A PC12 pilot reported temporary loss of control at 26;000 feet due to severe updrafts and downdrafts. Pilot regained control and diverted to a nearby airport.

Narrative

Part 91 Aircraft Owner Pick Up. Trip was planned several days in advance. Flight planned route was ZZZ1-ZZZ. Forecasted Weather Showed a Low Pressure System; IFR to MVFR conditions were forecasted with high ceilings. The forecast held as expected at departure time. Accompanying me was a Commercial Rated CSEL and CMEL Pilot that has been mentored for several months. After an uneventful climb; cruise with mostly smooth air the flight went as expected. Nearing the TOD phase; ZZZ METAR showed VFR conditions. During cruise over the mountains cloud tops visually appeared to be broken to overcast with tops gradually increasing to 20;000. After getting direct ZZZ [VOR] from ZZZ Center; clouds appeared to be non lenticular and did not appear to indicate severe turbulence. Shortly after turning direct ZZZ [VOR]; cloud tops appeared to slowly rise as high as our cruise alt of FL240. Having anticipated a descent through the cloud layer reduced power; speed and activated anti ice equipment. Immediately upon entering the clouds was an increase of airspeed of +10 Knots. Noticing the airspeed gain disengaged the autopilot to prevent an excessive gain or loss of air speed and excessive pitch while in alt hold mode. Airspeed upon entering the cloud was approximately 160 knots IAS. 10 knots above our VA of 150 Knots at our Weight of 9100 pounds at the time of incident. Immediately after the 10 knot increase; an instant airspeed loss of approx. 40-50 knots occurred along with two violent downdrafts. Immediately increased power; and held positive pressure/load on the aircraft. Following the airspeed loss and noted a 200 foot altitude loss followed by a 500 foot gain. Immediately reported the severe turbulence event to ZZZ Center and requested a climb to FL260 to get above cloud tops. Upon leveling off at 260 elected to request vectors to make sure no possible damage to the aircraft. No injuries occurred to both souls onboard. After a brief look at the wings for any warping or unusual damage and glance at interior elected to divert to ZZZ2. However a non structural inboard portion at the top of left wing near the weather seal seemed to look unusual. No unusual flight characteristics occurred. Out of an abundance of caution it was reported to ZZZ Center. Intentions were made to divert to ZZZ2. Weather was VFR there with a long runway and was the closest suitable airport east of the mountains. Upon being handed off to the next sector; ZZZ Center informed us that [priority handling] was provided on our behalf by ATC. A non event flaps up landing occurred at ZZZ2. After emergency vehicles made an inspection; taxied to the FBO. The FBO performed a Severe Turbulence Inspection and found no damage to the aircraft. The inspection included a visual inspection and flight data inspection from the ACMF files of the G Loads from the Aircraft. Highest G load recorded was Positive 1.1 G's with no negative G's being recorded. Inspection was noted in aircraft logbook and signed off. ZZZ Tower called after landing; reported no visual damage and was asked to call back once results of inspection were complete. After the inspection was completed; informed tower that the inspection revealed no damage. Following a stressful event; and further deteriorating weather PIC and pilot elected to return to ZZZ1 the following day. In conclusion the event showed how quickly conditions can change in mountainous areas. Even with seasoned experience flying in mountainous IMC conditions. Quick action and thorough planning made the outcome of the event positive.