Spirit Airlines flight 2037 encountered moderate turbulence during descent into Nashville International Airport (BNA), Nashville, Tennessee. One flight attendant (FA) was seriously injured and there were no passenger injuries. According to the flight crew, they became aware of developing weather in the terminal area during cruise at FL310. At that time, the airplane was approximately 160 nautical miles from any weather and the captain noted that towering cumulus clouds had been added to the automated meteorological briefing for BNA. The flight crew discussed weather mitigation strategies, including approach planning and potential windshear considerations. To reduce workload during the anticipated high-workload phase of flight and to provide advance notice to the cabin crew, the flight crew coordinated an early lavatory break. During this break, the captain advised the lead FA that the seat belt sign would be illuminated early and instructed the cabin crew to expedite descent preparations and be seated as soon as possible due to rapidly changing weather conditions. Descending through FL270, the seat belt sign was illuminated, and a public address (PA) announcement was made before the standard FL180 notification for the FAs to prepare for arrival. The flight crew stated this early announcement was made due to weather and elevated workload. Descending through FL180, air traffic control (ATC) issued an expedited descent clearance to 11,000 feet and a heading of 330°. While turning to the assigned heading, the flight crew observed convective weather directly ahead. The crew requested a right deviation, which was denied by ATC due to traffic; a left deviation was subsequently approved. The crew initiated a left turn but entered a pocket of turbulence before clearing the weather. At approximately 16,000 ft, the airplane encountered moderate turbulence lasting approximately 10 seconds. The autopilot and autothrottle remained engaged, airspeed remained stable, and turbulence penetration speed was maintained throughout the encounter. Following the turbulence encounter, the captain contacted the lead FA and was initially advised that all passengers and crew were fine. However later, during the approach, the lead FA informed the captain that one FA had been injured in the aft galley. Due to the late phase of flight, the crew elected to continue the approach and requested medical assistance upon landing. The injured FA reported that she was advised that turbulence was possible and that it would be prudent to expedite their descent preparations. She did not recall whether the seat belt sign was illuminated at that time. She had just returned to the aft galley and began emptying a coffee pot when the turbulence occurred. After landing, emergency medical services responded at the gate and transported the injured FA to the hospital. Medical evaluation revealed fractures to the left foot, multiple fractures to the right foot, a fracture to the tibia, and a fractured or bruised tailbone.