A bit more on mountain wave. In some circumstances, a localised area can have wave that has a vertical component the same as the wind speed. Unlikely though this might seem (need a diagram here – can’t find one to post), successively higher layers can pile up on each other, with a shorter local wavelength and more bending of the streamlines from horizontal to a steeper angle, until at a certain point they go vertical. In the lee of a Scottish Mountain, Morven (Aberdeenshire) on a day with about 10 knot wind, we found local lift of almost 10 knots for a while. Mountain waves are not always the classic sine wave shape.
This might be one explanation why even airliners experience up or down drafts which seem disproportionate to the general wind speed. As far as I know, however, this extreme is only found where the mountain shapes are right for it and at the same time the airmass characteristics (wind change with height, stable layer over unstable, etc.) lend themselves to this phenomenon.
I don’t know enough met to say if it could happen elsewhere than particular spots. I certainly don’t know what happens near jet streams.
It may be that the professionals don’t yet know that much, either. A lot of early work on wave came from gliding exploration of it, before the soundings and analysis/theories followed. For obvious reasons, there is not much gliding exploration of wave over the sea.
Chris N