Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 60; Flight Crew Type 60
Communication Breakdown; Situational Awareness
Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
General Physical Injury / Incapacitation
Human Factors; Weather
The B and C flight attendants were injured during the arrival phase of flight due to unexpected turbulence on Aircraft X. I briefed the flight attendants before boarding began about the convective weather that was moving through the area. Convective weather and turbulence was forecast for most of the route until north of ZZZ. I instructed them to remain seated until we called and expected a bumpy ride for the first hour of flight. After passing the ZZZ area; the ride became acceptable and I communicated with the flight attendants that it was safe to start service. The cabin in-flight service was performed and the flight conditions became smooth enough to allow the seatbelt sign to be turned off.We began the arrival phase flying th RNAV Arrival. The First Officer made a passenger PA at about FL230 and turned the seatbelt sign on. After passing FL180 I chimed the flight attendants to make their final walk through. Establishing contact with Approach; they reported areas of widespread rain showers in the area; but no reports of turbulence on the arrival. There were no reported turbulence plots in the weather packet north of the ZZZ area. The weather radar was operating and only showed a very small green return about two miles in diameter a few miles past the ZZZZZ intersection. I could visually see the area of the radar return. It was a single cumulus cloud with tops to about 16;000 feet. I did not see any rain falling underneath the build-up.The flight path on the ND showed our arrival track passing clear of the radar return on the southeastern side. After making the turn at ZZZZZ; I could see that our flight path was going to pass along the very outer edge of the cloud build-up and started to maneuver the aircraft further away. Given the short flying distance; I was unable to confirm the flight attendants were already seated. We just grazed the edge of the cloud build-up causing a momentary sharp jolt. I would estimate the intensity to be moderate lasting about two or three seconds. Beyond that point; the flight conditions were smooth.I asked the First Officer to call back and make sure the flight attendants were okay. The A Flight Attendant was seated and was unharmed. The B Flight Attendant had been standing in the aft galley and had been knocked to the floor. The C Flight Attendant who had been sitting in a passenger seat had hit his head on an overhead reading light causing a small cut on his forehead. A deadheading Flight Attendant assisted the B Flight Attendant in the aft galley while the C Flight Attendant remained in the passenger seat. The A Flight Attendant requested that we have Medical Personnel meet us at the gate.At that point we were at about 11;000 feet and 7-10 minutes from landing. I asked the First Officer to call the station directly and have paramedics meet the aircraft. An emergency was not declared. We landed uneventfully and taxied to the gate. Paramedics came onto the aircraft and helped the two flight attendants deplane. At that point; the A Flight Attendant said that a passenger not wearing their seatbelt had possibly struck their head and was requesting medical attention. Paramedics evaluated both flight attendants and the passenger. The passenger and flight attendants declined further medical attention. I contacted Dispatch and asked for the Chief Pilot on Call. I explained the situation and spoke with the Supervisor on Duty  and provided some basic details for event reporting requirements.My error was not seating the flight attendants earlier. Given our close proximity to building cumulus clouds on the arrival; there was a chance of possible turbulence that we should have expected. While we did a good job securing the cabin in the areas that were obviously convective; we let our guard down and got too close to a build up that seemed benign.
Until the time of the event; the descent into destination was uneventful. The passenger arrival announcement was made passing 23;000 feet; and at that time the seatbelt sign was turned on. The descent was flown in VMC and was smooth. Visually; some scattered summer buildups (cumulus) were observed. On the descent around 17;000 feet; on the [RNAV] Arrival; prior to the turn at ZZZZZ; a small cumulus cloud was visually observed at about 2 o'clock relative to the aircraft nose. The weather radar showed minimal return at that location with a small green dot a couple of MFD pixels in size. The small radar return appeared to be to the right (south) of the magenta (course) line suggesting that we would pass the cloud to the north.After the turn at ZZZZZ; now direct to ZZZZZ1; however; the cumulus cloud appeared at our twelve o clock position. The cumulus cloud appeared benign; the top reached about 17;000 feet no rain or virga was observed beneath the cloud and the radar continued to display only green returns. The Captain (Pilot Flying) then pressed the ATTEND button (dinged the flight attendants) and for comfort elected to not cross the cloud exactly in the center; but deviated a couple of degrees to cross the cloud at the outer edges.The cloud was entered around 12;000 feet and initially only some light chop was encountered. Then suddenly; just prior to exiting the cloud; a sudden; burst of moderate turbulence was encountered that lasted for only one second. The event passed almost as soon as it started and we immediately exited the cloud on the other side and entered completely smooth air once again. I made a quick call to the A Flight Attendant to make sure that the Cabin Crew was okay Initially the Flight Attendant informed me that they were okay and that they were all seated. However a couple of minutes later; the Cabin Crew called back and told me that one of the aft flight attendants was up during the event while the other aft Flight Attendant was in seat but without seatbelt on. I was then informed of possible injuries. At that time we were still on the arrival; just prior to RAVNN; so we continued while I called Station Ops informing them that we had Flight Attendant injuries and that we needed medical services at the gate. A few minutes later; we landed uneventfully and taxied to the gate where we were met by medical personnel.First; crews must not underestimate vertical (cloud) buildups no matter how benign they initially might seem. During summer activity; clouds can quickly and suddenly increase in intensity; therefore; we cannot completely rely on radar returns to assess vertical cumulus activity and severity. In this case most of the cloud had limited vertical activity except for one small; and therefore undetected region. Second; we must not assume that the cabin crew is seated with seatbelts fastened. A quick call to the back is all that is needed to quickly remind everyone to check seatbelts and to ensure everyone is seated.
Air carrier flight crew reported encountering moderate turbulence while flying through a cumulus cloud; which resulted in injuries to cabin crew.