On December 25, 2007, about 0918 Pacific standard time, a McDonnell Douglas DC-9-83, N943AS, encountered severe turbulence during descent for landing at Ontario International Airport (ONT), Ontario, California. The airplane, which was being operated under 14 Code of Federal Regulations Part 121 by Alaska Airlines as flight 464, was not damaged. Two flight attendants sustained serious injuries. The third flight attendant, the two flight crewmembers and the 109 passengers were not injured. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the regularly scheduled domestic passenger flight. The flight departed from Seattle, Washington, about 0703, and the intended destination was Ontario. Following the turbulence encounter, the airplane landed at ONT without further incident at 0925. According to information provided by Alaska Airlines, the flight was descending on the ONT ZIGGY 4 arrival in the vicinity of ZIGGY intersection at an altitude of 8,300 to 8,500 feet above mean sea level (msl) when it encountered severe turbulence. The two flight attendants who sustained injuries were both standing, completing final cabin duties in preparation for landing, and were knocked to the floor during the turbulence encounter. One of the flight attendants sustained multiple fractures to one ankle, and the other flight attendant suffered a head injury with loss of consciousness and concussion. In a written statement, the captain reported, "there was no turbulence listed in the hazard section of the preflight paperwork, but the Enroute section showed a couple of "5's" for the Tropopause [vertical windshear number] in California." (The vertical windshear numbers in the paperwork ranged from 00 to 06, with higher numbers indicating a greater chance of turbulence.) During a briefing with the first officer and the flight attendants, the captain told them, "the ride didn't look good for the climb, cruise, and descent into ONT." He also discussed the Santa Ana wind conditions expected on arrival at ONT. (Santa Ana winds are dry, warm, downslope winds in the Southern California area that blow in from the desert. They tend to be channeled through passes and canyons, locally increasing their speed.) The captain was the flying pilot. Although he anticipated turbulence, the departure, cruise, and initial descent into ONT were "smooth" and uneventful. At 13,000 feet msl on descent, he "chimed the flight attendants early," indicating they were to prepare the cabin for landing and then be seated. According to the captain, "a violent jolt" occurred with no warning as the airplane was descending through about 8,300 feet msl. In a written statement, the first officer reported that they "encountered previously unreported severe turbulence at approximately 8,500 feet msl." The preflight weather information in the briefing packet provided to the flight crew contained the actual and forecast weather for the departure, destination and alternate destination airports. The ONT Terminal Area Forecast (TAF) included in the document indicated the expected weather conditions for the flight's arrival were winds from 060 at 30 knots with gusts to 45 knots, visibility greater than 6 miles, and clear sky. The weather information provided also included a Pilot Report (PIREP) summary by state; no PIREPs were listed for the state of California. The document noted "no WSI hazardous weather" for the northwest U.S. as of 0600:24 and for the southwest U.S. as of 0600:25. (WSI Corporation provides weather information to Alaska Airlines.) The following National Weather Service (NWS) Significant Meteorological Advisories (SIGMETs) included the Ontario area and were issued before the flight's departure: SIGMET Victor 1 for Occasional Severe Turbulence below 12,000 feet was issued at 0200 and was valid until 0600. The turbulence encounter occurred within the area of this SIGMET. SIGMET Victor 2 for Occasional Severe Turbulence below 12,000 feet was issued at 0600 and was valid until 1000. The turbulence encounter occurred within the area of this SIGMET. These advisories were not included in the briefing packet. In response to written questions, the flight's dispatcher stated that as the airplane was nearing ONT, "a WSI sigmet for MOD TURB [moderate turbulence] was issued for the SOCAL [Southern California] region. This was not unusual, considering the Santa Ana wind conditions reflected in the TAF." He further stated that the "WSI sigmet was transmitted to several aircraft approaching the SOCAL region, but apparently [the accident aircraft] was not one of them." For further weather information, see the Meteorology Factual Report in the public docket for this accident. Review of 14 Code of Federal Regulations section 121.601 revealed that it requires an aircraft dispatcher, before beginning a flight, to provide the pilot in command with all available weather reports and forecasts of weather phenomena that may affect the safety of flight. This includes adverse weather phenomena, such as clear air turbulence, thunderstorms, and low altitude wind shear for each route flown and each airport used during flight. During the flight, the aircraft dispatcher must also provide the pilot in command any additional available information of meteorological conditions, and irregularities of facilities and services that may affect safety of flight. The airplane was equipped with a flight data recorder (FDR), which was sent to the Safety Board's laboratory in Washington, DC, for readout. The FDR data indicates that the airplane experienced a vertical acceleration of +0.4 G to +1.9 G, or 1.5 G total change. The NWS uses the following changes in vertical acceleration to quantify the intensity of turbulence: Less than 0.5 G, light turbulence; 0.5 to 1.0 G, moderate turbulence; 1.0 to 2.0 G, severe turbulence; greater than 2.0 G, extreme turbulence. For further details of the data recorded by the FDR, see the public docket for this accident.