CL65 CREW ON A VISUAL APCH TO SAV AFTER THE TWR IS CLOSED ENCOUNTERS WX BUT ELECTS TO CONTINUE.

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WE WERE ON AN IFR FLT PLAN; PART 121 SCHEDULED FLT TO SAV. WE WERE ESTIMATED TO ARRIVE SAV AT XA30. AT THIS LATE HR; THE TWR WAS CLOSED AND IT WAS NECESSARY TO GET THE FIELD WX OBSERVATION FROM ZJX. THE WX (RPTED BY AWOS) WAS A 20 MIN OLD OBSERVATION AND WAS VMC. CONDITIONS RPTED WERE: WINDS VARIABLE AT 5 KTS; SKY CLR; VISIBILITY 10 MI; TEMP 18 DEGS C; DEWPOINT 17 DEGS C; ALTIMETER 29.65. WHEN WE DEPARTED CVG; A LINE OF TSTMS WAS RPTED TO HAVE JUST PASSED OVER THE ARPT AT SAV. WHEN WE ARRIVED AT SAV; THEY HAD MOVED APPROX 20 MI E. WHEN WE WERE 10 MI FROM THE ARPT WE RPTED THE ARPT IN SIGHT; AS WE COULD SEE THE ROTATING BEACON. ZJX CLRED US FOR THE VISUAL APCH TO RWY 9 AND GAVE US A FREQ CHANGE. THE CAPT WAS THE PF; AND HE FLEW A DOWNWIND; BASE AND FINAL PATTERN. HE FLEW A VISUAL APCH BACKED UP WITH THE ILS TO RWY 9. AS WE LINED UP WITH THE RWY AND BEGAN DSNDING; IT BECAME APPARENT THAT THERE WAS A LOW-LYING SCATTERED TO BROKEN LAYER WITH BASES AT 500-600 FT AGL; AND WE WERE UNABLE TO REMAIN CLR OF THE CLOUDS. THIS INADVERTENT ENTRY INTO IMC WHILE ON A VISUAL APCH WAS CAUSED MOSTLY BY THE EXTREME DARKNESS OF THIS AREA (NO MOONLIGHT AND THIS AREA W OF THE ARPT IS UNDEVELOPED AND EXTREMELY DARK). A CONTRIBUTING FACTOR WAS THE LACK OF A HUMAN WX OBSERVER ON THE FIELD. THE AWOS WAS RPTING VMC; WHICH MAY HAVE BEEN THE CASE DIRECTLY OVER THE SENSOR PLATFORM; BUT WAS DEFINITELY IN ERROR. AFTER LNDG UNEVENTFULLY; WE CLOSED OUR IFR FLT PLAN WITH ZJX AND ISSUED A PIREP THAT CONDITIONS WERE MVFR AT THE ARPT AT BEST; FOR THE BENEFIT OF LATER ARRIVING ACFT.