Date: 201603
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: Turbulence
Aircraft: B737 Undifferentiated or Other Model
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: Situational Awareness
Second reporter function: Pilot Not Flying; Captain
Anomaly: Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; General Physical Injury / Incapacitation
Contributing factors: Weather
Primary problem: Weather

Narrative: We leveled off at FL270; cruising at Mach .794 with the autopilot off. It was smooth. It was a clear night. After we leveled off; the Captain made his PA announcement to the passengers and turned the Fasten Seat Belt (FSB) off.Center gave us a 'Pilot Discretion' to FL240. At that moment we encountered what we presumed to be a momentary pocket of Moderate turbulence. I believe the aircraft banked approximately 25 - 30 degrees to the right; and the altitude fluctuated from maybe 100 feet up to between 60 and 160 feet down. I leveled the wings and was right back at altitude not thinking anything about it; as it smoothed out as quickly as it came. This was the second flight of the day. The Captain went to the briefing room; I went to the gate. The plane was on the ground; but not at the gate. I used my IPAD to receive and review the flight papers; reviewed WSI (Weather Services International) and loaded the flight plan into Jeppesen FD-PRO. The dispatch weather in the flight papers showed no indication of a turbulent ride; and both airports and the route were actually and forecasted VMC conditions upon planned arrival time. On WSI; after loading the route; I received the WSI briefing. Then I reviewed the WSI Radar picture; FPGs; SIGMETs; Radar Summary and the Rapid Precision Mesoscale (RPM) Forecast Turbulence. The RPM Turbulence forecast showed an area of turbulence at FL270 well south of our route. All indications from WSI; DX weather; and the absence of PIREPS along our route of flight indicated to me a nice flight. Our preflight briefing in the cockpit corroborated what we both reviewed while apart.As the Pilot flying; the takeoff out of ZZZ was VMC; and the climb all the way to F270 was smooth. We were planned with a CI 155 and above 10000 MSL accelerated to 335 knots. I was hand flying the aircraft.Turbulence was momentary. We both thought instantly that it was a wake of an aircraft. I do not recall seeing anything on TCAS and I saw nothing in the sky. The captain inquired me if I was going to start down as he was putting FL240 into the MCP; I stated that it was smooth and that I would wait; and in that moment we encountered another pocket of Moderate turbulence and told him we are starting down. The Captain turned the FSB sign on again and mentioned to check seatbelts. All this happened within 10-15 seconds.At that same instance; I happened to be monitoring the service interphone and I heard a Flight Attendant (FA); ask 'if there is any doctor onboard to raise their hand because we have a severely injured FA on board.' The captain was letting Center know that we were leaving FL270 and reported the turbulence as Moderate to severe. In this same moment; I engaged the 'B' Autopilot and 'LVL CHG' foreseeing what is about to happen and told the captain we have a severely injured FA in the back. He assigned me the aircraft and the radios and stated that we will [advise ATC]. I suggested we report the turbulence as severe and then [advise ATC] which he did to ZZZ Center.All of this took place in less than a minute. It all happened extremely fast and we were back in smooth air out of FL267 on the way down to FL240. I believe present speed was Mach .794 and the aircraft went to Control Wheel Steering (CWS) Pitch; I believe because we were near the barber pole. I overrode the CWS; causing the Autopilot to disconnect and simultaneously pulled the thrust levers back; targeting for a 280 KIAS descent; reengaged the autopilot with a 'VS' pitch mode of something greater than 1500 ft/min rate of descent.Center asked for the status of the FA and our intentions. The Captain and I discussed continuing to ZZZ looking at the distance to the airport from the fix page and it was 105 NM. Exercising CRM; I entered an alternate airport into the fix page and we were about 160 NM away. I agreed with his decision and stated to Center that we are continuing to ZZZ and to have paramedics and ambulance standing by; because we have a severely injured FA onboard.The captain informed me that he was going to call dispatch and that he will be off; and I said to call him using 'call me MD'.We were handed off to the next controller; and upon checking in I confirmed with him that he knew we were an emergency aircraft with a severely injured FA onboard. I did let the Captain know that I plan on max speed all the way to ZZZ. We were informed by the lead FA that the injured FA will need to go to the hospital right away. I relayed this to Center. Because of her injuries and under our emergency authority; we did not comply with the FAR restriction of 250 KIAS below 10000 MSL. We were cleared to 11000 MSL than to 4000 MSL. With approach control; we were asked if we want straight in to the runway. I asked for the winds; '210/6'. I said yes. We were cleared discretion to 2000 MSL and pulled the runway data via ACARS. When the Captain was done with his communications I informed him of being cleared straight in and to reference the ACARS printer for the landing data. We briefed for the ILS; and ran the approach descent and approach checklists. The ILS was not operational; but we used FMC map mode and performed a VMC landing in VMC conditions. Approach lights and PAPI were operational.The ambulance and police were standing by the Gate upon landing. I believe there were 3 additional passengers who also required medical attention according to the FA's during debrief.


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


Synopsis: During flight a B737 encountered severe clear air turbulence which caused an injury to a Flight Attendant. The flight crew advised ATC and landed at the destination airport.