TS and CBs - Big No NO
I think this case simply outlines a great misconception about Thunderstorms and Cumulonimbus. If you fly around SE Asia, youll find all the guys there avoid these things like the plague...not only because down on that end of the golfball you CANT fly over them, but also because they spray all sorts of crap everywhere...litterally...
The problem here in Europe is that nowadays aircraft can actually fly over the tropopause, and therefore over what many people incorrectly perceive to be the tops of these things...
But you have to remember, Thunderstorms have got the power of atleast 1 hiroshima bomb dispersed in the space of an hour through the various processes each storm undertakes - the most important of these is the storm's own growth.
NASA aircraft have recorded updraughts of up to 12,000 ft/min in the hearts of small to medium storms, so you can only imagine how these nasties let rip when they get uplifted over terrain (note; the Alps and Pyrennes)
Not only are conditions trecherous inside these storms, but such conditions dont get much different on the top or downwind. The biggest danger is flying over them, because as youll find will happen is before you know it, a massive updraught of cloud from a daughter cell embedded will rise up and swallow you whole - what I think must have happened in this case.
And not only that, hail balls - indeed ranging from the size of golf balls to basketballs - are carried up in the cloud...would you honestly think that with speeds of up to 120 kmph these things will suddenly stop at the top of the cloud and fall back down...
the momentem these storms generates has been known to kick ice debris up to 4,000 to 5,000 feet above the top of clouds.
So, in conclusion...stay WELL clear of CBs and TSs - EVEN if you think you can clear the tops of them.