Generally speaking in these situations it is pilot error to fly into an
area of known thunderstorms. And they will blame him, appropriately,
even if he didn't know, but it was forecast (you're supposed to get a
briefing before you go). There is a *small* possibility that the
thunderstorms were not forecast from weather that built up very
suddenly.

The NTSB is chock full of accident findings whose cause is pilots
venturing into to bad or deteriorating weather conditions. But you
would like to think that a pilot of this stature would not make such a
foolish mistake. The rest of the facts will become known soon enough.

Tom