"severe and unexpected turbulence".
I assume therefore we are not talking about a Cb, but CAT?
If so, it is, of course over-forecast [or certainly used to be when I hung up the crystal ball and fir-cone with seaweed] but "unexpected" does surprise me.
Do not modern aircraft have on-board detection systems. If so, how good are they?
I ask in all sincerity, as I used to lecture on CAT to the Met Office College and also the RAF but am sadly out-of-date with developments.
As for seat belts, I know enough about CAT to always use mine except for brief trips to the lavatory.