour, a third!
The clouds extended from horizon to horizon, as in the article.
Reminds me of independence day: <a href="http://www.takoma.com/ssthenagain/Alien%20Ship.jpg" rel="nofollow">http://www.takoma.com/ssthenagain/Alien%20Ship.jpg</a>