On June 25, 1999, approximately 1945 central daylight time, a Boeing 737-3H4 transport category airplane, N650SW, operating as Southwest Airlines Flight 1257, was undamaged during an in-flight encounter with turbulence near Lafayette, Louisiana. A flight attendant working in the aft galley was seriously injured. The 137 passengers, 2 flight crewmembers, and 2 other flight attendants were not injured. The airplane was owned and operated by Southwest Airlines Co., of Dallas, Texas. Instrument meteorological conditions prevailed for the Title 14 Code of Federal Regulations Part 121 flight for which an instrument flight rules (IFR) flight plan was filed. The scheduled domestic passenger flight was dispatched from the Houston Hobby Airport (HOU) at approximately 1918, with the New Orleans International Airport/Moisant Field (MSY) near New Orleans, Louisiana, as its intended destination. The captain, who was the non flying pilot, reported that while in cruise flight at FL330, approximately 125 miles west of MSY, he became aware of thunderstorm activity over MSY after receiving the current ATIS for MSY. The captain stated that he informed the first officer of the weather, and the first officer turned on the seat belt sign and made an announcement to the passengers that the flight was about to initiate a descent into MSY. He briefed the passengers about the possibility of encountering turbulence while in the vicinity of thunderstorms during the descent and approach phase of the flight. The captain further stated that several minutes later, the flight entered an area "of stratus-like clouds" with a small cell approximately 20 miles to the left of their flight path. The captain reported that the airplane experienced a "sudden jolt" which he described as "similar to crossing the wake of another 737 at 90 degrees." The jolt was followed by 10 to 12 seconds of light to occasional moderate turbulence. The turbulence ended when the airplane exited the clouds, and smooth air prevailed for the remainder of the flight. A flight attendant in the aft galley of the airplane reported that she was reaching inside one of the galley cabinets when the turbulence occurred. She reported that "the plane dropped, she went up in the air and when she came down, she landed on her ankle." The injured flight attendant called the captain on the intercom and reported her injury. The captain asked her to try to reach the jump seat and strap in, which she did. An uneventful landing was made at MSY. The flight attendant was transported to a local hospital. The injuries to the flight attendant were initially reported as minor. Three days after the occurrence, the NTSB learned that she had sustained "a broken distal fibula that was displaced."