THE PILOT OBTAINED A PREFLIGHT WEATHER BRIEFING WHICH PRESENTED A WEATHER PICTURE OF MID LEVEL CLOUDS, TURBULENCE AND GOOD VISIBILITIES. FLIGHT PRECAUTIONS WERE NOTED FOR COASTAL MOUNTAIN OBSCUREMENT DUE TO LOW CLOUDS. RAIN SHOWERS AND LOW CEILINGS WERE FORCASTED FOR THE PERIOD ABOUT 8 HOURS AFTER ETA OF THE FLIGHT. THE ACCIDENT SITE IS AT THE SOUTHWESTERN END OF A SMALL VALLEY WHERE IT NARROWS TO A CONSTRICTED PASS WHICH IS SURROUNDED BY HIGH COASTAL HILLS. SEVERAL WITNESSES REPORTED THAT HEAVY RAIN, LOW CLOUDS AND VISIBILITIES OF ONE HALF MILE IN FOG WERE PRESENT IN THE VALLEY. THE AIRCRAFT WAS SEEN TO FLY DOWN THE VALLEY AT ABOUT 500 FEET AGL OVER A HIGHWAY. AS THE AIRCRAFT APPROACHED AN OIL REFINERY AT THE SOUTHWESTERN END, IT WAS SEEN TO SUDDENLY TURN TO THE RIGHT JUST BEFORE THE REFINERY FLAME STACK AND COLLIDE WITH THE TOP OF A MOUNTAIN RIDGE.