A lowered cloud feature under a thunderstorm base that often precedes strong tornadoes is a wall cloud.
A wall cloud is a localized, persistent lowering beneath the base of a cumulonimbus thunderstorm, usually near the storm’s strongest updraft. In a supercell, it commonly forms beneath the rain-free base where warm, moist inflow meets cooler air from a downdraft. Condensation makes the lowering visible as air rises and becomes saturated.
Rotating wall clouds are especially important because they signal a low-level mesocyclone and are the part of many supercells most likely to produce a tornado. Tornadogenesis may follow within minutes, but timing varies widely. A rotating wall cloud can persist without producing a tornado, so it is a warning sign rather than proof that one will form.
The main visual confusion is a shelf cloud. A wall cloud is an inflow feature that usually slopes inward toward the precipitation area; a shelf cloud is an outflow feature associated with a gust front and generally projects outward.