AN MD80 EXPERIENCES UNEXPECTED SEVERE TURB RESULTING IN A CABIN ATTENDANT INJURY.

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WE FIRST FLEW INTO DSM FROM THE W AND THEN ABOUT AN HR LATER DEPARTED FROM DSM. WE HAD BEEN CLRED DIRECT TO DVC AND TO CLB OUT FROM FL310 TO FL350. THERE WERE VISIBLE CLOUDS TO OUR FRONT AND EXTENDING TO THE SW. THE RADAR SHOWED PRECIPITATION (GREEN) ABOUT 25 MI TO THE W; SW OF OUR POS. WE REQUESTED TO DEVIATE TO THE N (R OF OUR COURSE LINE) TO AVOID WHAT I THOUGHT MIGHT BE SOME LIGHT BUMPS AS WE CONTINUED OUR CLB TO CRUISE AT FL350. WE WERE CLRED TO DEVIATE AS NECESSARY TO THE N. ABOUT 30 SECONDS AFTER ENTERING THE CLOUDS; WITH NO PRECIPITATION AND NOTHING ON THE RADAR WE STARTED TO GET POUNDED BY TURB AND EXTREME UPDRAFTS. THE VERT SPD WAS PEGGED AT THE MAX OF 6000 FPM UP FOR SEVERAL MINS. THE UPDRAFTS WERE LIFTING 1 SIDE OF THE ACFT AND THEN THE OTHER. THE AUTOPLT WAS KNOCKED OFF ALMOST IMMEDIATELY AND I WAS HAND FLYING THE ACFT. THE ACFT ROLLED INTO AT LEAST AN 80 DEG BANK AT ONE POINT. THE FO MADE AN ANNOUNCEMENT TO TELL EVERYONE (INCLUDING THE FLT ATTENDANTS) TO TAKE THEIR SEATS. HE ALSO ADVISED ATC THAT WE WERE ENCOUNTERING SEVERE TURB AND THAT WE WERE TURNING EVEN FURTHER TO THE R TO HOPEFULLY MOVE AWAY FROM THE TURBULENT AIR. THE ACFT WAS SHAKING SO VIOLENTLY THAT IT WAS DIFFICULT FOR THE FO TO TALK ON THE RADIO. WHILE KEEPING ATC AND THE CREW INFORMED THE FO ALSO WAS KEEPING ME INFORMED OF AIRSPD READOUTS; ACFT HEADING AND BANK ANGLE. AFTER ABOUT 5 MINS WE EXITED FIRST THE CLOUDS AND THEN THE TURBULENT AIR WELL TO THE NW OF OUR ORIGINAL POS. THE SEATBELT SIGN HAD BEEN ON DURING THIS EVENT AND ONLY 1 PAX WAS OUT OF HER SEAT IN THE LAVATORY. SHE HAD EXITED THE LAVATORY EARLY AND STRAPPED INTO THE FIRST AVAILABLE SEAT. ONE OF OUR FLT ATTENDANTS TWISTED HER BACK WHILE TRYING TO KEEP A SERVING CART ON THE GND. THE SERVING CARTS WERE TOSSED INTO THE AIR SEVERAL TIMES DURING THE ENCOUNTER WITH THE TURBULENT AIR. AFTER THE EVENT THE FLT ATTENDANTS SPOKE WITH EACH PAX AND FOUND THAT THERE WERE NO INJURIES TO ANY PAX. I USED COMMERCIAL RADIO TO RPT THE INCIDENT TO OUR COMPANY DISPATCH AND MAINT CTL.