COMMUTER LTT CANCELS IFR WHEN EXECUTING CIRCLE TO LAND. ENCOUNTERS IMC; DOES MISSED APCH.

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ON APCH INTO SONOMA COUNTY ARPT IN NORTHERN CA; WE DECIDED TO SHOOT THE ILS 32 AND THEN CIRCLE TO RWY 14 AS THE WINDS WERE VARIABLE 110 AT 8 TO 140 AT 4. WX AT THE ARPT AT THE TIME WAS BASED ON AN AWOS SYS AND WE WERE EXPERIENCING LIGHT TO MODERATE CHOP WITH RAIN. AWOS RPTED 1300 SCATTERED; 2200 BROKEN; 7000 OVCST AND 10 MI VISIBILITY. WITH THIS GOOD WX BUT WITH A TAILWIND TO A SHORT RWY WE ELECTED TO CIRCLE. WE BROKE OUT AT 1200 FT WITH VISIBILITY OF 5 MI. AT OUR CIRCLING MINIMUMS WE CIRCLED W OF THE FIELD ON A R DOWNWIND (COPLT FLYING). BASED ON HAVING THE ARPT IN SIGHT WITH THE GOOD AWOS RPT WE CANCELLED OUR IFR CLRNC. 2/3 OF THE WAY AROUND THE PATTERN WE INADVERTENTLY (NIGHT RAGGED CEILING) REENTERED IMC CONDITIONS WITH NO OPTION; BUT TO GO MISSED APCH (SONOMA COUNTY IS AT THE END OF A VALLEY WITH RISING TERRAIN). WE RECONTACTED ZOA FOR ANOTHER IFR CLRNC AND INFORMED THEM WE WERE FOLLOWING THE MISSED APCH PROCS. MORAL OF THIS STORY: DON'T CANCEL IFR ON ANY CIRCLE TO LAND UNLESS ABSOLUTELY NECESSARY.