On May 13, 1998, at 0839 eastern daylight time, a Fokker F-28 MK100, N1400H, operated by American Airlines as flight 1275 experienced moderate turbulence 20 miles east of Ft. Wayne, Indiana, during a descent through FL250. The #1 flight attendant received a fractured ankle. The captain, first officer, #2 flight attendant, and 35 passengers were not injured. The 14 CFR Part 121 flight was operating on an IFR flight plan. The airplane was not damaged. The flight originated from Washington, D.C., at 0710 eastern daylight time. The flight continued to Chicago, Illinois, where it landed uneventfully at 0803 central daylight time. The captain reported they were experiencing "light chop" while descending through FL250 at 250 knots. He reported they were deviating south around a small cell when moderate turbulence was encountered. The captain reported he called the flight attendants to tell them to take their seats. The #1 flight attendant answered the call stating that they were already sitting down. The captain stated that the #1 flight attendant then came into the cockpit and informed him that she had fallen and hurt herself during the turbulence encounter. The captain reported he instructed her to sit down and he coordinated with the #2 flight attendant for her to assume the duties of the #1 flight attendant for the remainder of the flight. The #1 flight attendant reported that the seatbelt sign had been turned on and she was completing her cabin check when it started getting "bumpy." She stated that she and the #2 flight attendant sat down in passenger seats near the back of the airplane. She stated the captain called back on the interphone using two rings. She stated that she and the #2 flight attendant remained seated until they heard the interphone ring a second time. The #1 flight attendant reported she got up to answer the call. She reported, "I ended up flying through the air and then landing on my knee, and elbow with my ankles going every which way. I then got up to the back jumpseat and realized I was pretty shaken up and my head and ankles were hurting." She reported she then informed the captain of the situation and that she needed medical attention. SIGMET 20C was issued at 0355 edt for Illinois and Indiana. The outlook for this SIGMET covered the area of the turbulence encounter and was valid from 0555 edt until 0955 edt. The outlook stated that thunderstorm activity was expected to slowly weaken during the period. A strong inflow of warm and moist air had ceased with a loss of the low level jetstream and the southerly air mass over the region had begun to gradually stabilize. The upper system providing dymanic support for convection was moving northeastward across the upper Great Lakes into Canada. SIGMET 1E was issued at 0555 edt for Indiana and Michigan. The outlook for this SIGMET covered the area of the turbulence encounter and was valid from 0755 edt until 1155 edt. The outlook stated that widely scattered thunderstorm activity was expected to continue to slowly weaken during the rest of the morning. Both of these SIGMETs were included in the dispatch weather information provided to the flightcrew of N1400H prior to the flight.