THE CAPTAIN REPORTED THAT AT THE BEGINNING OF THE DESCENT FOR LANDING HE TURNED ON THE SEATBELT SIGN AND INFORMED THE CABIN CREW TO SECURE THE CABIN EARLY BECAUSE OF SUSPECTED TURBULENCE. HE DID NOT URGE THE FLIGHT ATTENDANTS TO BE SEATED IMMEDIATELY, BECAUSE NONE OF THE WEATHER INFORMATION HE RECEIVED AT THE TIME INDICATED ANYTHING OTHER THAN THE POSSIBILITY OF LIGHT TURBULENCE. ABOUT 15 MINUTES LATER, WHILE DESCENDING THROUGH FLIGHT LEVEL 200, THE AIRPLANE BRIEFLY ENCOUNTERED WHAT THE CAPTAIN DESCRIBED AS MODERATE TURBULENCE. ONE OF THE FLIGHT ATTENDANTS WAS STILL SECURING THE CABIN AT THE TIME AND FELL BACKWARDS ONTO A STOVE IN THE GALLEY, SUSTAINING A FRACTURED SHOULDER. WEATHER INFORMATION RECEIVED AFTER THE ACCIDENT REVEALED THAT MODERATE TURBULENCE OR SIGNIFICANT WEATHER REPORTS WERE NOT FORECAST FOR THE ALTITUDE OF OCCURRENCE BY THE NATIONAL WEATHER SERVICE. Probable Cause: AN UNEXPECTED ENCOUNTER WITH UNFORECAST MODERATE HIGH LEVEL WINDSHEAR AND TURBULENCE AS THE FLIGHT ATTENDANT WAS PERFORMING HIS DUTIES DURING DESCENT.