The private pilot said the helicopter's turbine engine lost power after flying through an area of moderate to heavy snow. The pilot entered an autorotation and applied excessive aft cyclic during the touchdown in a field, which caused the main rotor blades to flex down and sever the tailboom. Weather reported in the area included snow and freezing fog, with a temperature of 16 degrees F. The helicopter was equipped with an automatic engine re-ignition and engine failure warning system, which was required per the rotorcraft flight manual (RFM) for operation in falling or blowing snow. The pilot said this system was armed at the time of the engine failure and the engine-out warning light illuminated and an audible tone sounded before the power loss. The RFM also stated that flight into known icing conditions was prohibited and fuel had to meet anti-icing capability of JP-4 when operating at 40 degrees or less. A review of fueling records revealed that no anti-icing additive was added to the fuel. The pilot was aware of the icing conditions, but he continued the flight. Examination of the engine found no mechanical anomalies that would have precluded normal operation.