A DEICE EQUIPPED C210 DEPARTED A MOUNTAINOUS ARPT AND AT CRUISE ALT ENCOUNTERED ICING SEVERE ENOUGH TO CAUSE A BUFFET INDUCED DSCNT.

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I WAS FLYING OUT OF JACKSON; WY; ENRTE TO ZZZ1 WITH 3 COLLEAGUES; LOTS OF BAGGAGE; AND FULL FUEL. THE AIRPLANE WAS AT FULL GROSS WT; AND THE TKOF ELEVATION WAS 6400 FT. THE TURBO HANDLED TKOF AND INITIAL CLB WITH NO PROBS. MY PREFLT WX BRIEFING INDICATED THAT WE'D BE FLYING THROUGH 2 COLD FRONTS; ONE OF WHICH WAS JUST PASSING THROUGH JACKSON. WX WAS 6000 FT OVCST WITH THE USUAL AIRMET FOR ICING AND RPTS OF LIGHT RIME ICING IN THE CLOUDS. WE WERE ASSIGNED THE TETON 2 DEP DIRECT KICNE INTXN; DIRECT POCATELLO VOR. THAT DEP REQUIRES A CLB TO 14000 FT TO CLR THE MOUNTAINS. THE MOCA IS 10000 FT ALONG THAT RTE; WHICH WILL BECOME IMPORTANT LATER IN THIS STORY. WE ENTERED THE CLOUDS AT 12000 FT MSL AND DID NOT PICK UP ICE IMMEDIATELY. HOWEVER; WHEN WE REACHED KICNE INTXN AND TURNED W TOWARDS POCATELLO; WE BEGAN PICKING UP LIGHT RIME ICE. THAT DIDN'T BOTHER ME AT FIRST; BECAUSE IT WAS CONSISTENT WITH WHAT HAD BEEN FORECAST AND RPTED. HOWEVER; THE ICING QUICKLY BECOME MODERATE IN INTENSITY AS WE CLBED THROUGH 13000 FT. OUTSIDE AIR TEMP WAS ABOUT -4 DEGS C. MY AIRPLANE HAS THE FACTORY-INSTALLED 'FULL DEICE' SYS; WHICH INCLUDES PITOT HEAT; ELECTRIC PROP DEICE; BOOTS ON THE LEADING EDGES OF THE WINGS AND HORIZ STABILIZER; AND A REMOVABLE; HALF-HT WINDSCREEN HOT PLATE. INITIALLY; THE BOOTS DID A GOOD JOB OF CRACKING THE ICE OFF THE WINGS AND STABILIZER. HOWEVER; I NOTICED THAT THE BOOTS DIDN'T REMOVE ALL THE ICE; AND IT WAS QUICKLY BUILDING ON UNPROTECTED SURFACES. BY THE TIME WE CLBED TO 14000 FT; THE AIRSPD HAD BEGUN TO DECAY DUE TO ICE ACCUMULATION ON THE AIRFRAME. AT THIS POINT; POCATELLO WAS ABOUT 60 NM AHEAD AND I KNEW THAT THE WORST ICING WOULD BE OVER THE MOUNTAINS ENRTE. ONCE WE GOT WITHIN 10 NM OF POCATELLO; WE'D BE OVER THE SNAKE RIVER PLAIN AND ICING WAS UNLIKELY TO BE A PROB. SO; I DECIDED TO PRESS AHEAD RATHER THAN RETURN TO JACKSON. AS THE AIRSPD CONTINUED TO DECAY; THE AUTOPLT PITCHED THE AIRPLANE UP TO A HIGHER AND HIGHER ANGLE OF ATTACK. I KNEW THIS WASN'T GOOD; SINCE IT EXPOSED THE UNPROTECTED LOWER SURFACES OF THE AIRFRAME TO ICING. FINALLY; AT ABOUT 85 KIAS; THE AIRPLANE BEGAN TO BUFFET. IT SEEMED TO BE COMING FROM THE TAIL; AND I IMMEDIATELY THOUGHT 'TAIL STALL!' I DISENGAGED THE AUTOPLT AND PITCHED THE NOSE DOWN TO PREVENT A STALL. I CALLED ZLC; DECLARED AN EMER; AND TOLD THEM I COULD NOT MAINTAIN ALT AND WAS DSNDING TO ESCAPE MODERATE ICING CONDITIONS. THEY CLRED ME DOWN TO 12000 FT IMMEDIATELY AND TO 10000 FT WITHIN 50 MI E OF POCATELLO. THE AIRPLANE PICKED UP SPD AND BY USING THE BOOTS PERIODICALLY; I WAS ABLE TO MAINTAIN 13000 FT. SURE ENOUGH; WE FLEW OUT OF THE WORST ICING ONCE WE GOT CLR OF THE MOUNTAINS. AFTER THAT; THE CLOUDS THINNED OUT AND I WAS LEFT TO CONTEMPLATE THE FROZEN SCENERY ARRAYED ON MY AIRFRAME AS IT SLOWLY SUBLIMATED AWAY. THE TOTAL ACCUMULATION ON UNPROTECTED PARTS OF THE PLANE WAS ABOUT 1 1/2 - 2 INCHES.