Murus

A localized, persistent, and often abrupt lowering of cloud from the base of a Cumulonimbus from which tuba (spouts) sometimes form. 

Usually associated with a supercell or severe multicell storm; typically develop in the rain-free portion of a Cumulonimbus and indicate an area of strong updraft. Murus showing significant rotation and vertical motion may result in the formation of tuba (spouts). Commonly known as a 'wall cloud'.