C-GFKS, a Beech A100 aircraft operated by Thunder Airlines, was departing runway 25 at Terrace Bay Airport, ON (YTJ) on a medevac flight to Thunder Bay Airport, ON (YQT). During the takeoff roll, the left wing struck a four foot high windrow (snowbank). The pilot lost directional control of the aircraft and it exited the south side of runway 25 at the midpoint. The aircraft came to rest approximately 30 feet off the side of the runway. There were no injuries, but the nose gear was sheared off and the aircraft was substantially damaged. A NOTAM had been issued for the runway stating that there were windrows four feet high, 10 feet inside the runway lights on both sides of the runway. The NOTAM also stated that the cleared portion of the runway was covered with ¬ inch of loose snow over 60 percent compacted snow and 40 percent ice patches and that braking action was fair to poor. The takeoff was being conducted at dusk in conditions of poor lighting and contrast. Crosswind was not a factor.