I shot this on a hike in the Blue Ridge Mountains in North Carolina 3 years ago. I could never get a definitive answer as to what this is...downdraft, formation of a tornado, collapsing cloud? We had gotten Tornado and severe thunderstorm warnings throughout that day.
5/10/14 Severy, Kansas Low-Precipitation Supercell. This was the second low-precipitation supercell of that day, which shortly after taking this photo, produced a brief rope tornado right before the sun set. Best structure I have personally seen in my six years of chasing.