DURING A PREFLIGHT BRIEFING, THE PILOT WAS ADVISED THAT THE MOUNTAINS WERE OBSCURED BY CLOUDS, THE OUTLOOK WAS FOR MARGINAL VFR CEILINGS, AND THE WEATHER CONDITIONS IN NORTHERN CALIFORNIA WERE DETERIORATING FASTER THAN FORCASTED. THE PILOT TOOK OFF AT 1333 PST WITHOUT FILING A FLIGHT PLAN. LATER, THE AIRCRAFT WAS OBSERVED PROCEEDING ALONG A HIGHWAY AT ABOUT 100 FT AGL. A WITNESS REPORTED THAT AFTER THE AIRCRAFT PASSED ABOVE HIM, IT MADE A 180 DEG TURN. THE PLANE THEN FLEW INTO A BOX CANYON AREA, ADJACENT TO THE HIGHWAY. AFTER ENTERING THE BOX CANYON, THE AIRCRAFT WAS IN A LEFT TURN WHEN IT CRASHED INTO THE SIDE OF AN UPWARD SLOPING HILL ABOUT 50 FT BELOW AND ADJACENT TO THE HIGHWAY. THE WITNESS STATED THAT THE PASS (WEST OF THE CRASH SITE) WAS OBSCURED WITH CLOUDS AND THAT THE PREVAILING OVERCAST WAS ABOUT 200 FT AGL. THE SOUNDS FROM THE ENGINE WERE REPORTED AS NORMAL.