Hogan Air

3-4 inch snow accumulation on aircraft prior to flight. Pilot said he swept snow off except an area he could not reach. Heavy snowfall, wet snow reported as aircraft was prepared for departure, during runup, taxi and takeoff. Pilot advised by tower that visibility was 1/8 mile with fog and snow. Published takeoff minimum for far 135 opn was 1/2 mile. After riking snowplow located 150 feet left of runway, aircraft continued over embankment and came to rest inverted. Ice and snow fnd on upper tail surface. Elevator trim indicated full nose down. Aileron trim ind left bank. Witnesses said engines sounded normal, landing lights on, 600 feet visibility from position of snowplow. Pilot said 35 inches map when descending. Map increased with additional throttle. No engine roughness.