A DH8 CREW ON VECTORS FOR A VISUAL AT TPA DEVIATED FROM ASSIGNED HDG BY 40 DEGS TO STAY CLR OF WX; WITHOUT ADVISING ATC.

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TPA WAS USING RWY 36L&R FOR LNDGS. WE WERE ADVISED TO EXPECT VECTORS FOR A VISUAL APCH TO RWY 36L SINCE WE WERE APCHING THE FIELD FROM THE NW. A TSTM SW OF THE FIELD DICTATED THAT APCHS WOULD HAVE TO BE MADE BTWN THE OM AND THE THRESHOLD. RAIN WAS LIGHT IN THIS AREA BUT LIGHTNING WAS PRESENT. WE HAD BEEN CLRED TO DSND TO 1600 FT ON A HDG OF 110 DEGS. ONCE WE WERE BELOW THE BROKEN CLOUDS; I NOTICED THE CAPT STARTING TO VEER L OF THE ASSIGNED HDG. I ASKED IF HE WANTED ME TO ASK FOR DEVS TO THE L AND HE SAID NO. AS HE CONTINUED TURNING L PAST A HDG OF 100 DEGS; I WENT AHEAD AND TOLD TPA APCH THAT WE NEEDED TO DEVIATE SLIGHTLY L. THEY WERE BUSY AND DIDN'T REPLY (OR THEY WERE TRYING TO FIGURE OUT HOW TO APPROVE THAT WHILE VECTORING US FOR SUCH A CLOSE APCH). THE APT KEPT TURNING UNTIL HE WAS 40 DEGS L OF THE ASSIGNED HDG. A FEW SECONDS LATER HE TOLD ME TO RPT THE FIELD IN SIGHT AND REQUEST CLRNC FOR THE VISUAL TO RWY 9. THOUGH BECOMING FLUSTERED BY THIS POINT; I DID AS INSTRUCTED AS WE CONTINUED TOWARD RWY 9; WHICH WAS IN SIGHT ABOUT 3 OR 4 MI AHEAD. AN ACFT WAS LNDG ON RWY 26L. APCH SAID 'STAND BY;' THEN CLRED US FOR THE VISUAL TO RWY 9 AND HANDED US OFF TO TWR A FEW SECONDS ALTER. WE LANDED WITHOUT FURTHER INCIDENT. HUMAN PERFORMANCE CONSIDERATIONS: THE CAPT IS WELL KNOWN IN BASE AS BEING EXTREMELY WX SHY DUE TO PREVIOUS UNHAPPY EXPERIENCES WITH LIGHTNING AND TURB. I HAD WITNESSED AN INCREASE IN THE INTENSITY OF HIS CONCERN AND VIGILANCE DURING PREVIOUS LEGS OF THIS TRIP; BUT HE HAD FOLLOWED CLRNCS AND COMMUNICATED HIS INTENTIONS VERY WELL UP UNTIL THEN. I WAS PROBABLY NOT ASSERTIVE ENOUGH IN RPTING HIS HDG DEVS TO HIM; BUT I DON'T THINK IT WOULD HAVE CHANGED ANYTHING. I FOUND OUT AFTER THE FACT THAT HE WAS FOLLOWING HWY 60 TO THE ARPT TO AVOID LIGHTNING AND TURB BY THE WIDEST MARGIN. FACED WITH A 2-EDGED SWORD (LIGHTNING AND ATC CLRNC); I ATTEMPTED TO ASSIST WITH THE CAPT'S UNKNOWN INTENTIONS AS QUICKLY AS I COULD ADAPT TO THE CHANGING SIT.