A B737-700 FLT CREW EXPERIENCES A RWY EXCURSION AS THEIR ACFT GOES INTO THE OVERRUN DURING A NIGHT OP DOWNWIND LNDG ON A SET RWY AT MSY; LA.

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ON APCH INTO MSY; WE EXPERIENCED WX DIFFERENT FROM ATIS. ATIS AT CRUISE WAS: WIND 080 DEGS AT 5 KTS; VISIBILITY 9 SM; 2500 FT BROKEN. THIS DATA WAS USED FOR OPC LNDG CALCULATION. RWY 19 WAS IN SIGHT 20 NM OUT AND WE WERE CLRED FOR A VISUAL APCH. ON APPROX A 2 MI FINAL; WE ENCOUNTERED A RAIN SHOWER AND FLUCTUATING AIRSPD. FO LANDED APPROX 2000 FT AFTER FLOATING DUE TO AN INCREASED TAILWIND. INITIAL REVERSE THRUST WAS 78% WITH MEDIUM BRAKING. I INSTRUCTED HIM TO INCREASE REVERSE THRUST DUE TO ANTICIPATED SLIPPERY RWY. REVERSE THRUST AND BRAKING WAS INCREASED. WHEN IT BECAME APPARENT THAT THE ACFT WAS NOT DECELERATING NORMALLY; I TOOK CTL OF THE ACFT AT APPROX 90 KTS WITH 2000 FT OF RWY REMAINING. THE ANTISKID CYCLED MULTIPLE TIMES WITH MAX EFFORT BRAKING AND WE ENTERED THE OVERRUN AT APPROX 20 KTS. IT IS POSSIBLE THAT SOME HYDROPLANING OCCURRED; BUT I WOULD NOT HAVE EXPECTED IT WITH THE MINIMAL AMOUNT OF WATER ON THE RWY. WHEN THE ACFT CAME TO A STOP; I ACCOMPLISHED A 180 DEG TURN AND TAXIED TO THE GATE. DUE TO THE ABNORMAL DECELERATION; WE MADE A PIREP RPTING POOR BRAKING CONDITIONS AND THE RAIN SHOWER ON SHORT FINAL. TWR INFORMED US THAT THE WINDS WERE NOW 090 DEGS AT 9 KTS (9 KT TAILWIND). TWR THEN TEMPORARILY CLOSED THAT RWY DUE TO THE POOR BRAKING CONDITIONS. I HAD THE FO PERFORM A THOROUGH EXTERIOR INSPECTION OF THE ACFT. NO DAMAGE NOTED. MSY ARPT OPS PERFORMED A RWY AND OVERRUN INSPECTION WITH NO DAMAGE NOTED. I SPOKE WITH TWR CTL ON THE PHONE AND WAS INFORMED NO RPT WAS NECESSARY (ON THEIR PART) DUE TO AN ACFT ENTERING AN OVERRUN. I ALSO SPOKE IN PERSON TO THE ARPT OPS PERSONNEL WHO CONDUCTED THE OVERRUN INSPECTION AND HE CONFIRMED NO DAMAGE. I CONTACTED THE CHIEF PLT ON ALL DISPATCH AND MAINT CTL. DIRECTOR OF FLT OPS ALSO JOINED IN ON THE CONFERENCE CALL. A DECISION WAS MADE TO HAVE CONTRACT MAINT PERFORM AN EXTERIOR INSPECTION. LOGBOOK WAS SIGNED OFF WITH NO DISCREPANCIES NOTED. AFTER CONFERRING WITH THE DIRECTOR AND THE CHIEF PLT; WE CONTINUED. THE FLT ATTENDANTS AND PAX WERE UNAWARE THAT ANYTHING ABNORMAL OCCURRED DURING THE LNDG. I SUBSEQUENTLY BRIEFED THE FLT ATTENDANTS ON THE SIT. I CONSIDERED RECONFIGURING TO FLAPS 40 DEGS; BUT DECIDED NOT TO SINCE WE WERE BELOW 1000 FT AGL. IN THE FUTURE; I WILL INSURE THE ACFT IS NOT FLOATED LONG; ESPECIALLY WITH THE UNEXPECTED WET RWY. I WILL DIRECT THE PF TO MORE AGGRESSIVELY APPLY REVERSE THRUST AND BRAKING EARLY IN THE LNDG SEQUENCE. THE FO AND I BOTH DISCUSSED IN GREAT DETAIL WHAT WE COULD HAVE DONE DIFFERENTLY TO INSURE THIS NEVER HAPPENS AGAIN. IT WAS A VALUABLE YET PAINFUL LEARNING EXPERIENCE FOR BOTH OF US. RECOMMEND THAT IN 'PT' SIMULATIONS; REJECTED TKOF EVENTS ARE ACCOMPLISHED ON A NEAR BALANCED FIELD CONDITION (STOPPING MARGIN 500 FT OR LESS); SO EACH PLT CAN EXPERIENCE WHAT IT'S LIKE TO PERFORM A MAX EFFORT BRAKING ON A WET RWY WITH ANTISKID CYCLING. SUPPLEMENTAL INFO FROM ACN 613550: AT ABOUT 3200 FT REMAINING AND 95 KTS; I CAME ON THE BRAKES. AT 2000 FT REMAINING; I CAME ON THE BRAKES WITH A GREATER SENSE OF URGENCY; FEELING THAT WE WERE NOT SLOWING DOWN AT A NORMAL RATE. AT 1000 FT IT WAS APPARENT THAT WE WERE NOT GOING TO BE ABLE TO STOP SMOOTHLY AND SAFELY WITHOUT GOING INTO THE OVERRUN. LOOKING BACK; THIS ALL COULD HAVE BEEN AVOIDED IF I WOULD HAVE LANDED 1000 FT WITH 40 DEGS FLAPS; AND USED MODERATE BRAKING.