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Shelf Clouds and Roll Clouds: Not Tornadoes, But Still Scary and Ominous (PHOTOS) By Jon Erdman Published Jun 23 2015 11:26 AM EDT weather.comA roll cloud near Lake Hendricks, Minnesota, on June 20, 2015. (Facebook/Thomas L. Stritecky) A shelf cloud, also known as an arcus or arc cloud, may be the most...