THE PLT & HIS WIFE DESIRED TO RETURN TO THEIR FLAGSTAFF, AZ, HOME FOLLOWING A WEEKEND AT STOCKTON, CA., & THEY DEPARTED SCK IN VFR WEATHER CONDITIONS. THE WEATHER AT THE DESTINATION ARPT HAD BEEN BELOW VFR MINIMUMS ALL MORNING & IT WAS FORECAST TO REMAIN POOR THROUGHOUT THE NIGHT. THE PLT DID NOT REQUEST A WEATHER BRIEFING PRIOR TO TAKEOFF, & HE DID NOT REQUEST WEATHER FROM ANY FACILITY WHILE ENROUTE. AFTER FLYING FOR ABOUT 3.5 HRS & APPROACHING FLG, IFR WEATHER WAS ENCOUNTERED. DESPITE THE FACT THAT IT WAS AN HR PAST SUNSET & THE PLT WAS NOT INSTRUMENT RATED HE ELECTED TO CONTINUE THE FLIGHT. THE FLG CONTROLLER REPORTED HE NEVER SAW THE ACFT & HE NEVER ISSUED THE PLT A CLEARANCE TO ENTER THE CONTROL ZONE. THE PLT COLLIDED WITH TREES & TERRAIN 4 MI WEST OF FLG WHILE DESCENDING AT A 60 DEG ANGLE IN A 40 DEG BANK. INVESTIGATION REVEALED NO PRE-IMPACT MALFUNCTION OF THE ACFT ENG. DUE TO THE ACFT DAMAGE, THE FLT CONTROL CONTINUITY COULD NOT BE ESTABLISHED.