AN EMS HELI PLT FOUND HIMSELF IN UNFORECAST ICING.

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I WAS REQUESTED TO CHK WX FOR A MEDICAL FLT FROM XXX TO PICK UP A PATIENT IN YYY; THEN DELIVER THAT PATIENT TO ZZZ. WX AT YYY WAS FORECAST TO BE 1500 FT OVCST AND BETTER THAN 6 MI AT THE TIME OF MY ARR. DUE TO SPARSE POPULATION ALONG THE FLT RTE; I KNEW IT WOULD BE DIFFICULT TO MAINTAIN VFR IN ACCORDANCE WITH FAR 135.207 WHICH REQUIRES SUFFICIENT SURFACE LIGHTS TO MAINTAIN VFR. SO I ELECTED TO FILE AN IFR FLT PLAN FROM WWW TO YYY. I OBTAINED MY INITIAL WX BRIEF VIA A PERSONAL COMPUTER. THERE WAS AN AIRMET FOR LIGHT OCCASIONAL MODERATE RIME ICE BTWN 4000 FT AND 7000 FT. THIS CONDITION WAS TO HAVE ENDED AT LEAST 1 HR PRIOR TO THE START OF THIS FLT. MY WX BRIEFING CONTAINED NO PIREPS FOR ICE ALONG THE RTE. I CONTACTED FSS TO SEE IF THE AIRMET HAD BEEN AMENDED AND TO VERIFY THAT THERE WEREN'T ANY ICING PIREPS. IN ADDITION TO NO AMENDMENTS OR PIREPS; THE BRIEFER INDICATED THAT THE FREEZING LEVEL IN THE VICINITY OF YYY WAS AT 9000 FT; HOWEVER THE SURFACE TEMP WAS 41 DEGS F. I DEPARTED XXX ENRTE TO WWW TO PICK UP ADDITIONAL FUEL FOR THE FLT. EN RTE I UPDATED THE WX BRIEFING AND FILED AN IFR FLT PLAN WWW TO YYY VIA VAB. THE MEA ALONG THIS RTE IS 3500 FT; SO I INCLUDED IN THE REMARKS SECTION THAT MY REQUESTED ALT BE EMPHASIZED. I DEPARTED WWW; WITH A CLRNC; AND CLBED TO 3500 FT. THE OUTSIDE AIR TEMP AT THE ALT WAS PLUS OR MINUS 0 TO PLUS 1. I CONTINUED TO CHK THE WIPER BLADES FOR ICE BUT FOUND NONE. ABOUT 30 MINS INTO THE FLT; THE TEMP DROPPED TO MINUS 1. I REQUESTED AND RECEIVED AN IMMEDIATE CLRNC BACK TO WWW FOR A VOR 5 APCH. ABOUT THIS TIME THE MEDICAL CREW INFORMED ME THAT WE WERE PICKING UP SOME ICE ON THE WING STUBS. UPON LNDG; I FOUND 3/4 OF AN INCH OF ICE ON THE SKID CROSS TUBES AND WIRE CUTTERS. ALSO THERE WAS ABOUT 1/2 INCH ON THE MAIN ROTOR PUSH PULL TUBES AND MAST. NO ICE WAS FOUND ON THE ROTOR BLADES. THE APCH AND LNDG PRESENTED NO PROBS. I SUSPECTED THAT THE BLADES WERE FREE OF ICE AS THERE HAD BEEN NO REDUCTION IN AIRSPD NOR INCREASED PWR REQUIREMENTS TO MAINTAIN ALT. THE LESSON LEARNED IS THAT DURING THE SEASONAL WX TRANSITION; ONE MUST BE MORE CONSERVATIVE.