What happened
Prior to departure, the pilot received a preflight weather briefing that indicated marginal VFR and IFR conditions along the proposed route, with specific precautions regarding thunderstorms. An instrument flight rules (IFR) flight plan was filed, and the aircraft departed as planned.
During the flight, air traffic control radioed information about a convective SIGMET. This weather advisory described a line of thunderstorms extending up to 45,000 feet across the pilot's route. The pilot indicated an intention to continue along the current route and stated he would check back later for further updates.
Approximately 20 minutes later, the pilot was warned that he was about to enter heavy precipitation and was offered an alternate escape route. His final radio transmission was "OK FOUR ONE POP." A few minutes after this transmission, his transponder code disappeared from radar displays.
The aircraft was at 9,000 feet MSL when it experienced inflight separation of both wings.