LOSS OF ACFT CTL -- WX FACTORS -- ACR HITS A TSTM AND LOSES 800 FT AND HAS SEVERAL INJURIES.

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WHILE ON A DEV FOR TSTMS USING RADAR AND ATC AUTH WE FLEW BTWN 2 SUBSTANTIAL CELLS ABOUT 40 NM APART. WE WENT IFR BTWN THE CELLS WITH A GOOD RIDE UNTIL JUST COMING OUT THE BACKSIDE. WE SUDDENLY WENT VFR TO SEE A LARGE CUMULUS CLOUD IN OUR 12 O'CLOCK POS -- TOO LARGE AND TOO CLOSE TO AVOID; HAD POSSIBLY 2-3 SECONDS OF VFR BEFORE CUMULUS CLOUD PENETRATION. WE WERE AT FL390; TOP OF CUMULUS CLOUD ESTIMATED AT FL410; ATTEMPT TO WARN FLT ATTENDANTS WAS ALMOST SIMULTANEOUS WITH CUMULUS CLOUD ENTRY AND INEFFECTIVE. RADAR PAINTED NO ACTIVITY PRIOR TO ENCOUNTER. TURB WAS SEVERE FOR APPROX 8-10 SECONDS; ACFT VIOLENTLY DSNDED TO FL382 AND WAS RECOVERED TO ASSIGNED FLT LEVEL WITHIN 45 SECONDS. 2 FLT ATTENDANTS SUFFERED BROKEN BONES; 1 PAX ALSO INJURED; AFTER ASSESSING OUR SIT; DECISION TO DIVERT TO STL (90 NM AWAY AT TIME OF DECISION) WAS MADE; MEDICAL ASSISTANCE MET ACFT AND INJURED WERE DISPATCHED TO HOSPITAL. RADAR RANGES WERE VARIED; TILT WAS APPROPRIATE. RADAR PICTURE DEPICTED PRIOR TO CUMULUS CLOUD ENCOUNTER INSTILLED CONFIDENCE WITH VISUAL PICTURE PRIOR TO IFR PORTION OF FLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS FO WAS FLYING A B767-200 WHEN THE UPSET OCCURRED. THEY WERE INITIALLY ON J-80 WHEN THEY SAW A LINE OF TSTMS THAT SEEMED TO BE GROWING. THEY TURNED TO THE N TO SEE IF THEY COULD GET AROUND THEM AND SOON REALIZED THAT THE CIRCUMNAVIGATION WOULD TAKE THEM AT LEAST 150 NM N OF COURSE. JUST THEN THEY SPOTTED A SPACE ABOUT 40 NM WIDE BTWN 2 LARGE CELLS. THE CREW COULD SEE THAT THE TOPS OF THESE CELLS WERE WELL ABOVE THEIR ALT; BUT THE GAP WAS STABLE AND THERE WAS APPARENTLY NOTHING BEYOND THEM OR AT LEAST NOT ON RADAR OR VISUAL INDICATIONS. THEY ENTERED SOME CIRRUS CLOUDS AS THEY APCHED THE GAP. AS THEY EXITED THE GAP THEY BROKE INTO THE CLR AND SAW A LARGE; RAPIDLY GROWING AND SHARP EDGED CUMULUS CLOUD DIRECTLY IN FRONT OF THEM. THE CAPT BARELY HAD TIME TO MAKE THE 'FLT ATTENDANTS BE SEATED NOW' ANNOUNCEMENT BEFORE THEY HIT THE CELL. THEY WERE IN SEVERE TURB FOR ABOUT 5 SECONDS THEN IT BECAME MODERATE TO SEVERE FOR ABOUT 5 SECONDS THEN THEY WERE IN THE CLR AND IN VERY SMOOTH AIR AGAIN. 1 PAX AND 3 FLT ATTENDANTS WERE INJURED AND THE ACFT WAS DIVERTED TO STL TO SEEK MEDICAL ASSISTANCE. THE PAX RETURNED TO THE ACFT TO CONTINUE TO SFO AFTER EMER TREATMENT. 2 FLT ATTENDANTS HAD BROKEN PELVISES AND WERE REMOVED TO THE HOSPITAL. THE PAX WERE THEN FLOWN ON TO SFO WHERE IT WAS DISCOVERED THAT THE 3RD FLT ATTENDANT'S ANKLE WAS BROKEN INSTEAD OF SPRAINED AND SHE WAS SENT TO A HOSPITAL FOR TREATMENT.