A Batik Air Boeing 737-800 aircraft, PK-LBY, experienced severe turbulence while en route between Jakarta International Airport (CGK) and Medan International Airport (KNO). One passenger and one flight attendant were seriously injured. The aircraft departed from Jakarta at 15:25 LT (08:25 UTC). During flight preparation, the pilot noticed the weather forecast for Medan indicated possibility of thunderstorm and visibility 4 km. During descent, the pilot turned the aircraft to avoid clouds and when noticed potential turbulence and the SIC turned the fasten seat belt sign on. At 09:59 UTC, the aircraft encountered severe turbulence and the autopilot disengaged unintentionally. The turbulence lasted for approximately 20 seconds. The vertical acceleration was between 2.334 G and -0.482 G at an altitude of approximately 24,500 feet. The pilot flew manually for 17 seconds, continued descent and reengaged the autopilot. After the turbulence, the flight attendants (FA) check the cabin condition and found one passenger injured in the aft lavatory and FA-5 injured on the aft galley. The aircraft continued descent the pilot contacted the company operation personnel to inform them that the aircraft encountered severe turbulence and two occupants were injured and requested medical assistance on arrival.A Batik Air Boeing 737-800 aircraft, PK-LBY, experienced severe turbulence while en route between Jakarta International Airport (CGK) and Medan International Airport (KNO). One passenger and one flight attendant were seriously injured. The aircraft departed from Jakarta at 15:25 LT (08:25 UTC). During flight preparation, the pilot noticed the weather forecast for Medan indicated possibility of thunderstorm and visibility 4 km. During descent, the pilot turned the aircraft to avoid clouds and when noticed potential turbulence and the SIC turned the fasten seat belt sign on. At 09:59 UTC, the aircraft encountered severe turbulence and the autopilot disengaged unintentionally. The turbulence lasted for approximately 20 seconds. The vertical acceleration was between 2.334 G and -0.482 G at an altitude of approximately 24,500 feet. The pilot flew manually for 17 seconds, continued descent and reengaged the autopilot. After the turbulence, the flight attendants (FA) check the cabin condition and found one passenger injured in the aft lavatory and FA-5 injured on the aft galley. The aircraft continued descent the pilot contacted the company operation personnel to inform them that the aircraft encountered severe turbulence and two occupants were injured and requested medical assistance on arrival.