The Chinook Arch (a stationary cloud formation downwind of the Rockies) should move E and diminsh this aftn. #mtwx pic.twitter.com/UdfRRjOnBF — NWS Great Falls (@NWSGreatFalls) September 20, 2015 As pointed out in a Tweet from NWS Great Falls (above), a “chinook arch” mountain wave cloud feature had formed in response to strong westerly... Read More