DURING A DARK NIGHT VFR FLIGHT, THE ACFT ENCOUNTERED A SEVERE DOWNDRAFT AND INSTRUMENT METEOROLOGICAL CONDITIONS. WITH FULL POWER THE ACFT DESCENDED, STRIKING TREES BEFORE IT STRUCK THE GROUND. DURING THE PILOT'S PREFLIGHT BRIEFING AND IN-FLIGHT BRIEFING HE WAS TOLD THAT FLIGHT PRECAUTIONS WERE PRESENT BECAUSE OF ICING, TURBULENCE, MOUNTAIN OBSCURATIONS, AND IFR CONDITIONS. IN A POST ACC STATEMENT THE PLT SAID HE WAS DESCENDING FROM 8000 FT MSL TO LAND AT MEDFORD OR FOR REFUELING WHEN HE ENCOUNTERED A STRONG DOWNDRAFT. THIS DESCENT CONTINUED UNTIL ABOUT 2000 FT AGL AT AN AIRSPEED OF 50 MPH. DUE TO LOW AIRSPEED AND TURBULENCE THE PLT DECIDED AGAINST TRYING A 180 DEGREE TURN. IMC WX CONDITIONS WERE ENCOUNTERED IN THE FORM OF FOG AND SNOW. THE PLT TURNED FROM 310 DEGREES TO 300 DEGREES AND BEGAN A CLIMB. AT APRX 6100 FT MSL ANOTHER DOWNDRAFT WAS ENCOUNTERED AND THE ACFT DESCENDED AT ABOUT 60 MPH UNTIL TREES WAS CONTACTED AT APRX 5100 FT. THE WINGS AND TOP OF THE ACFT WERE TORN OFF BUT THE 2 OCCUPANTS SURVIVED.