Date: 201702
Local time of day: 1201-1800
Location: KZAK.ARTCC, HI
Flight conditions: IMC
Weather elements / visibility: Turbulence
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A KZAK
ATC / advisory: Center KZAK
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Speed All Types; Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: General Physical Injury / Incapacitation
Contributing factors: Weather
Primary problem: Weather

Narrative: Our flight was originally scheduled to ZZZ.  We incurred an extensive maintenance delay.  Due to crew legality issues; we were routed to HNL; to continue HNL-ZZZ after laying over in HNL. On our HNL-ZZZ segment on; approximately 1:10 into the flight (80 nm west of CEBEN); we encountered a sudden-onset jolt of moderated turbulence.  At the time; the seatbelt sign was on and the flight attendants had been seated in their jumpseats. Flight attendant was seated on the aft jumpseat. During the jolt; she fell from her jumpseat to the floor. After the jolt subsided; we called the flight attendants to check on them and the passengers.  No passengers were injured.  The flight attendant who fell down reported that she felt she had bruised her elbow and shoulder during her fall.  She said that at the moment of the jolt; she had been reaching for the interphone to call the lead flight attendant to discuss another matter.  While doing so; she was in an awkward position and was not fully strapped into her jumpseat harness.  She reported that the combination of her not being in the harness; being in an awkward; unbalanced position; and the airplane encountering a sudden-onset jolt; led her to be knocked off balance and fall from her jumpseat.We continued to check on the injured flight attendant as the flight progressed.  Eventually we exited the turbulence; and the other flight attendants prepared to begin their service.  Prior to them initiating service; we held a joint discussion among all crewmembers and determined that the injured flight attendant would not participate in her normal duties; but rather ice her injuries.  We also determined that she desired to have medical personnel meet her when the flight arrived in ZZZ.  She did not desire further medical help during the flight. We initiated a phone patch with Dispatch to report the situation and request that medical personnel meet the flight in ZZZ to assess the flight attendant's injuries.We consulted weather/turbulence information from several sources prior to and during the flight.  The weather information in our release showed a forecast for light to occasional moderate turbulence between FL300 and FL400 in this location at this time.  The SABRE package showed no turbulence PIREPS for our cruise segment.  The app on our IPAD also showed occasional moderate turbulence during this time window: one extending south/west of the event location from FL300 to 400; and the other in the vicinity of CIVIT from FL320 to FL380.  Furthermore; the app forecast light-to-moderate turbulence at FL285 until approximately halfway between CEBEN and CIVIT; and those same models showed that 'higher was worse' for the first half of the flight.Based on these forecast and reports; we requested an initial altitude of FL290 while on the ground in HNL.  We reported this altitude to dispatch via ACARS prior to entering oceanic airspace; and dispatch concurred. We kept the flight attendants in their jumpseats throughout the climbout due to reports of moderate turbulence from Honolulu center.  Upon arriving at FL290; we experienced continuous light chop; with occasional brief moments of borderline moderate.  We advised the flight attendants to remain in their jumpseats and made a PA advising the passengers to remain seated with seatbelts fastened. Another aircraft was on the same airway; about 10 minutes ahead of us; at FL320.  Upon entering Oceanic airspace; we contacted them and requested ride information.  Initially; both aircraft were experiencing continuous lite chop with brief moments approaching moderate.  Around 50 minutes into the flight; the chop seemed to decrease in intensity but not disappear entirely.  We queried the aircraft; and they reported that they were nearly smooth.  We called the flight attendants and asked them how they felt about the ride in the back.  They indicated that they still didn't feel that it was smooth enough to begin their service; and that they would remain intheir jumpseats.We were in a thin layer of clouds during daytime; the ocean surface was visible out the side windows.  I had previously set my radar to 160 nm range and 1 degree down to scan for weather; none was observed.  We continued to have visible contact with the surface throughout the brief turbulence; based on these facts; I am certain that the turbulence was not related to either convective weather or a small 'build-up.'  Our release package showed a 140 kt jetstream crossing our flight path in this area at FL390; well above our altitude of FL290.  Our winds at the time were approximately 250 at 70 knots; I did not observe any change to either wind direction or velocity during the turbulence event.  The tropopause was forecast to descend from FL560 to FL400 in this area--again well above our altitude.  I observed no change in our OAT during the event. During the turbulence; we experienced a momentary overspeed of approximately 5 kts above Vmo for approximately 2 seconds.  We made the necessary electronic logbook writeup. We asked the other aircraft if they had had any significant turbulence or 'jolts' in this area.  They reported that they had not.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported a turbulence jolt at cruise attitude that injured a Flight Attendant.