On December 22, 1996, about 1212 hours Pacific standard time, two cabin attendants on American Airlines Flight 407 sustained injuries during a severe turbulence encounter about 75 miles east of San Jose, California, as the aircraft descended through 17,500 feet. The aircraft, a McDonnell Douglas MD-83, N590AA, was owned and operated by American Airlines, Inc., under 14 CFR Part 121 as a non-stop regularly scheduled domestic passenger flight from Chicago to San Jose. The aircraft was not damaged. One cabin attendant sustained serious injuries and another sustained minor injuries. The remaining two pilots, two cabin attendants, and 115 passengers were not injured. The flight originated at Chicago on the day of the accident at 0932 central standard time. According to statements from the flight and cabin crews, and, transcripts of the air-to-ground communications between the flight and ATC, the flight had been in moderate chop for some time during the initial descent. The cabin seat belt sign had been illuminated for about 20 minutes and all passengers were secure in their seats. The cabin crew had just completed a beverage service and were stowing galley equipment at the time. As the aircraft was descending through 17,500 feet inbound to San Jose, one severe turbulence jolt was experienced. The injured cabin crew members were standing in the aft galley; one was able to grab a hand hold while the other one was thrown against the ceiling and back to the floor. The cabin attendant who was thrown to the ceiling sustained a compound fracture of one ankle, which required surgery to repair. The second cabin attendant was treated at the hospital for soft tissue injuries and released. Review of the weather reports disclosed that SIGMET Papa 13 was issued at 221656 and forecast moderate to occasional severe turbulence in the southern portions of California below 18,000 feet. Center weather advisories 01 and 02 were in effect for the time and location of the encounter and predicted moderate to occasional severe turbulence below 22,000 feet. The dispatch weather package provided to the flight crew at Chicago was reviewed. The date/time group for the package preparation was noted to be 221501, prior to the issuance of SIGMET Papa 13 and both center weather advisories. No SIGMETS were noted for the terminal portions of the flight.