The pilot and his passengers had flown from Van Nuys, CA to Palm Spring, CA during the afternoon. After arrival, they spent several hours on the ground. At 1740, the pilot called a FSS to obtain a weather briefing for a flight to Concord, CA. During the briefing, he was advised that VFR flight was not recommended. The forecast indicated deteriorating weather conditions due to an approaching weather system off the coast of California. At 2153 PST, the aircraft departed westbound. Radar services were terminated 8 mi west of Palm Springs and no further radio communications were made. The aircraft failed to reach its destination and a search was initiated. Progress of the search was delayed due to thunderstorms. The aircraft was found where it had struck steep terrain in canyon foothills at the 3400 ft level. An investigation indicated the plane crashed in a slightly nose low, left bank attitude. No preimpact, mechanical problems were found. The Ontario, CA weather was 2200 broken, 4 mi vis with fog.