AT NOON ON 5/21/83, THE PLT DEPARTED RIVERTON, WY ON A FLT TO OGDEN, UT TO TRANSPORT A MINISTER BACK TO RIVERTON FOR A WEDDING. AFTER ARRIVING AT OGDEN, HE OBTAINED A WX BRIEFING FOR THE RETURN FLT. HE WAS WARNED OF POSSIBLE THUNDERSTORMACTIVITY, TURBULENCE, LOW CEILINGS & STRONG, GUSTY WINDS. THE NON-INSTRUMENT RATED PLT WAS TOLD THAT IN THE VICINITY OF LANDER & ROCK SPRINGS, WY, THERE WERE RAIN SHOWERS, AND WITH THUNDERSTORM ACTIVITY, VFR FLT MAY NOT BE POSSIBLE. THE PLTFILED A VFR FLT PLAN & STATED HE WOULD 'TAKE A LOOK-SEE.' AT 1729, HE TOOK OFF. THERE WAS NO EN ROUTE RADIO CONTACT, EXCEPT WHEN HE REQUESTED & WAS PROVIDED THE RIVERTON WX. AT APRX 1930, A RESIDENT NEAR THE CRASH SITE HEARD A SMALL ACFTFLYING LOW. SHE REPORTED THUNDER, LIGHTNING, THICK FOG & HEAVY SNOWFALL AT THAT TIME. WHEN THE ACFT DID NOT ARRIVE AT RIVERTON, A SEARCH WAS BEGUN. IT WAS FOUND NEAR A MOUNTAIN PASS WHERE IT CRASHED ON A RIDGE IN A 45 DEG NOSE DOWN, WINGSLEVEL ATTITUDE. THERE WAS VIRTUALLY NO MOVEMENT AFTER IMPACT. NO PREIMPACT/MECHANICAL FAILURE/MALFUNCTION WAS REPORTED.