I think if my ASI was showing essentially 0, my stall warning wasnt going off, and my VSI was pegged (but the plane seemed relatively controllable) I would assume I was stable. Flying into a thunderstorm could (in theory) wreak havoc with your instruments, going from a standard pressure of 29.92 (used for operations above 18k) into what could possibly have been 27.XX pressures. That would indicate a rapid descent and indicate a much lower altitude.