Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Total 20000
Situational Awareness
Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
General None Reported / Taken
Weather
Turbulence report flight from ZZZ to ZZZ1. I informed the flight attendants I didn't want to take chance and for them to stay seated for the beginning of the fight. After leveling off 28;000 feet and looking ahead the flight looked smooth. I informed them they could get up and perform service. Approximately about ten minutes later ATC informed us of a small build up that was ahead. So we both looked and did not see it; other than a bunch of haze due to looking into the sun and some stratus clouds that looked like they were below us. We asked ATC for 30;000 feet due to what he said; then we turned the radar on and by the time this was done you could see it was straight ahead and building fast. ATC said we could make a right turn; so I started climbing and made a right turn; but it was growing quite rapidly so I turn the autopilot off to help try to avoid this small build up and we caught just the tops of the clouds. That's when we encountered moderate turbulence; so I contacted the flight attendants as soon as possible to get an evaluation of what their condition was. They informed me that two had fallen and hurt their legs and the cart had fallen on its side. I asked them if they needed medical assistance and if we needed to land to get medical assistance; they said no. I checked back with them every 15 minutes to make sure they were still OK. I asked again if they needed medical assistance they said no; that they were just going to rest for a few. I informed dispatch of the situation. Also prior to landing in I ZZZ1 contacted operations to inform the flight attendant supervisor on call to meet the aircraft at the gate.  Clouds were deceiving not normal that stratus clouds will have this type of outcome .99.9% of the time it's always a cumulus clouds.
Pilot reported moderate turbulence that caused injuries to flight attendants.