Gallery of Outflow
Wall or Shelf Cloud?
[Part 1 of 4] A shelf cloud develops in piecemeal fashion as a high-based supercell becomes outflow dominant. The low-pressure area internal to the storm still was forcing rotation--in this case, of that ragged cloud lowering. If that feature looks like it can't make up its mind whether to be a wall cloud or a shelf cloud, there is good reason. It was rotating, but also, being undercut by cold outflow. The circular moat of texture in the surrounding cloud base is a sign of that rotation, even in a still image such as this, and indeed was turning slowly. Soon it would become part of a classical, forced-ascent driven shelf. In the distance, we see the wide cloud base of another supercell soon to be plowed by outflow.
12 NNW Sheridan Lake CO (18 Jun 11) Looking WNW