ACR CHANGING FROM VISUAL APCH TO ILS APCH RECEIVES GPWS SIGNAL.

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FLT DEPARTED MSP FOR PDX. FLT WAS NORMAL UP TO THE APCH AND LNDG PHASE. FLT WAS CLRED FOR THE BONUL 2 ARR; BONUL AT 10000 FT; 250 KTS. OUR FMC HAD BEEN PROGRAMMED FOR THE BONUL ARR ALL THE WAY TO REEDI; THEN THE MILL VISUAL RWY 28R WITH A POINT PDX 079/9.0 AT 3000 FT. AS A BACKUP WE HAD THE ILS TO 28R. THE WX; AS MUCH AS I CAN REMEMBER; WAS 150 SCATTERED 10 MI VISIBILITY; WIND 320 AT 6. IT SEEMED TO BE AN EASY VISUAL APCH TO RWY 28R. AT THE PDX 079/9.0 WE COULD NOT SEE THE ARPT. A FOG BANK HAD FORMED OVER GOV AND MCGUIRE ISLAND. AS WE TURNED TO 240 DEGS; APCH ASKED US IF WE HAD THE FIELD; NO WE DID NOT. TURN TO HDG 210 DEGS AND DSND TO 2000 FT FOR ILS TO 28R. ALL OF THIS WAS TAKING PLACE ON OR NEAR THE LAKER INTXN. AS WE TURNED TO 210 DEGS; I REPROGRAMMED THE FMC WITH A DIRECT INTERCEPT INTO LAKER. WE PASSED THROUGH THE COURSE NBOUND. WE WERE THEN TURNED BACK TO 320 DEG HDG. AT THIS POINT WE HAD A VISUAL ON THE RWY. WE WERE CLRED FOR THE VISUAL. AS WE LINED UP WITH THE RWY; WE GOT TERRAIN; TERRAIN FROM THE GPWS; AND A CALL FROM TWR THAT WE WERE 3 MI; 800 FT. ALL OF THIS HAPPENED WITHIN SECONDS AND I FELT COMPLETELY OUT OF THE LOOP. FACTORS I BELIEVE THAT CAUSED THIS OCCURRENCE: WE WERE VMC WITH FOG BLOCKING VISUAL TO RWY. OUR SPD WAS 210 KTS. WE WERE TOO FAST AND TRYING TO SLOW. WITH MY ATTN ON SETTING THE COURSE INBOUND ON THE FMC; SLOWING THE ACFT BY PUTTING THE GEAR DOWN; PUTTING FLAPS DOWN; PLACING THE BUG BACK TO APCH SPD; A RADIO CHANGE FROM APCH TO TWR AND MONITOR A HAND FLOWN VISUAL APCH. I WAS AS I SAID EARLIER; OUT OF THE LOOP. YOU MUST REMEMBER THAT ALL OF THIS OCCURRED IN LESS THAN A MIN. AFTER THE GPWS TERRAIN; GS WAS CAPTURED AND NORMAL APCH AND LNDG. I MADE A COMMENT TO TWR ABOUT THE FOG AND HE SAID THEY WERE GOING TO CHANGE OVER TO 10 APCHS.