However, not all wall clouds rotate. Rotating wall clouds usually precede strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. The shelf cloud is associated with the gust front. Rising cloud motion often can be seen in the outer part of the shelf cloud, while the underside often appears turbulent, boiling, and wind torn. A roll cloud is a low horizontal tube-shaped cloud associated with a gust front.
Roll clouds are relatively rare.
They are completely detached from the thunderstorm base or other cloud features. The possibility of a tornado forming beneath a shelf or roll cloud is quite small. The main threat is damaging straight-line winds. Wall cloud vs. shelf cloud A big challenge as a spotter will be to discern between a shelf cloud and a bonafide wall cloud. A shelf cloud signifies an area of downdraft and outflow while a wall cloud indicates an area of updraft and inflow .