RWY EXCURSION; LOSS OF ACFT CONTROL.

Narrative

DURING THE FLAIR ON LNDG UNEXPECTED RAIN REDUCED OUR VIS TO 0. ACFT HYDROPLANED OFF THE LEFT SIDE OF RWY. AFTER ABOUT 1500' WE COULD SEE ENOUGH TO RETURN TO THE RWY. VIS WAS GOOD DURING APCH AND FAIR ON THE ROLLOUT END. WHAT WAS UNUSUAL ABOUT THIS IS THE SMALL AREA OF VERY HEAVY RAIN IN THE T/D ZONE. WE WERE LNDG ON RWY 27L AND DURING THE APCH YOU COULD SEE THE FULL LENGTH OF RWYS 26L AND R; WHICH DECEIVED US AS TO HOW HARD THE RAIN WAS ON RWY 27L. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES EXTREMELY UNUSUAL CONDITIONS. A LINE OF ISOLATED SMALL CELLS EXISTED DIAGONALLY ACROSS THE ARPT. ACFT HAD DSNDED THROUGH AREA ANTICIPATING GUSTS AND INSTEAD ENCOUNTERED FAIRLY SMOOTH AIR. ON APCH COULD SEE THE ARPT FOR 10 MI. ACFT IN TRAIL LNDG AHEAD HAD NO PROBS. RPTR'S ACFT FLARED AT MOMENT WHEN RAIN BECAME EXTREMELY HEAVY CUTTING VIS TO 0 FOR OUT 2000'. 12 KT XWIND MUST HAVE BEEN MUCH GREATER WITHIN THE RAIN AREA. WITH 0 VIS CREW COULD NOT KNOW THERE WAS A NEED FOR MORE CORRECTION. ATL IS KNOWN FOR STANDING WATER ON RWY. TWR HAD NO WAY TO KNOW INTENSITY OF THE RAIN TO GIVE WARNING. LEFT MAIN WHEEL ONLY WAS OFF RWY.