On July 7, 1994, at 1659 central daylight time, a Boeing 737-2H4, N60SW, encountered moderate turbulence near South Bend, Indiana. One flight attendant was seriously injured. Four crew members and 100 passengers were not injured. Instrument meteorological conditions (IMC) prevailed for Southwest Airlines Flight 1490, scheduled Title 14 CFR Part 121 flight to Cleveland-Hopkins International (CLE) Airport, Ohio. According to the pilot/operator report, radar was used to avoid weather as the aircraft approached South Bend, Indiana. There had been no pilot reports of turbulence and the company dispatcher weather package did not report turbulence. As the flight climbed through FL200, moderate turbulence was encountered for several seconds. The aircraft then entered visual meteorological conditions and smooth air. The captain stated that the unexpected turbulence started and stopped so quickly that he did not have time to warn the flight attendants. A flight attendant in the aft galley fell to the floor injuring her back. A paramedic on the flight assisted the flight attendant. The seat belt sign was illuminated at the time the turbulence was encountered. The captain made the decision to continue to their destination because the weather they had passed through was getting worse, and the flight attendant's injury was not thought to be severe. As the flight approached Cleveland (CLE) thunderstorms were developing over the airport causing landing delays, so the flight was diverted to Detroit Metropolitan Wayne County Airport, Detroit, Michigan. Paramedics met the airplane at the gate and transported the flight attendant to the hospital.