SUPER MD80 ENRTE DFW FL330 50 MI E MAF ENCOUNTERED SEVERE TURB RESULTING IN INJURY TO PAX. SEAT BELT SIGN AND WARNING HAD BEEN ISSUED WELL BEFORE ENTERING THE TSTM AREA. PAX WAS RETURNING FROM THE RESTROOM WHEN THE TURB HIT. DIVERTED TO IAH FOR MEDICAL ASSISTANCE. PAX ELECTED TO CONTINUE TO DFW.

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ENCOUNTERED SEVERE TURB IN IMC AT FL330. SEAT BELT SIGN TURNED ON WELL IN ADVANCE OF CLOUD PENETRATION. NO SEVERE TURB FORECAST. RADAR DID NOT SHOW ANY RETURNS WITHIN 80 NM. CHKED ALL RANGES AND TILTS. RADAR WAS WORKING NORMALLY. CTR HAD ADVISED OTHER ACFT HAD TRAVERSED AREA WITH NO PROBS. ADVISED FLT ATTENDANTS TO BE SEATED (VIA PA) WHEN CLOUD PENETRATION WAS IMMINENT. ENCOUNTERED SEVERE TURB IMMEDIATELY AFTER GOING IMC. FLT ATTENDANTS RPTED PAX HAD BEEN IN RESTROOM AND WAS TRYING TO RETURN TO HER SEAT WHEN SEVERE TURB BEGAN. SHE WAS APPARENTLY THROWN OFF HER FEET; AND COMPLAINED OF INJURY TO HER HEAD; NOSE; AND KNEE. DISPATCH WAS NOTIFIED VIA RADIO OF PAX INJURY. MEDICAL PERSONNEL MET ACFT AT IAH (WX DIVERSION FROM DFW). PAX ELECTED TO REMAIN ON ACFT AND CONTINUE TO DFW. SUPPLEMENTAL INFO FROM ACN 352303: THE RADAR ON THE MD80 HAS BOTH A WX AND A WX OR TURB MODE. WE WERE SWITCHING RANGES AND MODES PERIODICALLY TO PREVENT - - WE HOPED -- GETTING INTO THE SIT IN WHICH WE FOUND OURSELVES. THE TURB LASTED APPROX 3 MINS; THEN INTERMITTENTLY FOR A FEW MORE MINS.