WX RPTING EQUIP PROB ASOS.

Narrative

THE ASOS HAS BEEN IMPLEMENTED AT THE MGM ARPT AND WE HAVE BEEN USING IT FOR APPROX 3 MONTHS. THE WX OFFICE HAS CLOSED AND A GROUP OF CONTRACT EMPLOYEES HANDLE THE HRLY AND SPECIAL OBSERVATIONS. ON NOV/XX/95; I WAS WORKING LCL CTL AND WAS THE CIC IN THE TWR. WE HAVE ONLY ONE CTLR IN THE TWR AFTER XX00 PM. BECAUSE IT WAS QUIET; I HEARD THE ALL WX LNDG SYS BEEP WHICH NORMALLY MEANS A SPECIAL OBSERVATION. THE HR BEFORE HAD SHOWED VISIBILITY GREATER THAN 10 MI. THIS OBSERVATION (SPECIAL) SHOWED 2 1/2 MI. THE RADAR CTLR HAD A TWIN INBOUND AT THE TIME WHICH WOULD HAVE BEEN ON A VISUAL APCH EXCEPT NOW THE WX WAS IFR. I LOOKED OUT THE WINDOWS AND ESTIMATED THE VISIBILITY TO BE 3 MI OR BETTER. I IMMEDIATELY CALLED THE CONTRACT PEOPLE AND ASKED WHY I WASN'T ALERTED TO THEM DROPPING THE VISIBILITY AND EXPLAINED THAT IT AFFECTED OUR OP. HE STATED THAT THEY HAD NO PROCS TO NOTIFY US OF OBSERVATIONS THAT CHANGED FROM VFR TO IFR. I SAW ACFT 8 MI FROM THE RWY. THE ASOS IS LOCATED ON THE W END OF THE ARPT AND WE HAD REDUCED VISIBILITY AND FOG ON THE W END BUT THE PREVAILING VISIBILITY WAS 3 MI OR MORE. ONLY THE WEST 1/3 OF THE RWY WAS COVERED WITH FOG. THE REST OF THE ARPT WAS CLR. I TALKED WITH THE CONTRACT WX PERSON AGAIN AND ASKED HIM TO TAKE ANOTHER OBSERVATION VISUALLY AND SHOW THE VISIBILITY AS 2 1/2 MI SINCE THE FOG DID APPEAR TO BE MOVING IN (VISIBILITY WAS BTWN 3 AND 4 MI). HE PUT OUT ANOTHER OBSERVATION AND SHOWED VISIBILITY 1 1/4 MI. WHEN I ASKED HIM WHY NOT 2 1/2 (WHICH PREVAILING WE HAD IT EASY) HE SAID HE HAD TO GO BY THE LOWER VISIBILITY WHICH WAS WHAT THE ASOS SHOWED. WE HAD AN MD88 ON APCH AND THE CONTRACT WX PERSON CALLED ME AND STATED THAT THE VISIBILITY ON THE ASOS WAS LESS THAN 1/4 OF A MI. HE WAS FIXING TO PUT IT OUT ON AN OFFICIAL OBSERVATION. I LOOKED OUT THE WINDOW AND SAW THE MD88 ON FINAL 8 MI OUT. THE PREVAILING VISIBILITY WAS 2 MI AT TWR LEVEL. THE FOG WAS 50 FT ABOVE GND LEVEL AND THE RWY WAS VISIBLE EXCEPT ON W END AT THE ASOS LOCATION. I TOLD THE CONTRACT WX PERSON NOT TO PUT OUT LESS THAN 1/4 MI. HE OBLIGED ME BUT LATER TOLD ME THAT HE WAS REQUIRED TO PUT OUT THE LOWER VISIBILITY AND WAS STICKING HIS NECK OUT BY NOT DOING IT. I TOLD HIM THAT THE PLT COULD NOT HAVE LANDED IF WE PUT OUT LESS THAN 1/4 OF A MI. WE HAVE NO RVR FOR RWY 28 SO THE PLT HAD TO HAVE 1/4 OF A MI OR MORE. I HAD THE PLT OF THE MD88 CALL ME AND ASKED HIM ABOUT THE WX. HE HAD THE RWY IN SIGHT THE WHOLE APCH. THE ASOS DOES NOT WORK IN THESE SITS.