FLC OF MD88 ENCOUNTERS SEVERE TURB AT CRUISE AT FL370. NO ADVANCE ALERTS BUT CUMULO NIMBUS BUILDING AROUND THE AREA. DSNDED TO AVOID THE TURB.

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ENCOUNTERED EXTREME TO SEVERE TURB OVER ELM GROVE; LA; AT FL370 DURING THE CRUISE PHASE OF FLT. NO RPTED TURB WAS INDICATED ON OUR FINAL WX RPT FOR OUR FLT LEVEL OR RPTED BY ATC. THE WX RADAR WAS ON AND INDICATED A CLUSTER OF CUMULO NIMBUS S OF OUR COURSE BY 30-40 NM. I REQUESTED AND RECEIVED CLRNC TO DEVIATE FURTHER N. AN UNDERCAST WAS BELOW WITH CLOUDS RISING TO APPROX FL380 AHEAD. AS WE STARTED TO APCH THE CLOUDS THAT WERE RISING AHEAD; I ASKED FOR THE FLT ATTENDANTS TO BE SEATED. THE SEAT BELT SIGN WAS ALREADY ON. WHILE STILL IN CLR AIR; WE STARTED TO PICK UP LIGHT TO MODERATE TURB. NO RADAR RETURNS WERE VISIBLE DIRECTLY AHEAD; AFTER ADJUSTING THE RADAR TO OBTAIN THE PROPER TILT FOR THE 40 MI AND 80 MI SCALES THAT I ALTERNATED BTWN. THE WIND WAS 250 DEGS AT 139 KTS. WE WERE UPWIND FROM THE CUMULO NIMBUS. UPON ENTERING THE CLOUDS AND GOING IMC; WE IMMEDIATELY ENCOUNTERED SEVERE TO EXTREME TURB. BOTH AUTOPLT AND AUTOTHROTTLES DISCONNECTED AND WE WERE +/-500 FT AND +/-10-15 KTS FROM FLT PATH. A PIREP WAS GIVEN AND A CLRNC TO FL290 ENSUED. THE ACFT WAS COMPLETELY UNCTLABLE EXCEPT IN THE VERT MODE. I MANAGED TO TURN ON CONTINUOUS IGNITION BUT HAD EXTREME DIFFICULTY FOCUSING ON THE INSTS; WITH ALL THE INTENSE SHAKING. WE WERE IN SEVERE TO EXTREME TURB FOR 60-120 SECONDS UNTIL BELOW FL290. NO PRECIP WAS NOTICED OTHER THAN ICE CRYSTALS. WE FINALLY LEVELED AT FL210 WHERE WE CONTINUED TO EXPERIENCE LIGHT TO MODERATE TURB IN CLR AIR FOR THE NEXT 10-15 MINS. THE COPLT SUFFERED A SPRAINED THUMB DUE FROM THE VIOLENT MOVEMENT OF THE CTL YOKE. ONE FLT ATTENDANT HAD A STRAINED/BRUISED BACK; BEING OUT OF HER SEAT; NOT HEARING MY PA TO BE SEATED. ONE OTHER FLT ATTENDANT WHO DID HEAR MY PA AND WAS SEATED CUT HER FINGER ON THE FOOD/BEVERAGE CART TRYING TO STEADY IT AS IT BOUNCED FLOOR TO CEILING IN THE AFT GALLEY. NO OTHER INJURIES WERE NOTED; BUT THE FLT ATTENDANTS DESCRIBED THE INTERIOR 'AS IF A BOMB WENT OFF' WITH BROKEN GLASS; FOOD; LIQUID; PAPERS; BLANKETS; PILLOWS; CLOTHING; ETC; EVERYWHERE.