Mountain wave clouds are produced by the winds at or near the ridgetops blowing perpendicular to the mountain chain. For example, a wind from the west blowing across the north-south oriented Cascade Mountains will at times produce mountain wave clouds east of the mountains.

The wave clouds can have various forms. Some of the more impressive forms are known as Altocumulus Standing Lenticular, or ACSL. These often take the form of pancakes, or even stacks of pancakes, and at times have been mistaken for unusual aircraft.

On the 20th of April, the weather pattern was rather unusual, with strong easterly winds blowing over the Bitteroot Mountains of north Idaho. The resulting ACSL clouds were a rare site for the Spokane area.