The pilot's failure to maintain clearance from the terrain during low-level maneuvering at night. Factors included the pilot’s improper in-flight decision to continue into an area that was forecasted to possibly contain snow, fog, and low ceilings. Additional factors included his failure to obtain a weather update while en route, a dark night, mountainous terrain, trees, snow showers, fog, and the pilot’s inadvertent flight into instrument flight rules conditions while on a visual flight rules flight.