Date: 201807
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: Mixed
Light: Daylight
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Physical Injury / Incapacitation; General Maintenance Action
Contributing factors: Weather; Human Factors
Primary problem: Weather

Narrative: While at level cruise of FL320; we approached a very narrow and rapidly developing isolated thunderstorm. Enroute controllers typically do an exceptional job of calling out severe weather and no such callout was made to alert of the weather. Equally important is the failure of this cell to appear on airborne radar until effectively no time remained to steer clear of it.I was actually sending an ACARS message to Dispatch to query about enroute weather ahead when I heard the Captain call out an area of weather straight ahead. Initially; he verbalized the weather as yellow; which typically denotes light precipitation; but this was strange primarily because it wasn't showing up on radar prior to such close proximity [and] also the fact that light precipitation doesn't typically exist at such altitudes. Almost like a countdown; the intensity of the weather ahead increased from yellow to red to magenta. From first sign of encountering the weather that led to the upset; was less than 10 NM. The Captain attempted to obtain clearance to deviate; but the frequency was cluttered with other aircraft requiring deviations due to the large number of buildups. By the time the severity of the weather (which led to the event) became apparent; a turn would have been impossible and we concluded that the implications on passengers; staff; and the aircraft would have been far worse if we encountered a severe weather upset while in a bank. For better or worse; we also concluded that the event would be over quite quickly.From start to finish; we estimate that we gained and lost roughly 1;000 feet of assigned altitude. I do know that we came within 300-400 feet and roughly 3-5 NM of [another air carrier] aircraft; who was given a brisk turn (40 degrees) when we were in the downside of the event. The event began with an aggressive altitude gain; followed by an equally aggressive altitude loss. So; the close proximity to the [other] jet was near the conclusion of the event.As soon as aircraft control and flight path/assigned altitude were reestablished; the Captain called the flight attendants. Initial reports were of no passenger injuries and very minor injuries of one flight attendant. The second call revealed more extensive injuries [involving] the two aft flight attendants. Up to this point; the Lead Flight Attendant played down the injuries (based on statements made by the affected individuals) and that we could continue to destination. The third communication revealed '...one flight attendant was 'out of commission';' at which point the Captain made the decision to return to [departure airport].Emergency medical staff awaited the arrival of the flight and the aircraft underwent maintenance inspections following such g-loading forces. The deferral of the No 2 Gen; which was a somewhat complicated deferral procedure for required action items each leg; also restricted operations above FL330. Had we not been constrained by this altitude limit; we would have easily topped this convective buildup. Based on the absence of an alternate; I conclude we could have climbed to at least FL360; if not FL380; based on weight.




Synopsis: A320 First Officer reported returning to departure airport after a Flight Attendant was injured during a thunderstorm encounter.