Originally Posted by Rohirrim
That is a huge mountain wave cloud. Formed by wind hitting the mountain, redirecting it up over the top. As it comes off its blowing the tops of the cloud out. The bottom of the cloud deck is blocked by the mountain so it is staying in tact.
You see these, although much less pronounced over the mountains in Denver when we have downslope winds out of the west. Creates a to of turbulence for aircraft.