Read somewhere that the worlds worst turbulence is over Japan.
This was well documented many years ago when the US dropped the atom bombs. The weather patterns were examined in great detail before the attack and possible severe turbulence was one of the problems.
Once when flying from Krakow to Warsaw in 735. The cabin crew were collecting the rubbish during decent at about 20,000ft. Seat belt signs were on.We went into a cloud and all hell broke loose. The crew were spreadeagled in the aisles holding the seat frames, pax were screaming, we were thrown about like rag dolls with no chance to stay on the seats.
It lasted for about 5 minutes but seemed like hours.
I was not happy as I had to go back up in an ERJ to Milan but it was ok.