On June 3, 2007, approximately 1300 mountain daylight time, a Boeing 757-251 transport category airplane, N523US, operated by Northwest Airlines, Inc., as flight 1221, encountered moderate turbulence during descent into the Denver International Airport (DEN), Denver, Colorado. There was no damage to the airplane. Visual meteorological conditions (VMC) prevailed at the time of the accident. The scheduled domestic passenger flight was being operated under the provisions of Title 14 Code of Federal Regulations Part 121 on an instrument flight rules flight plan. The captain, first officer, 3 flight attendants, and 167 passengers were not injured. One flight attendant sustained serious injuries. The flight originated from the Detroit Metropolitan Wayne County Airport (DTW), Detroit, Michigan, at 1225 eastern daylight time, and was en route to DEN. At 1255, Convective SIGMET (significant meteorological information) 50C was issued and valid until 1455 for the states of Colorado, Wyoming, New Mexico, and Kansas. The SIGMET indicated that area thunderstorms were moving 310 degrees at 20 knots with tops at 41,000 feet mean sea level (msl). According to the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the captain reported that "It was daylight VMC on arrival to Denver. We had the [weather] radar on and had it on for quite awhile as we had monitored and avoided numerous CBs (cumulonimbus clouds) along our flight route. In the Denver area we could see visually and on radar, CBs building over the mountains west of Denver. The CBs were no factor. There was some traffic in the area, but we never had a visual on another aircraft, only the communication between [air traffic control] and the other aircraft. Wake turbulence was not a factor. All checklists and normal flows were completed. All was normal. We were on a right downwind for [runway] 35L at 11,000 [feet msl)]. We were given a turn to a base and a descent to 9,000 [feet msl] we were given another turn to a dogleg and a descent to 7,000 [feet msl]. All was normal. At some point, we were cleared for a visual to 35L. We were in moderate turbulence between 10,000 and 7,000 [feet msl]. I extended the spoilers to possibly get through the turbulence quickly. The approach and landing were normal. After landing, we were advised by the lead flight attendant that a flight attendant in the aft galley was hurt, possibly with a broken leg." The first officer reported that no turbulence was forecasted or reported during the descent. According to a statement provided by the injured flight attendant, she was assigned to door 2 jump seat and had been providing service to the cabin from the aft galley. The attendant heard the double chime for the attendants to complete the final cabin check and then return to their respective seats. The attendant completed the check in a few minutes and was returning to the aft galley. The attendant reported that "it seemed that we were descending quickly so I handed the [trash] bag off to [another flight attendant], seated at door 4, so I could return to my jump seat. As I began to move toward the aisle, the plane dropped down slightly, which caused my feet to lift off the floor. This has happened to me before and I was able to brace myself by holding on to a passenger seat, or simply sitting in a passenger seat. I attempted to brace myself by reaching for the galley counters, however it was too far of a stretch and I wasn't able to hold myself very steady as my feet returned to the floor. Also, as my feet were returning to the floor, I felt the plane shift slightly from side to side. The motion of the plane coming back up as I was coming down, and shifting from side to side, I believe is what caused my leg to snap."