Mountain Waves (MTW)
Mountain waves occur when air is being blown over a mountain range. As the air hits the upwind side of the range, it starts to climb, thus creating a smooth updraft which turns into a turbulent downdraft as the air passes the crest of the ridge. From this point, for many miles downwind, there will be a series of down- and updrafts accompanied by moderate to severe turbulence.
Source: ICAO
See also:
Downdraft and Microburst (MB)
Mountain Waves (MTW)
Thunderstorms, CB / TCU
Turbulence Grade Classification
Clear Air Turbulence (CAT) Encounter
Wind Shear
Link to an explanatory article on Wikipedia here