Roll cloud

A Roll cloud is a relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base, unlike the more common shelf cloud. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms, a cold front or line squalls. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes.