You can sometimes see the mountain waves in the high level clouds, often 50 miles or more downwind. I've seen this in the lee of the San Bernadino mountains in LA when a strong Santa Ana wind has been blowing, and the (severe) turbulence has existed as far as Catalina Island. Makes for interesting flying.
There was a case in the US of a glider / tug combo meeting a mountain wave. The tug disintegrated, the glider landed safely and it is estimated that the g forces exceeded 14g.