AN SMA PLT HAD HIS DOOR OPEN INFLT. HE DEPARTED IMC FOR VMC AND HIS ASSIGNED ALT FOR A LOWER ALT WITHOUT ATC CLRNC. HE LANDED SAFELY WITHOUT DAMAGE OR INJURY.

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DEPARTED PWT AT XA42 ON IFR FLT PLAN. CLBED TO ASSIGNED ALT 5000 FT; RADAR CONTACT AND IDENTED BY SEATTLE DEP 127.1; AND IN RADIO CONTACT. ENTERED IMC BTWN 4000-4500 DURING CLB AND WAS IN SOLID IMC AT 5000; WHICH WAS THE MEA. AFTER LEVELING AT 5000 AT APPROX XA50 THE CABIN DOOR POPPED OPEN WITH ATTENDANT NOISE; WIND IN CABIN CAUSING CHARTS TO FLAP AND FLY AROUND AND NOTICEABLE BUFFETING OF THE ACFT. TO REDUCE BUFFETING I SLOWED THE ACFT AND LOWERED LNDG GEAR FOR STABILIZING EFFECT AND NOTIFIED DEP CTLR OF THE PROBLEM. THROUGH THE RAGGED UNDERCAST I COULD SEE VFR CONDITIONS AT 4500-4000 BELOW AND; TO AID IN RESOLVING PROBLEM; ALLOWED ACFT TO DSND TO VMC CONDITIONS AT 4000. I DID NOT IMMEDIATELY REQUEST NEW ALT (4000 WAS BELOW THE MEA) BUT WAS TALKING WITH DEP CTLR AND WAS XMITTING MODE C. AFTER STABILIZING AND SLOWING ACFT AND ENTERING VMC CONDITIONS; I REVIEWED THE POH TO DETERMINE SERIOUSNESS OF THE SITUATION; DETERMINED I COULD CONTINUE STRAIGHT AHEAD APPROX 15-20 NM AND LAND STRAIGHT AHEAD AT OLM; AND ADVISED CTLR. CTLR CLRED ME TO 3000; PROVIDED WX AT OLM (WHICH WAS MARGINAL) AND ILS FREQ AND CLRED ME FOR STRAIGHT-IN ILS 17 AT OLM. HE ALSO ASKED IF I WAS DECLARING AN EMER. I ADVISED I WAS NOT; AFTER REVIEWING THE POH TO DETERMINE THAT I COULD SAFELY CONTINUED TO OLM TO LAND. I LANDED AT OLM CLOSED AND LATCHED THE DOOR; AND CONTINUED TO OLM TO LAND. I LANDED AT OLM. CLOSED AND LATCHED THE DOOR; AND CONTINUED THE FLT. I BELIEVE I WAS DISTR BY REFUELING PERSONNEL AT PWT WHILE IN PRESTART CHKLIST AND DID NOT CONFIRM THE DOOR WAS COMPLETELY LATCHED; WHICH IS A CHKLIST ITEM.