During a local flight, the pilot of the glider reported that he found a hole to climb above a cloud layer and rode mountain wave updrafts to 17,500 feet mean sea level (MSL). While cruising at 17,500 feet, the pilot observed multiple holes in the cloud layer below and could see the "entire east behind me was still open." About 20 to 25 minutes later, the pilot began hearing reports over the radio that "precipitation was moving in" and the "cloud deck was thickening" at the departure airport. The pilot decided to dive for the "the last two remaining holes" in the cloud layer, but during the descent his "primary window" closed and he continued south to the last remaining window. While flying in this window, the pilot reported that he was unable to maintain altitude and then was unable to maintain visual meteorological conditions due to the increasing cloud layers. The pilot descended about 2,000 feet while in instrument meteorological conditions for about two minutes and did not see "any sign of the cloud base." About 6,400 feet MSL (2,200 feet above ground level), the pilot decided that his "safest option" remaining was to "bailout." Subsequently, the pilot placed his right hand on his parachute "D ring" and then "jumped, arched, and pulled." The parachute opened and the pilot landed safely in the tree canopy below, and the glider continued flying for a few miles and impacted trees. The canopy and left wing sustained substantial damage. The pilot stated there were no mechanical malfunctions or failures with the glider that would have precluded normal operation. About 5 hours before takeoff, a terminal aerodrome forecast issued for an airport about 12 nautical miles northwest of the glider's flight path, called for a ceiling of 3,500 feet overcast during the glider's flight. About the time of the accident, 18 nautical miles southeast of the glider's flight path an automated surface observing system reported a cloud layer of 4,900 feet few and 5,500 feet few.