Mammatus (pronounced Ma-Ma-Tuss) clouds formed Tuesday, June 17th with thunderstorms that moved through. Mammatus form underneath the storm cloud and typically appear once the worst of the storm has passed.
Mammatus are typically harmless, and are not a sign of a tornado. This is a misconception. For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. If the water drops in the clouds are large, it could take hours for the clouds to fall apart.