Flight Crew Air Transport Pilot (ATP)
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control
Weather
Our cruise altitude was flight level 350. Our speed was .81 mach. We were on an Eastbound track. Flight conditions were IMC; obscured hazy layer; no radar images; and no other reports of turbulence on the track. Severe turbulence was experienced for 2-3 minutes. The autopilot disengaged; speed fluctuated 15-20 knots fast and slow. Our speed went 4-5 knots into red speed tape and we an received aural warning for approximately 3-4 seconds. The stick shaker was intermittently engaging during the event. We had an altitude loss of 250 feet due to the performance of the aircraft. No engine parameters were exceeded. The Captain was pilot flying and I was pilot monitoring. I was calling out airspeeds; altitudes; pitch and roll during the event. After the event contacted [Company] and maintenance. We issued a PIREP on 123.45 common radio and requested climb to FL360 via CPDLC. A logbook entry was entered for severe turbulence. Severe Turbulence was encountered. There were no signs of severe turbulence prior to event. I do not think this event could have been avoided. We did not have any warnings prior to the severe turbulence encounter. After the Captain and I debriefed the event we both had the immediate same thought.
[Report narrative contained no additional information.]
Air carrier flight crew reported encountering unexpected severe turbulence in cruise causing an overspeed warning alert.