C172 PLT FORGOT TO CLOSE HIS VFR FLT PLAN.

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WHAT FOLLOWS IS A DESCRIPTION OF EVENTS LEADING UP TO A PHASE OR LEVEL 2 ALERT FOR A MISSING ACFT AS A RESULT OF FAILURE TO CLOSE A FLT PLAN. THE MATTER WAS COMPOUNDED BY MCMINNVILLE RADIO'S FAILURE TO NOTIFY BOISE FSS OF A CHANGE (DIVERSION) AND THE ADDITION OF 1.5 HRS TO THE TRIP). I FILED (AND OPENED) A VFR FLT PLAN FROM PULLMAN; WA (PUW) TO NAMPA; ID (S67) ON DEC/XA/01. DEP WAS LISTED AS XA15Z WITH 2 HRS ENRTE. WX AT PULLMAN WAS CLR AND IT WAS FORECAST TO BE VFR AT BOISE AND ALL POINTS IN BTWN. I CONTACTED SLC FLT WATCH NEAR DONNELLY (DNJ). WX AT BOISE WAS 1600 FT CEILINGS; 10 MI VISIBILITY. 30 NM N OF BOISE; I PICKED UP ATIS; RPTING THE SAME. TYPICALLY; (I HAVE FLOWN THIS RTE 100+ TIMES) ONE CAN TRANSITION FROM CLR WX N OF BOISE TO OVCST VIA THE SNAKE OR PAUETTE RIVER DRAINAGES (HORSESHOE BEND OR EMMETT). THAT DAY; THE OVCST LOOKED TO BE A LEVEL LAYER TRANSLATING TO FOG IN HIGHER ELEVATIONS (N OF SQUAW BUTTE). SLC FLT WATCH RPTED MIST AT ONTARIO; SO I OPTED NOT TO CONTINUE VIA WESTERN APCH. I CLBED FROM 5500 TO 8000 FT TO RECONTACT SLC FLT WATCH AND GOT WX FOR MCCALL (MYL); LEWISTON AND PULLMAN. MCCALL WAS CLR AND WAS NEAREST. THE DEWPOINT SPREAD WAS 1 DEG C AND I WAS CONCERNED ABOUT GETTING STUCK. WX RPT GIVEN TO ME FOR LWS AND PUW WAS CLR; 10+ NM VISIBILITY AND GOOD DEWPOINT SPREAD. ALTHOUGH IT WAS 1 HR OLD (WHY; I DON'T KNOW AS EACH HAS AUTOMATED RPTING); I HEADED TO PUW. I TRIED TO RAISE BOISE RADIO ON 122.45; BUT NO ONE COPIED. I TRIED WHAT THE CHART LISTED AS ONTARIO RCO (MCMINNVILLE) AND THE TECHNICIAN TOLD ME TO TRY BOISE. I TRIED AGAIN; UNSUCCESSFULLY. I WAS CONCERNED ABOUT SPENDING TIME CIRCLING TO STAY IN RANGE WITH DARKNESS APCHING. I CALLED MCMINNVILLE BACK AND HE RELUCTANTLY AGREED TO RELAY A CHANGE IN FLT PLANS. I TOLD HIM I WAS ENRTE TO PUW (GAVE HIM INFO ON ORIGINAL PLAN) AND ASKED HIM TO EXTEND ME OUT ANOTHER 1.5 HRS. AFTER A WHILE; HE CAME BACK ON AND SAID HE HAD COMMUNICATED THE INFO TO BOISE BY LANDLINE. I RESUMED MY COURSE TO PULLMAN. UPON MY RETURN; I FAILED TO CLOSE THE FLT PLAN; RESULTING IN MUCH WASTE EFFORT FOR ALL INVOLVED. WHEN I RECEIVED A MESSAGE ON MY CELL TO CALL IN; I ULTIMATELY SPOKE TO SOMEONE IN BOISE WHO INFORMED ME THAT THE PLANE HAD BEEN LOCATED AT PULLMAN AT THE FBO. HE ALSO INFORMED ME THAT BOISE FSS HAD NOT BEEN CONTACTED BY MCMINNVILLE FOR THE CHANGE IN FLT PLAN. THIS; OF COURSE; COMPOUNDED THE SIT; BECAUSE AUTHS COULD HAVE STARTED BY CALLING THE PULLMAN CONTACT AND WOULD HAVE DONE SO 1.5 HRS LATER WHEN IT WOULD HAVE BEEN EASIER TO CONFIRM. NOT WITHSTANDING THAT FACT; I APOLOGIZE FOR NOT CLOSING THE PLAN AND REGRET THE DIFFICULTIES INVOLVED.