A Retreating Thunderstorm at Sunset

What type of cloud is that? This retreating cumulonimbus cloud , more commonly called a thundercloud, is somewhat unusual as it contains the unusual bumpiness of a mammatus cloud on the near end, while simultaneously producing falling rain on the far end. Taken in mid-2013 in southern Alberta , Canada , the cloud is moving to the east, into the distance, as the sun sets in the west, behind the camera. In the featured image , graphic sunset colors cross the sky to give the already photogenic cloud striking orange and pink hues. A darkening blue sky covers the background. Further in the distance, a rising , waxing , gibbous moon is visible on the far right.