Tornadoes all too often assemble beneath "wall clouds". A wall cloud is the isolated lowering of a thunderstorm's cloud base often in the southwest quadrant of such a storm and frequently situated within or close to a rain-free region at the back of the storm. These dramatic wall cloud photographs were captured by John Denk during Thursday evening's severe thunderstorms, dramatically illustrating why trained spotters across Chicago's southern suburbs were concerned about possible tornado development. John reports the wall cloud was, quite ominously, rotating. Tornadic circulations have been known to occur beneath wall clouds even before a complete funnel is visible. This wall cloud was spotted and photographed by John near 177th and Central Avenue just east of Tinley Park. Many thanks to John Denk for sharing these shots with us!
--By Tom Skilling, WGN-TV Meteorologist
Photos courtesy of John Denk
