A PC12 PLT ON AN IFR FLT PLAN FROM COS TO SBS ENCOUNTERED UNEXPECTED CONVECTIVE ACTIVITY AND ICING. HE DEVIATED AND DSNDED TO CLR THE WX.

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I WAS IFR FROM COS TO SBS ON SEP/TUE/05. THERE WAS CONVECTIVE ACTIVITY ALONG THE FRONT RANGE. I CHKED THE WX PRIOR TO MY DEP TO MY DEP AND IT SHOWED MOST OF THE ACTIVITY OVER THE FRONT RANGE; E OF MY TRACK. AFTER TKOF AND CLBING THROUGH 17000 FT; I ASKED FOR CLRNC TO DEVIATE. THE CTLR GRANTED CLRNC TO DEVIATE. TSTMS REQUIRED SEVERAL DEVS AND ALT CHANGES. ONBOARD RADAR INDICATED LIGHT PRECIP THROUGH A CORRIDOR BTWN CELLS. THIS QUICKLY CLOSED IN ON ME. I IMMEDIATELY ATTEMPTED TO CONTACT CTR BUT HE WAS BUSY AND I COULD NOT GET IN TO HIM. THE ACFT STARTED TO ICE (LIGHT TO MODERATE) WHICH REQUIRED AN IMMEDIATE COURSE CHANGE. THE CTLR STATED THAT I WAS IFR WHICH REQUIRED CLRNC TO DEV AND I SHOULD HAVE ASKED. I EVENTUALLY WAS CLRED TO DSND INTO VFR CONDITIONS AND CANCELED. CONTRIBUTING FACTORS: FAST MOVING WX SYS; CONVECTIVE ACTIVITY; RADIO CONGESTION; TURB AND ICE AND REPEATEDLY BLOCKED RADIO XMISSIONS. RADAR SUMMARY CHART SHOWED MOST OF THE ACTIVITY TO THE E OF MY TRACK. THIS QUICKLY CHANGED. I SHOULD HAVE CONSIDERED RETURNING TO MY DEP ARPT.