Background
The flight was operating under a restriction to maintain an altitude of FL250 because of an inoperative pressurization component. At the time, AIRMET 13C was in effect, indicating a line of severe thunderstorms with tops exceeding FL450 extending along and to the north of the planned flight route.
Incident Details
In response to the thunderstorm activity, the flight crew deviated to the south. Despite this deviation, the aircraft experienced moderate to severe turbulence on three separate occasions within a span of approximately ten minutes. Each encounter lasted between five and ten seconds. The crew characterized the turbulence as clear air turbulence, as the aircraft was not within clouds.
The captain reported that the aircraft was clear of clouds, no turbulence had been reported by air traffic control, and neither the aircraft's radar nor ATC radar showed any returns along the flight path. The nearest thunderstorm cell was estimated to be 35 to 40 miles away.
Injuries
One flight attendant, who was in the cockpit during the turbulence, reported sitting on the cockpit floor and holding onto armrests for stability. After leaving the cockpit, she responded to a call from another flight attendant who had been seriously injured during the incident.
Conclusion
The source provides no further details on the investigation or causal factors. The flight encountered multiple episodes of turbulence while deviating from thunderstorms, resulting in at least one serious injury to a crew member.