Wall Cloud - Supercell and Tornado Significance

Supercell and Tornado Significance

The wall cloud feature was first identified by Ted Fujita associated with tornadoes in tornadic storms. In the special case of a supercell thunderstorm but also occasionally with intense multicellular thunderstorms, the wall cloud will often be seen to be rotating. A rotating wall cloud is the area of the thunderstorm that is most likely to produce tornadoes, and the vast majority of intense tornadoes.

Tornadogenesis is most likely when the wall cloud is persistent with rapid ascent and rotation. The wall cloud typically precedes tornadogenesis by ten to twenty minutes but may be as little as one minute or more than an hour. Often, the degree of ascent and rotation increase markedly shortly before tornadogenesis, and sometimes the wall cloud will descend and "bulk" or "tighten". Tornadic wall clouds tend to have strong, persistent, and warm inflow air. This should be sensible at the surface if one is in the inflow region; in the Northern Hemisphere, this is typically to the south and southeast of the wall cloud. Large tornadoes tend to come from larger, lower wall clouds closer to the back of the rain curtain (providing less visual warning time to those in the path of an organized storm).

Although it is rotating wall clouds that contain most strong tornadoes, many rotating wall clouds do not produce tornadoes. Absent a low-level boundary, tornadoes very rarely occur without a rear flank downdraft (RFD), which usually manifests itself visually as a drying out of clouds, called a clear slot or notch. The RFD initiates the tornado, occludes around the mesocyclone, and when it wraps completely around, cuts off the inflow causing death of the low-level mesocyclone and tornadolysis. Therefore, in most cases, the RFD is responsible for both the birth and the death of a tornado.

Usually, but not always, the dry slot occlusion is visible (assuming one's line of sight is not blocked by precipitation) throughout the tornado life cycle. The wall cloud withers and will often be gone by the time the tornado lifts. If conditions are favorable, then, often even before the original tornado lifts, another wall cloud and occasionally a tornado may form downwind of the old wall cloud, typically to the east or the southeast in the Northern Hemisphere (vice versa in the Southern Hemisphere).