MLG FLC ENCOUNTERS A WALL OF RAIN AND ACFT DRIFT ON TOUCHDOWN FOR LNDG. L MAIN GEAR LEAVES THE RWY.

Narrative

VISUAL APCH WAS FLOWN TO RWY 31L DAL WITH THE ILS AS A BACKUP. THERE WAS TSTM ACTIVITY TO THE N OF THE FIELD NOTED ON RADAR AND VISUALLY. THE VISUAL WAS FLOWN ALL THE WAY DOWN TO THE THRESHOLD AT WHICH TIME AN UNEXPECTED WALL OF WATER WAS ENCOUNTERED APPROX 50 FT ABOVE TOUCHDOWN. AFTER THE 30 FT; 10 FT RADAR ALT CALLOUTS I SENSED A DRIFT TO THE L AT WHICH TIME I VOICED THIS TO THE FO (PF). HE STATED HE WAS CORRECTING. THE L MAIN TOUCHED DOWN ON THE RWY APRON AND LEFT THE RWY AND CONTINUED OFF THE RWY FOR APPROX 800 FT. THE ACFT WAS RIGHTED BACK ONTO THE RWY AND SLOWED AT WHICH TIME I TOOK COMPLETE CTL OF THE ACFT WITH 'I'VE GOT THE ACFT.' AT THIS TIME FOLLOWING THE INCIDENT; WE WERE STILL NOT CERTAIN WE HAD IN FACT LEFT THE RWY SURFACE. ACFT WAS AT THE GATE WITHIN 1 MIN AND 'PARKING THROUGH FLT' CHKLIST WAS COMPLETED AT WHICH TIME I LEFT THE COCKPIT TO DETERMINE BY INSPECTION IF THE ACFT HAD DEPARTED THE RWY. THIS WAS VERIFIED BY INSPECTION OF THE L ENG; MAIN GEAR; WHEEL WELL. I RETURNED TO THE JETWAY AND NOTIFIED MAINT AND DISPATCH. DISPATCH NOTIFIED CHIEF PLT ON DUTY AND FAA COM CTR.