On May 26, 2004, at 1720 central daylight time, a McDonnell Douglas DC-9-82 (MD-82), N573AA, operated by American Airlines as flight 1396, encountered turbulence during descent into the Lambert International Airport (STL), St. Louis, Missouri. One flight attendant received serious injuries and one passenger received a minor injury. The captain, first officer, 2 remaining flight attendants, and 82 passengers were not injured. The 14 CFR Part 121 scheduled domestic passenger flight was operating in instrument meteorological conditions on an instrument flight rules flight plan at the time of the encounter. The flight originated from Orange County, California, at 1228 pacific daylight time. The flight continued to its original destination of STL where it landed uneventfully. The captain's report stated they were on the TRAKE arrival into STL and they were deviating to the south due to weather. As they were descending at 280 to 290 knots through 12,000 feet they encountered an updraft which lasted about 2 seconds. They had just entered the cloud tops when this occurred. The cockpit crew was then informed that a flight attendant in the rear of the airplane was thrown off her feet and injured when they encountered the turbulence. Another passenger who was in the lavatory when the turbulence was encountered, reported that he hit his head. The flight crew declared an emergency. The captain reported they were immediately cleared for the approach and they landed on runway 12R at STL without incident. Emergency personnel met the airplane. The flight attendant was removed from the airplane and transported to the hospital where it was determined that she suffered broken ribs, a fractured wrist, and pelvic fractures. The passenger refused medical treatment. According to crewmembers, the seatbelt sign was on when the turbulence was encountered. Another flight attendant who was in the aisle reported, "The plane dipped severely and I nearly hit the ceiling. I came down hard on my knees. I immediately took the nearest pax [passenger] seat and strapped in." This flight attendant reported that a few minutes later a passenger came up the aisle to inform the number 1 flight attendant that the flight attendant in the rear of the airplane had been injured. The flight was deviating 15 nautical miles (nm) south of a thunderstorm when the turbulence was encountered. The airplane was 30 distance measuring equipment (DME) on the 247 Radial from the Forristell very high frequency omnidirectional range (FTZ VOR) or approximately 55 nm west-southwest of STL when it encountered the turbulence. Scattered thunderstorms were forecast for the area. Convective Sigmet 25C was issued at 1555 valid until 1755 for an area 50 miles south-southwest of Decatur, Illinois, to 20 miles east of Columbia, Missouri. This Sigmet reported a line of thunderstorms 35 nm wide moving from 260 degrees at 50 knots with tops to 35,000 feet. Tornadoes, 2-inch hail, and wind gusts to 60 knots were possible. This line of storms continued to move to the east-southeast. Convective Sigmet 33C was issued at 1755 valid until 1955 for an area 30 miles north-northeast of Quincy, Illinois, to 50 miles south of Terre Haute, Indiana, to 20 miles east-northeast of Springfield, Missouri, to 30 miles north-northeast of Quincy, Illinois. This Sigmet reported an area of severe thunderstorms moving form 250 degrees at 55 knots with tops to 41,000 feet. At 1735, another MD-80 pilot reported an encounter with severe turbulence at 11,000 feet, about 40 miles west-southwest of STL.