A340 CREW ENCOUNTERED SEVERE TURB AFTER INADVERTENTLY ENTERING A TSTM AT NIGHT.

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WE DEPARTED MINNEAPOLIS ST PAUL INTL ARPT AT XX42 CDT ENRTE TO INDIANAPOLIS. OUR RTE WAS DIRECT TO DLH J38 GRB J101 BAE DIRECT JOT DIRECT WORDY DIRECT VHP DIRECT. THIS RTE WAS FILED BY DISPATCH TO AVOID AN AREA OF TSTM ACTIVITY E OF MINNEAPOLIS. AFTER TKOF WE PROCEEDED TO DLH AND LEVELED OFF AT FL270. WE HAD BEEN FILED FOR FL330 BUT THERE WAS A COMPANY RPT OF LIGHT TO MODERATE TURB AT FL330 AS WELL AS PIREPS FROM OTHER ACFT RPTING MODERATE TURB AT THAT ALT. WE COULD SEE WHAT APPEARED TO BE SOME CLOUDS ABOVE US; POSSIBLY SOME 'BLOW-OFF' FROM THE TSTM ACTIVITY SE OF OUR POS. THIS REINFORCED OUR DECISION TO LEVEL OFF AT FL270. ABOUT 50 NM S OF DULUTH AT FL270 WE OBSERVED A LINE OF WX ON OUR RADAR SCOPE APPROX 80-100 MI W OF OUR POS. TO THE E; TOWARD GREEN BAY; THERE WAS NO INDICATION OF WX ON OUR RADAR. WE REQUESTED DIRECT ROUTING TO GRB. THIS OBSERVATION LED US TO BELIEVE OUR RADAR WAS OK. SHORTLY AFTER TURNING TOWARD GRB WE ENCOUNTERED IMC CONDITIONS AND SOME LIGHT TURB AS WELL AS SAINT ELMO'S FIRE. WE LEFT THE SEAT BELT SIGN ON AFTER ADDRESSING THE PAX. THE FLT ATTENDANTS BEGAN THEIR SVC SHORTLY AFTER OUR LEVELOFF. WE SCANNED THE RADAR AT MULTIPLE RANGES AND A VARIETY OF TILT ANGLES. NOTHING APPEARED ON THE RADAR IN FRONT OF THE ACFT. AT YY17 CDT WE ENCOUNTERED A VIOLENT UPDRAFT ACCOMPANIED BY MODERATE RAIN AND INCREASED ST ELMO'S FIRE. THE AUTOPLT DISCONNECTED ON ITS OWN AND WE ASCENDED 1400 FT IN CONTINUOUS MODERATE TURB. WE REQUESTED AND RECEIVED CLRNC FOR AN IMMEDIATE L 90 DEG TURN TO EXIT THE WX. AFTER RETURNING TO FL270 WE TRIED TO LOCATE THE AREA OF WX ON THE RADAR BUT COULD NOT. ATC STATED THAT THEY COULD NOT LOCATE ANY SIGNIFICANT WX EITHER. WE WROTE UP THE ACFT RADAR AND CONTINUED TO INDIANAPOLIS. ONE FLT ATTENDANT HAD A MINOR INJURY.