ACR FLC ENCOUNTER SEVERE TURB FROM TSTM. VERT ALT EXCURSION IS 3500 FT. FLC RECOVER AND LAND WITHOUT FURTHER DIFFICULTY.

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ACFT WAS ON A SCHEDULED FLT FROM LAX TO MDW. FORECAST WX FOR MDW CALLED FOR TSTMS IN THE AREA; HOWEVER THE CREW HAD TALKED TO DISPATCH AND DETERMINED LNDGS WERE STILL IN PROGRESS AT MDW. A LINE OF TSTMS WAS E TO W FROM ABOUT KIRKSVILLE; MO; E TO UNDETERMINED BOUNDARY. THE ALTERNATE FOR FLT WAS ORD AND WAS OPEN. THE FUEL STATE WAS SUCH THAT A DIVERSION AROUND THE WX WAS IMMINENT. ZAU INDICATED A BREAK IN THE LINE FROM JUST E OF QUINCY; IL. THE ACFT'S WX RADAR ALSO CONCURRED SO THE DECISION WAS MADE TO PROCEED THROUGH THE BREAK; AND CTR RECLRED FLT DIRECT TO ROCKFORD; IL; NW OF CHICAGO. THE RIDE THROUGH THE BREAK IN THE WX WAS ONLY LIGHT TURB AND ACFT WAS DSNDED BY CTR ROUTINELY AND HANDED OFF TO CHICAGO APCH CTL. AS THE ACFT PROCEEDED FROM ROCKFORD TO DUPAGE TO MDW; ACFT WAS DSNDED TO 3000 FT MSL; AND ASKED TO SLOW TO 180 KTS. BTWN ROCKFORD AND DUPAGE THE LINE OF TSTMS REAPPEARED AND LAY JUST S OF CHICAGO. MDW WAS LNDG ON RWY 31C; AND WAS STILL RPTING VMC AND OPEN FOR LNDG. AT THIS POINT THE ACFT WX RADAR SHOWED VIRTUALLY TOTAL ATTENUATION. SO DEVS WERE ASKED FOR CHICAGO APCH AND WERE GRANTED TO GO N AROUND THE WX. RADAR DISPLAY STAYED THE SAME NO MATTER WHAT RANGE SCALE OR DEG OF TILT. POS OF THE ACFT WAS NOW ABOUT 10-15 MI W OF MDW WHEN VIOLENT TURB WAS ENCOUNTERED. CAPT WAS FLYING THE ACFT WITH AUTOPLT AND AUTOTHROTTLES ENGAGED. SUDDENLY THE ACFT STARTED LURCHING VIOLENTLY; THE AUTOPLT TRIPPED OFF AND IVSI (INSTANTANEOUS VERT SPD INDICATOR) PEGGED OUT AT 6000 FPM UP. AIRSPD WENT OVER 250 KTS AND THE ALT WENT TO APPROX 6500 FT IN A FEW SECONDS (APPROX 10-15 SECONDS). THE CAPT WAS HOLDING FULL FORWARD ON THE CTL WHEEL CALLING FOR AUTOPLT BACK ON; ALT HOLD ON AND FOR FLAPS UP AND CALL FOR RECLRNC FROM APCH. THE CREW TRIED TO CALL APCH CTL BUT ALTDEV OCCURRED BEFORE CONTACT COULD BE MADE. THE ACFT EXITED THE PROB AREA AT ABOUT 6500 FT AND CONTINUED TO FLY NORMALLY. AFTER COM WITH CHICAGO APCH WAS RE-ESTABLISHED; THE DECISION WAS MADE TO DIVERT TO ORD WHERE WX WAS VMC. A NORMAL APCH AND LNDG WAS MADE AT ORD. ON GND AT ORD CAPT CALLED APCH CTL AND THEY SAID OUR ACFT 'LOOKED LIKE A PING-PONG BALL' ON THEIR RADARSCOPES. A DETAILED GND INSPECTION OF THE ACFT WAS ACCOMPLISHED AT ORD; AND NO DAMAGE TO THE ACFT WAS FOUND.