M20 PLT ENCOUNTERS RAPID ICE BUILDUP AND DECLARES AN EMER IN ORDER TO FACILITATE CONTINUED DEV OUT OF ICING CONDITIONS.

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WE WERE CRUISING AT 11000 FT MSL IN AND OUT OF THE CLOUDS ON V-458. ALTHOUGH THERE WAS NO AIRMET FOR ICING OVER THE RTE; LIGHT RIME ICING HAD BEEN RPTED BY A PLT AT THIS ALT AND LOCATION 2 HRS PREVIOUSLY; SO I WAS WATCHING THE OAT CLOSELY. WE HAD EXPERIENCED VERY SLIGHT RIME AND HAD REQUESTED CLB TO 13000 FT TO CLR THE CLOUD LAYER. ATC WAS UNABLE TO CLB US AND; AS WE ENTERED A DARK CLOUD; WE EXPERIENCED FORECAST FREEZING RAIN; RAPID ICE BUILDUP AND CONTINUOUS MODERATE TURB. I REQUESTED AND WAS GRANTED A 180 DEG TURN. WE ALSO REQUESTED AND WERE CLRED TO DSND TO 9000 FT MSL. AS WE COMPLETED THE TURN; ATC ADVISED THAT IF WE WISHED TO CONTINUE L WE WOULD NEED TO DECLARE AN EMER. AS I WAS STILL ABOVE 10000 FT AND VMC WAS AVAILABLE ON MY L WHILE THE SAME CLOUDS WE HAD JUST ENCOUNTERED WERE ON MY R; I ELECTED TO DECLARE AN EMER TO ENABLE THE CTLR TO GRANT MY HDG REQUEST. WE REMAINED VMC WHILE DSNDING AND SELECTING ANOTHER AIRWAY TO OUR DEST; THEN REQUESTED VECTORS TO THE AIRWAY AND CONTINUED AT 9000 FT; EXPERIENCING RAIN WITHOUT ICING. I LEARNED FROM THIS TO TAKE PIREPS SERIOUSLY; AND PLAN AN ALTERNATE RTE WHEN ICING IS RPTED OR FORECAST; SO AS TO REMAIN BELOW THE FREEZING LEVEL. I KNOW ANY EMER DRAMATICALLY INCREASES CTLR WORKLOAD AND IMPACTS OTHER ACFT WHO HAVE NOT DECLARED; AND WILL TRY TO AVOID SITS; WHICH COULD LEAD TO HAVING TO DECLARE AN EMER IN THE FUTURE. I APPRECIATED THE EXCELLENT HELP GIVEN BY THE CTLRS AS I RECOVERED FROM THIS MISADVENTURE.