Love the cloud shots. There's also a couple of pretty decent shots of lightning on that site. I tried to photograph lightning for a year before I had any success, and it was nowhere near as good as some of those shots. Running around in an open field, carrying a metal tripod while lightning is streaking across the sky is a rush.
Thanks for the link. Those clouds look like bad news. :)
Sometimes very ominous in appearance, mammatus clouds are harmless and do not mean that a tornado is about to form; a commonly held misconception. In fact, mammatus are usually seen after the worst of a thunderstorm has passed.