Frontier Airlines flight 1369 experienced convective turbulence during the descent from cruise altitude to land at Dallas-Fort Worth International Airport (DFW). The captain stated they were briefed and were expecting a mostly smooth flight from Chicago Midway International airport to the Saint Louis area, and then anticipated a “bumpy ride” the remainder of the flight to the DFW, and those were exactly the conditions they encountered. He said that about halfway through the flight it became rough, and they were allowed to deviate around some convective cells to get a smoother ride. Air traffic control (ATC) also gave an additional reroute to transition south of the weather and return to the same arrival transition east to DFW. The captain said that the ride became smoother, so he turned off the seat belt sign as the flight was descending on the arrival at DFW. During arrival, the flight was given a descent to flight level (FL) 240 and told to maintain 290 knots or better. It was mostly smooth, so the captain kept the seat belt sign off, but as the descent progressed, they started getting some light chop, and all of a sudden experienced “one big, unexpected bump”. The captain said he immediately turned the seat belt sign on and made a call to the flight attendants (FA) to get seated. He said the “A” FA told him that one crew member fell and hit her head and hip and was in pain and unable to get up. The first officer (FO) stated they were instructed to descend from FL280 to FL240 and during the descent the airplane flew through a cloud that looked “light with no apparent indications of adverse conditions”, but upon entering the cloud the ride went from smooth, to light, to moderate turbulence. There had been no pilot reports (PIREPS) prior to entering the descent, and the flight attendants had been preparing the cabin for landing and were about to be seated. The FO recalled that the seat belt sign was on throughout the descent. The FO said he immediately got on the public address (PA) system and told the flight attendants to be seated, and shortly thereafter they were informed that the “D” flight attendant had been thrown around and was injured. The flight crew informed ATC of the medical event, requested priority handling, and asked for paramedics to meet the airplane at the gate. ATC declared an emergency for the flight crew. The injured flight attendant was on the galley floor and could not get up. She was assisted by another flight attendant and two passenger nurses, and the flight landed otherwise uneventfully. After landing the injured flight attendant was given medical care and diagnosed with a broken hip.