On April 10, 2004, about 1835 central daylight time, a Boeing 737-300, N662SW, operated by Southwest Airlines as flight 2038, encountered turbulence while en route to the St. Louis International Airport, St. Louis, Missouri. During the turbulence encounter, a cabin attendant received serious injuries. No injuries to the passengers or other crew members were reported. No damage to the airplane was reported. The 14 CFR Part 121 scheduled domestic passenger flight was operating on an instrument flight rules flight plan. The weather conditions at the time of the occurrence have not been established. The flight originated from the William P. Hobby Airport, Houston, Texas, at 1753 According to a written report filed by the operator, the airplane was in "rough air" due to a storm front along the route of flight. The airplane had deviated around storm cells and had also varied altitude in an attempt to minimize the turbulence encountered. The operator stated that the seat belt sign was on and that an announcement was made for the flight attendants to take their seats. The operator stated that after a brief encounter with moderate turbulence near Little Rock, Arkansas, the flight crew was informed that a flight attendant had injured her shoulder. The report does not state whether the injury happened before, or after the announcement was made to the flight attendants. Upon arrival at the destination, the flight attendant walked off of the airplane and into the gate area for treatment. It was later found that the flight attendant had fractured her shoulder. In a written report, the pilot stated that while en route the airplane was in turbulence. He stated that the seat belt sign was on and as the turbulence began to intensify he made a public address announcement for the flight attendants to be seated. He stated that he was then informed that one of the flight attendants had been injured. A written report from the flight attendants indicated that the fasten seat belt sign was not on at the time of the injury. The National Transportation Safety Board was not informed of the extent of the injury received by the flight attendant until May 13, 2004. No cockpit or flight data recordings were obtained.