Sunflower

At 1027 the pilot received a weather briefing for Holbrook, AZ. The pilot was told of forecasts for "at least moderate icing in clouds or in precipitation above the freezing level" and the briefer stated "I guarantee you moderate to severe turbulence..." for a route into Holbrook. The pilot later got weather info for Amarillo, TX and filed an IFR flt plan to Amarillo. The aircraft departed at 1150. At 1204 the pilot reported he had an engine problem. Radar plots indicate the aircraft was maintaining a climb of about 500 fpm up to 11,200 ft. Thirty seconds later the aircraft had descended to 8,500 ft. Radio and contact was lost about 1207. Based on a weather study the probable meteorological conditions that existed about the time & in the area of the accident were moderate to severe turbulence below 12,000 ft msl & while in the clouds above freezing level the aircraft likely encountered moderate to severe rime icing. The aircraft was not certificated for flt into known icing conditions. The flight was transporting prisoners for various government agencies.