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Spectacular Mammatus Clouds over Hastings, Nebraska [hprcc.unl.edu]
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.
Here is a cloud that really looks like bad news [webwombat.com.au]. :)