The glider experienced a loss of lift and collided with a bench during a forced landing in a field. In a written statement, the pilot reported that prior to departure he spoke with a McMinnville Flight Service Station (FSS) briefer. He received a weather forecast indicating that the top of lift was to be 5,300 feet mean sea level (msl). The pilot further stated that a towplane launched the glider, releasing it 4 nautical miles southwest of the departure airport at an altitude of 2,700 msl. The pilot discovered several thermals in the area and ascended to 3,700 feet msl. Shortly thereafter, the glider encountered rain showers and began to lose altitude. Though the bad weather appeared as though it was dissipating, the glider continued to descend. The pilot opted to land in a plowed field. The glider touched down in the field farther down than the pilot anticipated. The pilot did not have a sufficient distance to stop before the glider's right wing collided with a robust bench.