It was interesting watching that weather sort of get stuck there and not move forward, eventually it did... But for a while it seemed like it was unable to. I'm guessing you're right and that it was some kind of pocket of dry atmosphere. Wish I understood how all that worked better, it's almost like there's a vortex or something where certain areas are charged differently for whatever reason. Whatever the case, it sure was a pretty show!
Being on the edge of the Colorado Plateau and the associated 2500 foot elevation change has a lot to do with it. I find it all fascinating and a real show to watch.