ACR X RADAR VECTOR BELOW MVA RECEIVED GPWS CLB. SYS ERROR.

Narrative

X WAS ON APCH TO RWY 33 AT ROA FOR AN ILS TO THIS RWY. ATIS VISIBILITY WAS 4 MI PREVAILING. I WAS ADVISED BY APCH CTL THAT THE ROA TWR WAS RPTING 1 MI VISIBILITY. THIS WAS BELOW MINIMUMS FOR ILS 33 ROA; 1 1/4 MI REQUIRED FOR THIS APCH. (AIM PROCS FOR VISIBILITY PREVAILING VERSUS TWR MEASUREMENTS.) I ASKED FOR VECTORS TO THE LDA RWY 6 AT ROA NEEDING 1 MI VISIBILITY FOR THIS APCH. I WAS AT 5000 FT MSL AND THROUGH THE CTRLINE FOR LDA RWY 6 HDG S AND E. I WAS GIVEN A TURN W TO PARALLEL DOWNWIND THE APCH COURSE FOR LDA RWY 6. THIS WAS ABOUT A 240 DEG HDG. AT ABOUT 12-15 DME ON THE LDA FREQ OF 111.1 FOR RWY 6 AT ROA; I WAS TURNED TO A HDG FOR A R BASE ABOUT 330 DEGS AT 5000 FT MSL. UPON ROLLING OUT ON BASE I RECEIVED A GPWS TERRAIN WARNING. THE VISIBILITY WAS VERY MARGINAL AND I IMMEDIATELY BEGAN A CLB TO 5500 FT MSL ADVISING THE COPLT TO TELL THE CTLR. ATC WAS ADVISED OF OUR GPWS TERRAIN WARNING IMMEDIATELY AND WE WERE CLRED TO 5500 FT MSL. I CONFORMED TO COMPANY PROCS FOR UNKNOWN GPWS TERRAIN PULL UP WARNINGS WITH CLBING MANEUVER. THE APCH TO ROA LDA RWY 6 WAS COMPLETED SAFELY.