Before the accident flight, the glider flight instructor observed adverse weather approaching the airport from the south. He wanted to demonstrate a crosswind landing to his student before ceasing glider operations due to the adverse weather. The glider released from the tow airplane around 1,000 feet above ground level and then proceeded on downwind for runway 29. While on final approach, the flight instructor had difficulty maintaining proper runway alignment due to a strong, gusting crosswind. As a result, he elected to land the glider in a grassy area immediately north of the runway. He noted that his forward visibility was limited by heavy rain. The glider impacted a taxiway sign during the landing, substantially damaging the left wing. Weather radar and surface observations indicated that the airport was affected by a thunderstorm shortly after the glider departed on the local area flight. The thunderstorm produced heavy rain, lightning, and strong winds. During the accident flight, the airport's automated surface observing system recorded a peak wind from 220 degrees magnetic at 42 knots. The glider's maximum demonstrated crosswind component for safe approach, landing and aerotow launching was 16 knots.