PLT OF AN SMA SEL ENCOUNTERED ICING CONDITIONS AND HAD TO DIVERT TO LAND AT A NEARBY LARGER ARPT; WITH AN ILS AND LONGER RWY.

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IFR FLT FROM NQA TO CNC. THE PREFLT WX BRIEFING INCLUDED AN AIRMET FOR POSSIBLE ICING IN CLOUDS IN NORTHERN MISSOURI AND IOWA; BUT CEILINGS WERE SUCH THAT FLT OUTSIDE ICING CONDITIONS WAS POSSIBLE. THE FLT WAS UNREMARKABLE UNTIL JEFFERSON; MO; WHERE I ENCOUNTERED A THIN OVCST DECK AT ABOUT 5000-6000 FT MSL. AT THE TIME; I WAS IN CRUISE FLT AT 4000 FT MSL WITH EXCELLENT VISIBILITY. SUBSEQUENTLY; I WAS CLRED TO PROCEED DIRECT FROM THE MCM VOR TO THE CLARITON NDB. I LOWERED MY ALT TO 3200 FT MSL TO KEEP WITH THE GOOD VISIBILITY I HAD. BY KIRKSVILLE; MO; THERE WAS SOME LIGHT SNOW THAT REDUCED VISIBILITY TO ABOUT 5 MI. I THEN FLEW OVER THE TOWN AND ARPT OF CENTERVILLE; IA; WITH THE BEACON IN SIGHT. I THEN PROCEEDED DIRECT TO THE CLARITON NDB; AND ABOUT 15 MI SE I SUDDENLY FLEW INTO THE OVCST LAYER. THE DSM ATIS WAS BROADCASTING. I WAS UNABLE TO PICK UP THE CLARITON AWOS; AND DIDN'T KNOW THE ARPT CONDITION UNTIL DSM APCH INFORMED ME OF AN 1100 FT CEILING WITH 3 MI IN LIGHT SNOW. AT THE ENRTE MEA OF 3100 FT; I BEGAN PICKING UP LIGHT MIXED ICE ON THE WINGS; AND BY THE TIME I REACHED THE NDB; ENOUGH ICE HAD FORMED TO CAUSE ME TO FEEL UNCOMFORTABLE SHOOTING A NON PRECISION APCH TO A SHORT RWY WITH ICE ON THE AIRFRAME. I THEN ASKED FOR VECTORS TO DSM; AND WHEN INFORMED THAT THE LONG RWY WITH AN ILS WAS CLOSED FOR SNOW REMOVAL; I DECLARED AN EMER TO GET THAT RWY. I MAINTAINED A HIGH SPD OVER THE FENCE AND LANDED UNEVENTFULLY. I FEEL MY COMFORT LEVEL WITH THE WX BEING BETTER THAN FORECAST FOR MUCH OF THE RTE LED TO A LEVEL OF COMPLACENCY THAT CAUSED ME TO CONTINUE WHEN I SHOULD'VE LANDED AT CENTERVILLE. THE ICE BUILT SO RAPIDLY; I REALIZED I NEEDED HELP GETTING THE ACFT ON THE GND. FLYING AN AIRPLANE WITH ANTI- AND DEICING EQUIP CAN LEAD TO A FALSE SENSE OF SECURITY; EVEN WHEN FLYING A LIGHT UNEQUIPPED ACFT; AS IN MY CASE. NEXT TIME I WILL BE MUCH MORE ATTENTIVE TO WX CONDITIONS.