One flight attendant in the aft galley was seriously injured and four others aboard the air carrier flight received minor injuries when the airplane encountered moderate to severe clear air turbulence in cruise at flight level 310 (FL310, 31,000 feet). About 30 minutes before the turbulence was encountered, the company flight dispatcher communicated to the flight crew that the flight was approaching an area of forecast moderate to severe turbulence associated with mountain wave activity between FL310 and FL390 (39,000 feet). The dispatcher recommended that the flight alter course and descend to FL280 (28,000 feet). Eighteen minutes before the turbulence was encountered the pilot requested a “ride report” at FL310 from air traffic control and was told to expect “occasional light chop, nothing real bad.” The pilot remained at FL310 and remained on the existing routing. Approaching the area of forecast turbulence, he illuminated the seat belt sign and advised the passengers of the potential for turbulence. He also advised the flight attendants to stow the galley equipment. Approximately 3 to 4 minutes later, the flight encountered turbulence. The injured flight attendant was one of two in the aft galley. The other flight attendant reported that when the turbulence started they were “jolted” a couple of times during which she recalled her feet came off the floor and then the turbulence subsided. They “scrambled” toward their jumpseats; however, they encountered additional, more severe, turbulence and it became impossible to hold onto anything. She recalled being thrown about the galley; hitting the ceiling, counters, and doors before being “slammed” to the floor. When the turbulence ceased, the flight descended to flight level 280 and continued to its intended destination without further incident. The airplane’s digital flight data recorder showed the aircraft encountered three vertical acceleration excursions between 1.449 g’s and -0.379 g’s over a 16-second period. Probable Cause: The failure of the pilot-in-command to properly evaluate a hazardous weather advisory and his failure to adequately alter course and flight altitude to avoid the area of hazardous weather.