What happened
While en route at flight level 350, the flight encountered severe clear air turbulence. Due to the intensity of the atmospheric disturbance, the crew diverted the aircraft to Reno, Nevada. At the time of the encounter, the seat belt signs were not illuminated.
The investigation
An examination of the flight data recorder was conducted to analyze the impact of the turbulence. Over a 6-second period, the recorded vertical acceleration (Nz) fluctuated between +1g, +1.5g, -0.07g, +1.3g, and +0.86g. Data also showed that the atmosphere at flight level 350 had been stable several minutes before the onset of the turbulence.
Findings
Investigators determined that there were no reports of clear air turbulence from other aircraft in the area, nor was the turbulence included in any weather forecasts. Additionally, the flight crew received little to no warning regarding the impending turbulent conditions.