A low, horizontal, rotating tube appearing ahead of a storm is a roll cloud.
A roll cloud is a rare type of arcus cloud, also known by the World Meteorological Organization’s cloud-atlas name volutus. It forms as air rises along a gust front, cold front, or sometimes a sea-breeze boundary, causing moisture to condense into a detached, tube-shaped cloud.
Its apparent rotation is horizontal rather than the vertical rotation associated with a tornado. Wind shear—changes in wind speed or direction with altitude—helps give the cloud its rolling appearance. A roll cloud can be several kilometres long and may move independently of the storm that produced the boundary.
The most famous example is Australia’s Morning Glory cloud, especially over Queensland’s Gulf of Carpentaria. The common mix-up is with a shelf cloud: a shelf cloud is wedge-shaped and attached to the parent thunderstorm, while a roll cloud is generally detached. A wall cloud, by contrast, is a lowered cloud base beneath a storm’s updraft and may rotate vertically.