The Boeing 737-800 encountered turbulence while descending from 37,000 feet to 24,000 feet during an arrival to Miami International Airport (MIA), Miami, Florida. The pilot flying (captain) stated that 26 minutes before the scheduled landing he asked the flight attendants to prepare the cabin earlier than usual, due to "weather south and west of MIA and along our route of flight." Ten minutes after the announcement the airplane encountered turbulence with altitude changes of plus or minus 500 feet within a few seconds. The pilot reported that both of the airplane's weather radar systems were turned on, and were scanning at 40 and 80 miles range respectively. He reported that the returns showed no convective activity in the immediate area. The flight attendants reported preparing the cabin for arrival, per the pilot's request. Two flight attendants were finalizing their duties in the aft galley when the airplane encountered turbulence and they were thrown to the ceiling. After they came down they attempted to open the jump seat and secure themselves, but again were thrown to the ceiling. During the sequence, one of the flight attendants sustained a fractured ankle.