B767-400 FLT CREW ENCOUNTERS SEVERE TURB WHILE TRANSITING BTW WX CELLS.

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THIS RPT IS SUBMITTED AS AN ALTDEV DUE TO A SEVERE TURB ENCOUNTER. AT FL390 AND APPROX 130 MI AWAY FROM A WX SYS; RADAR (AND VISUAL) INDICATED 3 CELLS; 2 BUNCHED TOGETHER (R SIDE) AND A THIRD (L SIDE) APPROX 30-40 NM FROM THE OTHER TWO. I FELT CONFIDENT I COULD TRANSIT THE GAP SAFELY IN VISUAL CONDITIONS ON TOP AND ELECTED TO PROCEED THROUGH THE GAP. I INSTRUCTED BOTH THE PAX AND FLT ATTENDANTS TO TAKE THEIR SEATS WELL IN ADVANCE. ALSO; WE GOT MINOR DEVS FROM MCI CTR IF NEEDED. RADAR (0-1 DEG) INDICATED LITTLE TO NO ACTIVITY IN THIS GAP. MINS PASSED AND AS WE TRANSITED THE GAP; I VISUALLY NOTICED THE CLOUDS WERE HIGHER THAN PREVIOUSLY THOUGHT AND REALIZED IT WAS CLOSING AND RISING. I WOULD PENETRATE SOME CLOUDS VERY CLOSE OFF THE NOSE THAT LOOKED 'HARD.' I WAS NOW NOT SURE HOW MANY MI I WOULD HAVE TO GO THROUGH (RADAR STILL NOT PAINTING MUCH) AND REALIZED I WOULD NOT STAY ABOVE SO I TURNED TO THE L INITIALLY THINKING I WOULD DO A 180 DEG TURN AND CALL CTR (EMER IF NECESSARY). WE STARTED TO ENCOUNTER MODERATE TURB AT THIS TIME. AFTER APPROX 30-40 DEGS OF TURN; I REALIZED I WOULD BE IN A WORSE SITUATION AS WE WOULD BE TOO CLOSE TO THE ISOLATED CELL ABEAM AND NOW BEHIND TO MY L (STILL VISUAL); SO I TURNED BACK R TO ORIGINAL COURSE AND IT WAS AT THIS TIME WE ENCOUNTERED SEVERE TURB AND THE BEGINNING OF A 1500 FT LOSS OF ALT. BEGINNING WITH THE FIRST TURN; THERE REALLY WAS NO ADEQUATE TIME TO INFORM ATC. WHEN CLR WE INFORMED CTR OF BOTH OUR ALTDEV AND SEVERE TURB ENCOUNTER. AN ACFT BEHIND ME WAS TOLD TO TURN AND NOT FOLLOW MY COURSE. THERE WERE NO SEPARATION ISSUES. THIS EVENT'S DURATION LASTED NO MORE THAN 1-1/2 MINS AND WE WERE BACK IN THE CLR SHORTLY THEREAFTER. COM WITH THE FLT ATTENDANTS WAS ESTABLISHED; THERE WERE NO INJURIES TO EITHER THEM OR PAX AS ALL WERE IN SEATS. IT WOULD HAVE BEEN BEST TO AVOID SYS ALTOGETHER; BUT AFTER MAKING MY DECISION I WISH I HAD STAYED ON COURSE TO MINIMIZE TIME IN TURB. I AM GLAD THAT WE HAD ALL CREW AND PAX IN SEATS AS A PRECAUTION.