THE FLIGHT ENCOUNTERED TURBULENCE WHILE DESCENDING THROUGH 11,000 FEET ON INITIAL APPROACH TO MIAMI, FLORIDA. THE AIRCRAFT WAS OPERATING NEAR A LEVEL TWO RADAR ECHO AND THE CAPTAIN REPORTED THEY WERE CROSSING A NARROW BAND OF CLOUDS AND WERE IN CLOUDS AT THE TIME OF THE TURBULENCE ENCOUNTER. AFTER EXITING THE CLOUDS THE CAPTAIN OBSERVED A THUNDERSTORM CELL 7 TO 8 MILES SOUTH OF THEIR POSITION. NO SIGNIFICANT WEATHER WAS SHOWING ON THE AIRCRAFT'S RADAR AT THE TIME OF THE ENCOUNTER ACCORDING TO THE CAPTAIN. A FLIGHT ATTENDANT WHO WAS EXITING THE AFT LAVORATORY WAS LIFTED OFF THE FLOOR DURING THE TURBULENCE ENCOUNTER AND SUSTAINED AN ANKLE INJURY WHEN SHE LANDED. SHE WAS WEARING 2-INCH HIGH HEEL SHOES AT THE TIME OF THE ACCIDENT. SHE STATED THE SEATBELT SIGN WAS ON AT THE TIME AND ALL PASSENGERS WERE SEATED. THE COCKPIT CREW HAD MADE NO ANNOUNCEMENTS FOR FLIGHT ATTENDANTS TO BE SEATED.