AN SMA ENCOUNTERED INFLT WX ICING AND FREEZING RAIN.

Narrative

THE BASIC PROB WAS FLYING IN UNFORECASTED CONDITIONS. HAVING GOTTEN MY WX BRIEFING FROM DUATS; THE FORECAST FOR WINDS. A LOFT SHOWED A TEMP OF PLUS 3 DEGS C AT 6000 AND WOULD RISE DURING THE DAY. SURFACE TEMP WAS PLUS 2 DEGS C AT CDW. A LIGHT RAIN WAS FALLING. HVY RAIN WAS FORECAST FOR THE EARLY P.M. I PLANNED TO RETURN PRIOR TO THAT AFTER TKOF. THE TEMP DROPPED TO MINUS 4 DEGS C AT 4000 FT. MIXED ICE BEGAN TO ACCUMULATE. I INFORMED ATC THAT I WISHED TO RETURN TO CDW. HE WANTED TO VECTOR ME TO THE LOC APCH. THIS WOULD HAVE MEANT A TURN TO THE N WHERE COLDER CONDITIONS WERE FORECAST AND FLYING THROUGH CLOUDS AND PERHAPS ACCELERATE THE ACCUMULATION OF ICE. SINCE CDW WAS VFR (1300 FT AND 5 MI) I ASKED FOR A DSCNT AND A VISUAL APCH. THE CTLR DSNDED ME TO THE MVA (1500 FT). I WAS STILL IN THE CLOUDS. HE TOLD ME HE WOULD DECLARE AN EMER FOR ME AND DSND ME DOWN TO 1300 FT; WHICH HE DID. AT 1300 FT I WAS ABLE TO MAKE VISUAL CONTACT WITH THE FIELD. THE LESSON TO BE LEARNED IS; DON'T DEPEND ON TEMP FORECASTS IF CLOSE TO O DEGS C DURING PRECIPITATION. SINCE THIS FORECAST WAS OFF BY AT LEAST 5 DEGS C.