CAPT OF AN ACR LTT QUESTIONED THE LEGALITY OF CONTINUING AN INST APCH WITH OVER AN HR OLD SPECIAL WX RPT.

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XA45 AFTER TKOF FROM HOT FOR A 30 MIN LEG TO HRO; WITH 5 HRS OF FUEL ON BOARD; THE WX IN HRO WENT DOWN; SO WE PROCEEDED TO OUR NEXT STOP BPK (15 MIN LEG). AT ABOUT XB45 WE MISSED AT BPK. WE HAD SUFFICIENT VISIBILITY AT BOTH PLACES TO SHOOT APCH. WE DECIDED TO GO BACK TO HOT. EVIDENTLY; THE WX OBSERVER PUT A SPECIAL OBSERVATION RPT (SA) IN AT XC00 AND WENT HOME. 15 MINS OUT OF HOT; ZME ADVISED US THAT THE HOT WX WAS EXPIRED; GAVE US LIT WX AND A PIREP OF A COMPANY ACFT THAT JUST LANDED. WE WERE ALSO TALKING TO THE COMPANY ACFT. THE (COMPANY ACFT) WERE ABOUT 10 MINS AHEAD OF US; THEY SAID 'WE BROKE OUT AT 1800 FT' AND THE 'VISIBILITY WAS GOOD.' FIELD ELEVATION IS 540 FT SO WE HAD OVER A 1000 FT CEILING. WE WENT AHEAD AND LANDED AT ABOUT XD15 WITH NO PROB. I BASED MY DECISION TO LAND ON THIS: THE SPECIAL OBSERVATION RPT WAS 1 HR 10 MINS OLD. WE HAD A PIREP OF AN ACFT 10 MINS AHEAD. WE DID HAVE KAVOURS WX THAT WAS CURRENT. THE APCH PLATE HAD MINIMUMS TO USE WITH LIT ALTIMETER SETTING WHICH WE HAD. I STILL HAD 3 1/2 HRS OF FUEL ON BOARD. MY ORIGINAL ALTERNATE WAS DFW; IT WAS ONLY 1 HR AWAY. I KNOW THEY PUT SPECIAL OBSERVATION RPTS IN EVERY HR; BUT I CAN'T FIND WHERE IT SAYS THEY ARE ONLY GOOD FOR 1 HR. NO ONE QUESTIONED WHAT WE DID; I'M JUST NOT SURE IF IT WAS 100 PERCENT LEGAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE WAS CONCERNED ABOUT THE LENGTH OF TIME THAT THE SPECIAL OBSERVATION WAS VALID FOR HIS INST APCH. HE WAS ADVISED THAT THIS WOULD HAVE NO LEGAL AFFECT ON HIS FLT SINCE HE WAS ALREADY ENRTE SINCE IT WOULD AFFECT ONLY THE PREPLANNING OF THE FLT PRIOR TO DEP. OF COURSE; WHILE ENRTE; IF A LATER RPT (OFFICIAL) WAS RECEIVED PRIOR TO THE FINAL APCH FIX; THAT INDICATED THAT THE WX WAS BELOW ARPT MINIMUMS; THEN A PART 135 OR PART 121 INST APCH COULD NOT BE COMMENCED. HOWEVER; SINCE THE LATEST WX WAS INDICATED ABOVE MINIMUMS; THE RPTR WAS LEGAL TO MAKE THE APCH.