On February 13, 2019, about 1245 Alaska standard time, a Piper PA-32R-300 airplane, N40449, sustained no damage when it was involved in an accident near Anchorage, Alaska. One passenger received a serious injury, and the pilot and other passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 non-scheduled passenger flight. The pilot reported that during a visual flight rules flight from Lime Village Airport (AK2) to Merrill Field Airport (MRI), Anchorage, Alaska, the airplane encountered momentary severe turbulence as it descended through 3,500 feet mean sea level (msl) about 22 miles northwest of Anchorage. One of the two passengers, who was seated in the second row right seat, hit her head on the cabin ceiling. The flight landed at MRI without incident and the passenger was treated at a medical facility, where it was determined that she sustained serious injuries. The injured passenger stated that her lap belt was fastened, but she did not tighten it enough. She was familiar with the use of the lap belt and flew with the company in the accident airplane once or twice a month for many years. She recalled that the pilot provided a passenger briefing, but could not remember if seat belt use was included. The pilot reported that they experienced occasional light turbulence during most of the flight and that significant turbulence was not forecast. The unexpected severe turbulence lasted about 5 seconds. A review of National Weather Service graphical forecast products for the area revealed a turbulence forecast issued at 0900 and valid for the time of the accident for occasional to constant moderate turbulence below 6,000 ft msl. The forecast issued at 1200 and valid for the time of the accident indicated isolated moderate turbulence transitioning to occasional to constant moderate turbulence from the surface to 8,000 ft msl with low level wind shear. A review of pilot reports indicated that the pilot of a similar airplane type experienced continuous moderate turbulence in the area of the encounter from 900 ft to 500 ft about 15 minutes after the occurrence.