A "towering Cu that will rain itself out".
Not a phrase that I'm particularly familiar with, but I do understand what you're on about. The tops of these particular clouds on the day did not reach up to the anvil level.
I still wouldn't like to fly a light aircraft through or near to them, and now allocate a dollop of faith in the rain radar that these A320 types carry on board. They have the ability to just miss the worst bit.