AN E120 ACR FLC HITS SEVERE TURB AFTER LEAVING AN AREA OF TSTMS AND THEIR CABIN ATTENDANT IS INJURED. THEY THOUGHT THAT THEY HAD FINALLY GOTTEN OUT OF THE TURB WHEN THEY WERE IN CLR AIR.

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WHILE ATTEMPTING TO AVOID A SEVERE WX SYS WE DEVIATED APPROX 140 MI FROM OUR PLANNED COURSE. WE THOUGHT WE HAD CLRED THE WX AND TURNED ON COURSE AFTER NO LONGER PAINTING PRECIP ON OUR RADAR. AT THIS TIME WE HIT UNEXPECTED SEVERE TURB FOR ABOUT 20-30 SECONDS. THE PAX WERE SEATED; BUT THE FLT ATTENDANT UNFORTUNATELY WAS NOT. SHE INJURED HER NECK WHEN HITTING THE CEILING OF THE ACFT. THE TURB SOON STOPPED. WE CONTINUED THE FLT WITHOUT FURTHER INCIDENT. THE FLT ATTENDANT WAS ABLE TO PERFORM HER DUTIES; BUT DUE TO HER INJURY; HAD TO BE RELIEVED WHEN WE REACHED OUR DEST. I'M SUBMITTING THIS RPT BECAUSE I HAVE NEVER EXPERIENCED TURB LIKE THIS IN MY 9800 HRS OF FLYING. AT THE TIME IT SEEMED LIKE I WAS DOING THE RIGHT THING; BUT NOW I CAN SEE THAT I DIDN'T. ALTHOUGH I USUALLY HAVE THE FLT ATTENDANT SIT DOWN WHEN THERE IS DOUBT ABOUT THE POSSIBLE TURB; I DIDN'T DO IT THIS TIME. ALSO; I SHOULD HAVE GIVEN THE CLOUDS GREATER CLRNC. I MISJUDGED THE WX. THIS RESULTED IN AN INJURY TO OUR FLT ATTENDANT AND A FRIGHTENING EXPERIENCE FOR ALL OF US.