WHILE ESTABLISHED IN CRUISE FLIGHT CONDITIONS THE COMMERCIAL AIRLINER ENCOUNTERED AN AREA OF FORECAST THUNDERSTORMS. THE CAPTAIN ELECTED TO ATTEMPT TO FLY THROUGH THE AREA. AS THE AIRLINER WAS ATTEMPTING TO AVOID THE THUNDERSTORMS IT ENCOUNTERED ONE RAPID UP AND DOWN DRAFT. THIS CAUSED 2 SERIOUS INJURIES AND 19 MINOR INJURIES. THE FLT CONTINUED ON TO THE DESTINATION AND WAS MET BY MEDICAL PERSONNEL WHO TREATED AND EVACUATED THE INJURIED. VARYING ACCOUNTS WERE RECEIVED FROM PASSENGERS AS TO WETHER OR NOT THE SEAT BELT SIGN WAS ILLUMINATED PRIOR TO ENCOUNTERING THE TURBULENCE, HOWEVER, THE CABIN CREW REPORTED THAT THE LIGHT WAS ILLUMINATED AT THE TIME AND A SEAT BELT CHECK HAD BEEN MADE. NEITHERTHE FLT ATTENDANTS NOT THE PASSENGERS REPORTED HEARING A PA ANNOUNCEMENT PRIOR TO THE TURBULENCE. ONE FLT ATTENDANT WAS INFORMED BY THE FLT ENGINEER THAT THEY WERE APPROACHING AN AREA OF ROUGH WX BUT THIS INFO WAS NOT PASSED ONTO THE REMAINING CABIN CREW MEMBERS.