HDG TRACK DEV BY SMA PLT WHEN IN PROX OF TSTM ACTIVITY.

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DEPARTED SGJ ON AN IFR FLT PLAN FOR FAY; NC. PREFLT WX BRIEF INDICATED 30 SCATTERED TO BROKEN 80 OVCST FOR RTE AND LR- UNTIL SAV; GA. SOON AFTER DEP AND NEAR CRG; ENTERED IMC. ENCOUNTERED LIGHT TO MODERATE TURB. OTHER ACFT IN AREA RPTING TURB AND 200-300 FT ALTDEVS. AFTER CRG; I ENCOUNTERED MODERATE TURB WITH INABILITY TO MAINTAIN ALT. VMC/IMC. ATC ASKED ABOUT FLT CONDITIONS. STATED MODERATE TURB WITH ALTDEVS. I ASKED FOR HDG DEVS TO AVOID BUILDUPS. MODERATE TO HVY RAIN DEVELOPED. ENTERED INTO IMC CONDITIONS. UPON EXITING A CLOUD BANK ENTERED A SMALL AREA (1-2 MI) OF VMC. NOTICED AN UNUSUALLY DARK BUILDUP WITH BILLOWING SIDES AND BASES THAT POSSIBLY CONTAINED MODERATE TO SEVERE TURB. THIS WAS DIRECTLY IN OUR PATH. IMMEDIATELY ASKED ATC FOR 30-40 DEG W DEVS (WE WERE HDG NNE). ATC APPROVED. ENCOUNTERED 300 FT ALTDEVS DUE TO UPDRAFTS/DOWNDRAFTS. I INITIATED A 180 DEG TURN BACK TO E WITHOUT A CLRNC. IN THE TURN TOLD ATC WHAT I WAS DOING. I TOLD HIM I WAS DOING 'A 360' (SHOULD HAVE SAID 180 DEG). ATC SAID 'NEVER TO DO THAT AGAIN; TFC MAY HAVE BEEN INTRAIL.' A SIGMET WAS BROADCAST FOR MODERATE TO SEVERE TURB FROM CHS-JAX-ALMA. I DON'T REMEMBER IF THAT WAS BEFORE OR AFTER THE EVENT; BUT VERY NEAR THE SAME TIME. WE WERE STORMSCOPE EQUIPPED WHICH SHOWED ACTIVITY BEHIND BUT NEVER IN OUR PATH. ATC WAS RPTING LEVEL 3 ACTIVITY. CAUSE OF PROB: SURPRISED BY WX DIFFERENT FROM FORECAST. NO RPTS OF TURB; TSTMS; ICE. IN MY JUDGEMENT; AN IMMEDIATE DEV WAS NECESSARY BUT I FORGOT TO ASK FIRST. ATC WAS CORRECT. I DID HAVE 30 SECONDS TO ASK.