B737 FLT CREW DIVERTS TO IAD AFTER RAIN AND INOPERATIVE COMPONENTS MAKE LANDING AT DCA DICEY.

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I WAS THE CAPT ON FLT XXXXX ON JUL/XX/08 AND PF FROM ZZZ-DCA. ON ARR AT DCA; RAIN SHOWERS HAD CONTINUED TO AFFECT THE STATION AND THE TWR RPTED THE RWY AS BRAG (RJ 2 MINS EARLIER) WITH A WET RWY. RWY 1 AT DCA IS 6869 FT AND THE RWY REQUIRED FOR US AT 135000 LBS ACCORDING TO OUR LNDG DISTANCE CARD IS 5785 FT. ADDITIONALLY; WE HAD 2 MEL'S THAT AFFECTED LNDG DISTANCE; INOP #1 THRUST REVERSER (ROUNDED UP TO 200 FT FROM PERFORMANCE; LNDG) AND THE AUTO SPD BRAKE SYS OPTION #2; MEL 27-2B; 350 FT CORRECTION FOR FLAPS 40 DEGS; WET RWY. THAT REQUIRED 6335 FT AND LEFT US WITH A 'CUSHION' OF 534 FT; AND AT XA00 HRS ON THE 3RD LEG OF THE DAY; IT 'DID NOT FEEL RIGHT.' IT WAS IN MY JUDGMENT TO DIVERT TO OUR ALTERNATE. SO ON SHORT FINAL; IN THE RAIN; WET RWY; WE ELECTED TO DISCONTINUE THE APCH AND 'GO MISSED.' THE CONDITIONS WERE NOT GOING TO IMPROVE AND IAD WAS A CLOSE; SUITABLE ALTERNATE. IN THE EVENTS LEADING UP TO THIS; OUR AIRPLANE HAD THE REVERSER PLACARDED FOR SEVERAL DAYS. WE HAD FLOWN THE AIRPLANE ALREADY TWICE IN THAT DAY AND ON THE INBOUND LNDG AT ZZZ PRIOR TO OUR DCA FLT; WE HAD A 'SPD BRAKE DO NOT ARM' LIGHT AFTER LNDG. MAINT THEN PLACARDED THE AUTO SPD BRAKE WHICH WOULD PREVENT AUTO DEPLOYMENT ON LNDG. I HAD PHONED THE DISPATCHER IN OPS REGARDING THE WX IN DCA AND DISCUSSED WITH HIM THE CURRENT WX THERE (RAINING) AND WHAT LOOKED LIKE A SECONDARY LINE W OF THE ARPT. HE THOUGHT THAT THE WX WOULD IMPROVE PRIOR TO OUR ARR AND HE THOUGHT IT WOULD 'DIE OUT' AND NOT BE A FACTOR. WHEN I WENT TO THE AIRPLANE; MY FO WAS ALREADY IN THE BOOKS AND LOOKING AT LNDG DISTANCES WITH THE NEW MEL. WE DISCUSSED THE LNDG DISTANCES AND I CALLED THE DISPATCHER (WITH THE MECH WHO PLACARDED THE SPD BRAKES IN THE COCKPIT TOO) AND DISCUSSED FURTHER THE CONCERN ABOUT THE RAIN AND STOPPING DISTANCE AND THE NEW INFO REGARDING THE SPD BRAKES. HONESTLY; I CANNOT RECOLLECT THE CONVERSATION FULLY; BUT WAS CONVINCED TO CONTINUE BASED ON OUR DISCUSSION AND HIS THOUGHTS ON OUR ARR AT DCA. I FELT CONFIDENT THAT WE WOULD BE ABLE TO COMPLETE THE FLT. ENRTE; THE FO AND I CONTINUALLY CHKED THE WX AND SAW HEAVIER RAIN APCHING FROM THE W AND THAT THE RAIN WAS CONTINUING AT DCA. WE WERE 'RETHINKING' THE DECISION TO LAND WITH THE CONDITIONS AS THEY WERE. WE RAN THE NUMBERS AND SENT AN ACARS MESSAGE TO DISPATCH AS FOLLOWS: 'HEADS UP; IF DCA RWY WET MY CALCS SHOW US NEEDING 5785 FOR WET RWY 200 FT FOR INOP RVRS AND 360 FT FOR AUTO SPD BRAKE. 6345 RQD; LET'S LOOK AT IAD.' HE RESPONDED WITH THIS: 'GOOD EVENING; RWYS AT DCA ARE GROOVED; DCA TWR RPTS RWYS AS DAMP; THEY SAY FLT LNDG RWY 19 ROLLOUT TO INT AT RWY 22 OR TXWY A. THE RESTR FOR MEL 27-13B IS TKOF WT PLNTY; NOT LNDG. THE PNLTY FOR MEL 27-02B IS USABLE LNDG BEYOND THRESHOLD 6869 MAX PNLTY EVEN WET IS 350 FT.' BY THIS TIME; WE WERE GETTING PRETTY BUSY AND WE LOOKED AT EACH OTHER AND SAID; 'DAMP?' NOT EXACTLY SURE WHERE THAT FITS INTO THE CHARTS. WE DISCUSSED AMONG OURSELVES THAT IF THE RWY WAS 'WET' WE WOULD CHOOSE NOT TO LAND. WE WOULD CONTINUE TO THE ARPT AND SEE WHAT THE ACTUAL CONDITIONS WERE. CHKING IN TO THE TWR FREQ ON FINAL; THE FO ASKED SPECIFICALLY IF THE RWY WAS WET AT WHICH TIME THE CTLR RESPONDED IN THE AFFIRMATIVE. AFTER LNDG AT IAD AND WAITING FOR PERSONNEL TO ARRIVE; I WAS ON THE PHONE WITH THE DISPATCHER DISCUSSING PARKING AND PAX ISSUES; ETC. HE WAS WANTING US TO REFUEL AND CONTINUE BACK TO DCA. I INFORMED HIM THAT WE WERE NOT IN AGREEMENT WITH THAT IDEA. AFTER SOME DISCUSSION HE TOLD ME THAT IF WE WERE GOING 'FATIGUED' THAT I WOULD NEED TO CALL CREW SCHEDULING. I TOLD HIM THAT WHEN WE PARKED THE JET THAT WE WOULD BE FATIGUED AND THAT I WOULD CALL THE SCHEDULER. I DID CALL CREW SCHEDULING AND IT ENDED UP BEING A MOOT POINT; DIDN'T PARK UNTIL XA12 AND WE WOULD NEVER HAVE BEEN ABLE TO COMPLETE THE FLT ANYWAY WITHIN OUR DUTY DAY. SOLUTION? I LEARNED SOME THINGS FROM THIS FLT FOR SURE. I'LL BE MORE PROACTIVE AT THE 'FRONT END' CONCERNING THE CONDITION OF MY AIRPLANE AND THE CONDITIONS THAT I'LL BE FLYING INTO.ALTHOUGH THE FORECAST: TEMPO 0105 5SM -SHRA FM 0600 21004KT P6SM BKN030 BKN080 WASN'T SHOWING TOO MUCH; MY EYEBALLS INDICATED SOMETHING OTHER. ALTHOUGH MANY PAX WERE INCONVENIENCED; I BELIEVE WE MADE THE RIGHT DECISION IN FLYING TO THE ALTERNATE.