What rotating cloud formation often precedes a tornado?

A rotating cloud formation that often precedes a tornado is a wall cloud.

A wall cloud is a localized lowering beneath the rain-free base of a cumulonimbus thunderstorm. It forms where warm, moist inflow rises into the storm’s strongest updraft. As air rises and cools, water vapor condenses, creating the distinctive lowered cloud feature.

When a wall cloud rotates, it provides visible evidence of a low-level mesocyclone—the broad rotation inside a supercell thunderstorm. Many strong tornadoes develop from rotating wall clouds, which commonly appear about 10 to 20 minutes before tornadogenesis, although the timing can vary widely.

A wall cloud is not the same as a shelf cloud or funnel cloud. A shelf cloud is usually an outflow feature along a gust front, while a funnel cloud is a narrower rotating condensation feature extending downward from a storm cloud. A wall cloud can produce a tornado, but rotation alone does not guarantee that one will form.