On May 28, 2001, at about 0940 eastern daylight time, a Boeing 737-301, N349US, flight 351, registered to First Union National Bank Trustee and operated by US Airways Inc, as a 14 CFR Part 121 scheduled domestic passenger flight, from Atlanta, Georgia, to Pittsburgh, Pennsylvania, experienced an in-flight encounter with weather (turbulence) while climbing between flight level (FL) 220 to 240 in the vicinity of Toccoa, Georgia. Visual meteorological conditions prevailed and an IFR flight plan was filed. The airplane was not damaged. The airline transport rated pilot-in-command (PIC), first officer, 2 flight attendants and 116 passengers reported no injuries. One flight attendant sustained serious injuries. The flight originated from Atlanta about 17 minutes before the accident. The PIC diverted to Atlanta and landed with out further incident at 0955. The PIC stated they were in a climb following another airplane which reported no problems ahead, the seat belt sign was illuminated, and he had briefed the crew on the weather conditions before departing Atlanta. They had just come out of some rain showers and were in the clear between FL 220 to 240. All weather activity was located off the left wing to the north. The other jet located to their front had just requested a vector when they experienced some light to moderate turbulence. The fasten seat belt sign was on and the crew had been briefed on the weather conditions prior to the flight. A short time later, he was informed that a flight attendant had broken her ankle. He requested and received clearance to return to Atlanta. The injured flight attendant stated that before the flight the captain held a routine preflight briefing, and there was no specific warning of bad weather that she could recall. During the takeoff she occupied the forward jump seat with the "A" flight attendant. After the "two bell signal" she released her seat belt and started back to the aft galley to assist the "B" flight attendant. They removed the service cart and were preparing to serve when they felt a few bumps. They decided that they would wait awhile to begin the service due to the turbulence when aircraft suddenly plunged. She was standing by the 2L door and was thrown into the air. When she landed she felt pain in her right foot. A quick examination revealed that bones were protruding and that her leg was bleeding. The "B" flight attendant tried to assist her, and she informed her to advise the captain of the situation. Review of the NTSB Meteorology Factual Report revealed there were no severe weather forecast Alerts, Segments, Center Weather Advisories, or Airmets for turbulence over Georgia at the time of the accident and none was required. (For additional information see NTSB Meteorology Factual Report, an attachment to this report.) The Loral-Fairchild model F1000 solid state flight data recorder (FDR) was removed from N349US after the accident flight and shipped to the NTSB Vehicle Recorder Division's laboratory in Washington D.C. for readout and evaluation. The FDR data for the accident flight indicated that while the airplane was ascending from 21,500 to 22,500 feet pressure altitude, the airplane experienced dynamic accelerations for about 20 seconds. The maximum normal acceleration recorded during the event was 1.738 G's at FDR elapsed time 31,450 seconds. The minimum normal acceleration recorded during the event was 0.776 G's at FDR elapsed time 31,451 seconds. (For additional information see NTSB Flight Data Recorder Factual Report, an attachment to this report.) The FDR was released to US Airways on July 3, 2001.