wall cloud

A wall cloud, or pedestal cloud, is a cloud formation associated with thunderstorms. It is a marked lowering typically beneath the rain-free base (RFB) portion of a deep cumulus cloud (normally cumulonimbus but on occasion cumulus congestus), and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than that of the ambient cloud base.
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A localized, persistent, often abrupt lowering from a rain-free base . Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation . However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes , by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion. See supercell.
"Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.