THE RPTR CONTINUED IN MARGINAL VFR INTO UNFORECAST IFR; DECLARED AN EMER AND RECEIVED AN IFR CLRNC TO HIS DEST RDU.

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THE OCCURRENCES OF THE FLT WERE AS FOLLOWS: WHILE ENRTE TO RDU I ENCOUNTERED UNFORECAST IFR CONDITIONS AT APPROX 35 MI SE OF RDU. WHILE FLYING AT APPROX 1300 FT MSL; I WAS JUST ABLE TO PICK UP RDU'S ATIS INFO AND REALIZED THAT THE WX HAD DETERIORATED TO MUCH WORSE THAN WAS FORECAST. CONDITIONS WERE 4 OVCST 2 MI FOG AT RDU AT APPROX XA20 AM. I WAS ALREADY GREATER THAN 1/2 WAY TO MY DEST AND KNEW OF THE HIGH TWRS CLOSE TO MY RTE. BECAUSE IT WAS NIGHT; I DECIDED THAT IT WOULD BE UNSAFE AND ILLEGAL TO DSND ANY LOWER. A TURN BACK TOWARDS OAJ WAS NOT FEASIBLE. I HAD ENCOUNTERED IMC AND WAS MORE THAN 1/2 WAY TO MY DEST; THEREFORE NOT LEGALLY ABLE TO CONTINUE THE FLT UNDER VFR. MOREOVER; I WAS CONCERNED ABOUT THE SAFETY OF THE FLT DUE TO GND OBSTRUCTIONS NEAR MY POS AND THE ABSENCE OF FORWARD VISIBILITY. I AM A CURRENT AND RATED INST PLT. MY PART 135 TRAINING AND JUDGEMENT MADE ME REALIZE IT WAS NECESSARY FOR ME TO DECLARE AN EMER IN ACCORDANCE WITH REGS. I SQUAWKED 7700 AND CONTACTED GSB APCH ON 123.7; THE CLOSEST RADAR APCH FACILITY. I INFORMED GSB APCH THAT I WAS DECLARING AN EMER BECAUSE I AM VFR PART 135 PLT AND HAD ENCOUNTERED UNFORECAST IMC AND REQUESTED AN IFR FLT PLAN TO RDU. I BEGAN AN IMMEDIATE CLB AWAY FROM DANGER TO 4000 FT MSL; CHANGED MY SQUAWK CODE AND PROCEEDED DIRECT RDU AS INSTRUCTED BY GSB APCH. I WAS HANDED OFF TO RDU APCH AND COMPLETED THE ILS RWY 23R APCH INTO RDU WITHOUT INCIDENT. I THEN TAXIED TO COMMERCIAL CARGO OPS WHERE I IMMEDIATELY CONTACTED DIRECTOR OF OPS; AND I WAS INSTRUCTED TO REMAIN AT RDU BECAUSE THE FLT COULD NOT BE COMPLETED UNDER VFR PART 135 REGS. I HAVE OVER 800 HRS OF FLT TIME; AND HAVE BEEN TRAINED WELL FOR PART 135 OPS. COMPANY POLICY INSISTS THAT OUR PLTS OPERATE IN STRICT ACCORDANCE WITH OUR OPS MANUAL. I WAS CONCERNED FOR MY SAFETY AND REFUSE TO OPERATE CONTRARY TO PART 135 REGS. THIS IS THE REASON FOR MY ACTIONS. ACCURATE WX INFO IS PERTINENT TO ANY FLT. I NOW REALIZE THAT I DID NOT HAVE THE MOST RECENT WX PRIOR TO MY DEP. I CANNOT BE CERTAIN IF THIS WOULD HAVE MADE A DIFFERENCE BECAUSE I DO NOT HAVE ACCESS TO PAST WX AT THIS TIME. RDU COULD HAVE STILL BEEN MVFR UPON MY DEP AND DETERIORATED WHILE ENRTE. I WILL NOT KNOW. REGARDLESS; THE BRIEFER DID NOT MENTION ANY CONDITIONS LESS THAN MVFR AT THE TIME OF MY STANDARD BRIEFING. I DID INFORM HIM OF MY DEP TIME AND HE FAILED TO MENTION THAT THE WX WOULD BE UPDATED IN LESS THAN 1/2 HR. IF I WOULD HAVE KNOWN THIS; I WOULD CERTAINLY HAVE RECHKED BEFORE DEPARTING. THEREFORE; I FEEL THAT TO PREVENT RECURRENCE OF THIS OR SIMILAR SITS; WX BRIEFER SHOULD ALWAYS MAKE PLTS AWARE OF THE TIMELINESS OF WX UPDATES. BETTER WX EDUCATION FOR PLTS IS A FOUNDATION AS WELL.