B737 FLT CREW IS ISSUED A LOW ALT ALERT BY ATC DURING APCH TO HOU; DISTRACTING THEM ENOUGH THEY DON'T SELECT LNDG FLAPS UNTIL 500 FT AGL.

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THIS FLT WAS A 2 HR REROUTE DUE TO HOU WX. SHORTLY AFTER TKOF; THE ANTI-SKID INOP LIGHT ILLUMINATED. I SENT AN ACARS TO DISPATCH AND CHKED LNDG DATA FOR HOU AND OUR ALTERNATE OF SAT. BOTH WERE LEGAL FOR THE MEL. AFTER ENRTE REROUTES AND A COUPLE OF HOLDINGS WE WERE FINALLY CLRED TO HOU AND THE LOC RWY 22. TSTMS IN THE AREA; 9 MI; 2200 FT BROKEN; WINDS 200 DEGS 17 KTS GUSTING TO 28 KTS; ALTIMETER 29.50. WE BRIEFED THE APCH WITH THE FO FLYING. LNDG GEAR DOWN; FLAPS 15 DEGS WE STARTED DOWN FROM 1600 FT TO OUR MDA OF 500 FT. SHORTLY AFTER THE FINAL DSCNT WAS STARTED; TWR CALLED US WITH 'ALT ALERT; MINIMUM ALT FOR YOUR SECTOR IS 1500 FT; CHK YOUR ALTIMETER OF 29.50.' WE ARE VFR AND IT TAKES A LOT OF TIME TO DIGEST THE WARNING; LOOK AT THE TERRAIN DISPLAY; OUT THE WINDOW AT THE GND; OUR ALTIMETERS AND QUESTION OURSELVES WHAT IS WRONG. THEN ANOTHER ALERT FROM TWR; 'HVY RAIN SHOWERS ON THE APCH END OF THE RWY; WINDS 200 DEGS 20 KTS GUSTING 28 KTS.' NOW WE LOOK AT THE APCH END AND IT IS OBSCURED DUE TO THE HVY RAIN. NO PROB; WE'VE BRIEFED THE APCH. BUT WHAT WAS THE TWR TERRAIN ALERT ALL ABOUT? EVERYTHING LOOKS GOOD HERE AND NO TERRAIN WARNING IN OUR COCKPIT. I SETTLE DOWN ENOUGH AT 500 FT AND OUR MDA TO LOOK AROUND AND SEE WE WILL HAVE FLAPS 15 DEGS. FLAPS 40 DEGS IS SELECTED AND LNDG CHKLIST IS FINISHED. I TELL THE FO IT IS MY AIRPLANE AND I LAND. WITH THE ALERT DIVERSIONS FROM TWR; MY NORMAL FLOW WAS DISRUPTED. IF TWR HAD CONTINUED WITH ALERTS AND KEPT MY ATTN DIVERTED; THE FLAPS PROBABLY WOULD HAVE REMAINED AT 15 DEGS AND WE MAY HAVE ROLLED OFF THE DEP END OF THE RWY. FLAPS 30 DEGS OPC STOPPING MARGIN WAS 800 FT AND FLAPS 40 DEGS WAS 1100 FT. I'M STILL TRYING TO DEVISE MY OWN BACKUP LNDG CHKLIST.