Is it certain that unforecast CAT was the culprit? Often the sudden turbulence is the result of the crew failing to pick an undetected Cb top - unseen because the radar tilt was not optimum, or maybe the gain was in auto at high altiude instead of max gain. Max gain and judicious use of tilt is vital at high altitude to pick up the slightest hint of moisture which could give a clue to something nasty at the cruising level if at night or in cloud. And sometimes even the range is not been monitored closely by the crew because of preoccupation with other matters.