Horseshoe Vortex Cloud
Confirmed by our local meteorologist.

I photographed this cloud at the Rocky Mountain Arsenal Wildlife Refuge, on an extremely windy day with the Rocky Mountains in the background and a large storm cloud moving over the mountains.
The horseshoe vortex cloud is a very rare meteorological phenomenon and looks like an upside-down U shape.
It requires wind shear, thermal updraft, and a cumulus cloud and it dissipates quickly.

United States National Weather Service (2018) quote:
"As the updraft pushes flattish cumulus clouds up & a horizontal vortex develops from differential updraft speeds... As the vortex climbs, it's caught in the faster horizontal winds aloft, & the middle part of the vortex catches the faster speeds with the ends being slower."

As rare as this cloud is, a neighbor saw & photographed another horseshoe vortex cloud around noon on the same day that I photographed this one at 5:30 pm.