American Airlines flight 2516 encountered clear air turbulence during decent, while en route to Chicago O’Hare International airport (ORD), Chicago, Illinois. The flight was a regularly scheduled passenger flight from Orlando International Airport (MCO), Orlando, Florida to ORD. As a result of the turbulence, one flight attendant (FA) sustained serious injuries. The aircraft was not damaged, and the flight landed without further incident. The flight crew reported that while on the VEECK5 arrival at flight level (FL)290, near the top of the decent, air traffic control (ATC) advised of reports of moderate turbulence between FL240 and FL190. ATC suggested the flight begin its decent early to avoid the area, and the flight crew agreed. The flight had previously been smooth, and this was the first turbulence advisory they had received from ATC. The captain informed the cabin crew to expect moderate turbulence in about 10 minutes and advised them to prepare the cabin for landing and take their seats. The captain made an announcement for the passengers to remain seated. The seatbelt sign was on. While the cabin crew was preparing for landing, the airplane encountered severe turbulence as it descended through FL250. FA1 reached the forward jumpseat, while FA2 and FA3 reached nearby cabin seats. All 3 had difficulty reaching their seats and trouble securing their seatbelts due to the turbulence. FA4 was lifted up then down to the floor while attempting to reach a seat, injuring her left leg and ankle. She remained on the aisle floor until the turbulence ended, about 2 minutes after it began. Once FA1 confirmed it was safe to get up, she informed the captain about the injury to FA4 and went to check on the passengers with FA2. FA3 tended to FA4 and made a passenger announcement requesting medical assistance. A nurse who was a passenger on board rendered aid to FA4. The captain advised dispatch of the event and requested medical personnel meet the airplane at the gate. After landing, FA4 was transported to the hospital where she was diagnosed with multiple fractures to her left leg and ankle which required surgery. A post-accident weather analysis provided by the operator showed strong horizontal and vertical wind shear and gravity waves in the area indicating an environment conducive to clear air turbulence. Preflight weather planning predicted light to moderate turbulence between FL290 to FL360 in the area of the event. While there were no active Significant Meteorological hazards (SIGMETs) over the area of the event, a turbulence SIGMET was in effect between FL290 and FL360 for an area about 20 nautical miles southeast of the event. The Turbulence Auto-PIREP System (TAPS) had generated severe turbulence reports for altitudes between FL276 and FL244 in the hour prior to the event, and one pilot report of severe turbulence between FL200 and FL250 was issued a few minutes after the event.