I was watching the dark clouds roll by and noticed an area that began to lower.  I couldn't quite tell what it was as it only lasted about 5 minutes.  Here are some vidcaps showing it lower, expand into what almost looks like a non-rotating wall cloud, then retract and dissipate:

Visible bands in the tower as it rises straight up.
Cloud lowering appears.
Zoomed in on lowering.
Formation continues to lower...
Formation spreads out at bottom.
A few seconds later.
Grows thicker.
Retracts back up...

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