On April 29, 2005, approximately 0731 central daylight time, a twin-turbofan Boeing 737-700 airplane, N727SW, operating as Southwest Airlines flight number 2440, was undamaged during an in-flight encounter with turbulence while in cruise flight at Flight Level 410 (41,000 feet mean seal level) near Little Rock, Arkansas. The airline transport rated captain, airline transport rated first officer, two of the three flight attendants, and 15 passengers were not injured. One flight attendant was seriously injured. The airplane was registered to and operated by Southwest Airlines, of Dallas, Texas. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the scheduled passenger flight operating under 14 Code of Federal Regulations Part 121. The cross-country flight originated from the William P. Hobby Airport (HOU), near Houston, Texas, at 0637 and was destined for the Lambert-St. Louis International Airport (STL), near St. Louis, Missouri, where the flight landed without further incident. The 12,500-hour captain reported in the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) that while in cruise flight at FL410 (41,000 feet msl), there was no turbulence and the flight was above an overcast cloud layer for most of the flight. The captain stated that visibility was limited at times due to a thin layer of haze. As a precaution, the captain kept the fasten seat belt sign on and instructed the flight attendants to take their seats if the flight became "bumpy." Several minutes later, the flight encountered a thin layer of haze. Upon exiting the layer of haze, the flight crew noticed "an unusual cloud formation" directly ahead. The captain initiated a right turn as the flight encountered "two abrupt bumps and a gain in air speed." The captain added that the encounter lasted about 15-20 seconds, which was light to moderate turbulence. Following the turbulence upset, the captain was notified by a cabin crew member that a flight attendant sustained a leg injury in the aft galley, and would need medical attention upon arrival at STL. A staff meteorologist for the Safety Board reviewed composite reflectivity images for the immediate area of the turbulence encounter. The satellite imagery at 1215Z indicated radiative cloud top temperature of 218.3 degrees Kelvin (K) or -54.86 degrees Celsius (C). The winds aloft at FL410 were reported from 280 degrees at 127 knots. Convective significant meteorological information (SIGMETs) were issued from 0955Z through 1255Z for thunderstorm activity. Convective SIGMET 18C issued at 1155Z was current for a portion of Arkansas and Oklahoma, enclosed from 10ENE TUL-40WSW ARG-20ENE LIT-20NE MLC-10ENE TUL. For an intensifying area of severe embedded thunderstorms moving from 260 degrees at 35 knots, with tops to 40,000 feet. Hail to 1 1/2 inches and wind gusts to 60 knots were possible with these storms. Severe to extreme turbulence, lightning, microburst, severe icing, and localized IFR conditions was implied with the issuance of the advisory. The provided coordinates of the turbulence upset was located outside and immediately east of the convective SIGMET in affect.