Please note that this is NOT valid when climbing or descending
IOW, it is completely unenforceable
Same as in the UK.
Even well above mountain tops one can get caught in heavy sinking air (20 kts plus) and getting into wave rotor can be like being inside a tumble dryer with total loss of control.
What sort of wind speed would one need to see a 20kt vertical speed (2000fpm)?
I suspect that the winds produced in the Alps on a
day like this or this would result in virtually no vertical airflow.
This would be different (look how close the isobars are over the Alps). The troughs are just a bonus, for character building
However, I did the flight UK-Croatia at FL190 and, from my
writeup
The effect of the ~ 20kt northerly wind flowing over the mountains was only just felt at the level flown, with no turbulence most of the route and very light turbulence near the end.
so clearly altitude helps a lot. FL190 was about 8000ft above the general terrain.
Similarly, on
this trip, I was about 7000ft above the terrain and felt almost no turbulence despite a >30kt wind flowing straight across the Pyrenees.
This excellent leaflet from New Zealand is worth a read.
There is a very rough rule of thumb which goes something like: for every 10kt of wind speed across the ridges, you want to be 1000ft above the terrain, to keep the vertical speed of any downdraught below 200fpm. I think that is probably about right for low wind speeds. Above 30-40kt wind speeds, I reckon things are going to get a lot more exciting.
But in this case we don't know the aircraft type so, as usual, chewing away on insufficient information.