A GOV TWIN TURBOPROP DSNDS 500 FT BELOW HIS ASSIGNED ALT DURING WX AVOIDANCE MANEUVER WITH ZDC; DC.

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TURBULENT DAY WITH BUILDING CUMULO NIMBUS. REQUESTED ENRTE DSCNT IN ANTICIPATION OF SETTING UP FOR A VISUAL APCH TO OKU ARPT. CLRED TO A LOWER ALT WITH A STEP-DOWN MIDDLE ALT. WHILE DSNDING; DISCUSSING THE APCH SETUP AND ATTEMPTING TO MAINTAIN CLR OF CUMULO NIMBUS; PF INITIALLY DSNDED THROUGH STEP-DOWN ALT. CTLR QUERIED US AS WE PASSED 300 FT BELOW ASSIGNED ALT. IMMEDIATELY BEGAN CORRECTING BACK TO ASSIGNED ALT BUT WITH ESTABLISHED DSCNT RATE WE PASSED APPROX 500 FT BELOW ASSIGNED ALT BEFORE CLB ESTABLISHED BACK TO ASSIGNED ALT. WHEN QUERIED; PNF RPTED TO CTLR THAT WE WERE CORRECTING AND THAT PF HAD MISUNDERSTOOD STEP-DOWN ALT. NEXT LEG OF FLT FROM OKU TO RIC WHILE CRUISING AT 10000 FT BTWN CSN AND GVE; REQUESTED HIGHER ALT TO CLR LARGE BUILDING CUMULO NIMBUS. CTR DENIED HIGHER REQUEST. PF THOUGHT HE COULD CLR CUMULO NIMBUS TOP WHILE REMAINING WITHIN ALT TOLERANCES. HOWEVER; XING CUMULO NIMBUS TOP ACFT ENTERED UPDRAFT CAUSING ALT BUST. IMMEDIATELY DSNDED BACK TO ASSIGNED ALT UPON EXITING UPDRAFT. WX CLR AND VISIBILITY UNLIMITED ABOVE CLOUD TOPS. TOO CLOSE TO CUMULO NIMBUS TO CIRCUMNAV BY THE TIME IT WAS EVIDENT THAT ASSIGNED ALT WOULD NOT ALLOW ACFT TO CLR CUMULO NIMBUS TOP AND ANTICIPATED REQUESTED HIGHER ALT DENIED. WE SHOULD HAVE BEEN MORE ASSERTIVE IN EXPLAINING TO CTLR HOW QUICKLY AND WHY HIGHER ALT WAS NEEDED AND THAT WE ONLY NEEDED IT FOR A VERY FEW MI. SHORT OF DECLARING AN EMER; I FELT OUR ACTIONS TAKEN TO RIDE OUT THE UPDRAFT TURB WERE JUSTIFIED IN CONSIDERATION OF ACFT/AIR CREW SAFETY. I GUESS ANOTHER OPTION; THOUGH LESS PRUDENT; WOULD HAVE BEEN TO QUICKLY CANCEL OUR IFR CLRNC; CONTINUE VFR AND CLB/DSND AS NECESSARY PROVIDING OUR OWN OBSTACLE/ACFT SEPARATION.