Southwest Airlines flight 136 encountered turbulence while descending through 15,600 ft into Denver International Airport (DEN), Denver, Colorado, and a flight attendant fractured her ribs. During the brief for the flight preceding the accident flight, the captain notified FA ?A? that he would be requesting that the FAs take their seats about 25 minutes before their arrival at DEN. The captain stated that this was a routine part of his briefings for flights into DEN due to the unpredictable weather. FA ?A? stated that she had been busy with her duties and about 4 hours had passed since the briefing. Subsequently, she had forgot to share this information with the other FAs. About 35 minutes before landing, the captain asked the cabin crew to clean up the cabin and take their seats. All three FAs located vacant passenger seats in the cabin and were seated. When the captain ?double chimed? the cabin at about 16,000 ft (indicating the cabin should be secured early), all three FAs stood up to return to their jump seats. Shortly thereafter, the airplane experienced moderate turbulence. FA ?B? reported that as she was walking through the aisle, the turbulence occurred, and the airplane ?dropped?. She went into the air before falling onto a row of seats. As she attempted to sit in one of the seats, the airplane dropped again, and she fell to the floor. Following the turbulence, the first officer stated that he gave a pilot report (PIREP) to air traffic control at DEN and the flight proceeded to its destination and landed without further incident. After landing, the flight crew was informed that FA ?B? required medical attention. Emergency medical services arrived and transported FA ?B? to the emergency room where she was diagnosed with multiple rib fractures. A post-accident review of meteorological conditions showed no significant meteorological information (SIGMETs). Upper air model data at the time of the accident revealed limited wind shear around the accident altitude. Weather radar data from the National Weather Service station located in Watkins, Colorado (located 6 nautical miles southeast of DEN) showed no reflectivity that suggested convective activity or other precipitation.There was one applicable PIREP from about 85 minutes earlier that reported moderate turbulence between 10,000 ft and 13,500 ft (MSL) about 35 miles east-southeast of the accident location near DEN. Analysis of the aggregate weather information suggested the turbulence was likely mechanical in nature and caused by significant surface winds in the vicinity of mountainous terrain. Probable Cause: The flight?s encounter with mechanical turbulence.