THE PLTS HAD RECEIVED A WX BRIEFING FROM SEATTLE FSS EARLIER IN THE DAY FOR A FLT FROM SEATTLE TO PORT ANGELES. DUE TO THE TIME SPAN BETWEEN ARRIVAL & DEPARTURE AT PORT ANGELES, THE WX BRIEFING WAS NOT VALID FOR THE RETURN FLT. THE ACFT IMPACTED THE WEST SLOPE OF RAPIDLY RISING TERRAIN AT 1,780 FT MSL. THE GENERAL AREA IS UNPOPULATED & THERE ARE VIRTUALLYNO LIGHT SOURCES TO PROVIDE REFERENCE WHILE FLYING AT NIGHT. AN AIRLINE CAPTAIN REPORTED A SCATTERED TO BROKEN LAYER AT 1,800 FT MSL & AN OVERCAST LAYER AT 4,000 FT MSL. VFR TRAFFIC FROM PORT ANGELES TO SEATTLE NORMALLY FOLLOW A ROUTING WHICH DEPARTS PORT ANGELES EASTBOUND ALONG THE NORTH COASTAL AREA OF THE OLYMPIC PENINSULA. THE ROUTE THEN TURNS SE NEARDISCOVERY BAY & PROCEEDS INLAND OVER RELATIVLY LOW TERRAIN TOWARDS SEATTLE. UNDER FAVORABLE WX CONDITIONS TRAFFIC WILL ALSO PROCEED SE AT SEQUIM BAY (PRIOR TO DISCOVERY BAY). THE TERRAIN SOUTH OF SEQUIM BAY CLIMBS RAPIDLY AS THE NE CORNER OF THE OLYMPIC MOUNTAIN RANGE IS APPROACHED.