ACR FLC CANCELS IFR FLT PLAN WHILE STILL IN IMC.

Narrative

WHILE BEING VECTORED FOR AN ILS APCH INTO MKL WE HAD THE FIELD CLRLY IN SIGHT OVER 6 MI AWAY. THE WX WAS RPTED AS 4000 FT OVCST AND 2 1/2 MI OF VISIBILITY. ZME ASKED US TO CANCEL ASAP ON THE GND DUE TO TFC TO FOLLOW. WITHOUT CHKING WITH ME; MY FO CANCELED IFR AT THIS POINT BECAUSE HE HAD THE FIELD IN SIGHT. I SHOULD HAVE STOPPED HIM AND INFORMED CTR THAT WE WANTED TO REMAIN ON OUR IFR FLT PLAN; BUT BECAUSE WE SO CLRLY HAD THE FIELD SO FAR AWAY. I LET IT GO. CTR THEN HAD US CONTACT JACKSON RADIO FOR ADVISORIES. UPON CONTACTING JACKSON RADIO WE WHERE TOLD THE FIELD WAS IFR BECAUSE OF THE 2 1/2 MI VISIBILITY RPT. WE TOLD THEM THAT WE HAD THE FIELD OUTSIDE THE OM; AND A KING AIR TAXIING AND ALSO TOLD RADIO THAT VISIBILITY WAS MUCH BETTER THAN THE RPTED 2 1/2 MI. WE CONTINUED AND LANDED; BUT AFTER LNDG; I REMEMBERED THAT EVEN THOUGH WE HAD MORE THAN ENOUGH VISIBILITY; IT WAS THE RPT THAT MATTERED. 5 MINS AFTER OUR LNDG THEY CHANGED THE VISIBILITY TO 5 MI. MY FO AND I DISCUSSED THE PROB ON THE GND; AND I MADE IT CLR TO HIM NOT TO CANCEL IFR WITHOUT CHKING WITH ME FIRST; AND NEVER TO CANCEL WHEN THE CTL ZONE IS IFR. CONTRIBUTING FACTORS ARE: THIS WAS OUR 4TH CONTINUOUS DUTY OVERNIGHT IN A ROW. SECOND I HAD FLOWN WITH THIS FO FOR 3 WKS; AND HE HAD DONE AN EXCELLENT JOB; SO I WAS NOT MONITORING HIS ACTIONS LIKE I SHOULD FINALLY THE INACCURATE WX RPT CAUSED THE WHOLE PROB. IF IT HAD BEEN UPDATED PROPERLY; THE CTL ZONE WOULD NOT HAVE BEEN IFR.