http://denver.cbslocal.com/2012/07/29/a-rare-mountain-tornado-touches-down-near-mt-evans/

It doesn’t happen very often, but Colorado had a rare mountain
tornado touchdown near Mount Evans. Saturday afternoon at 2:51 pm. Thunderstorms
strengthening over the mountains west of Denver were strong enough to produce a
weak, short-lived twister. The National Weather Service has analyzed pictures
from the area and confirms that it was a bona fide tornado.