A B737 ON A VISUAL APPROACH TO BOI RECEIVED A TERRAIN WARNING FROM THE EGPWS.

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DSCNT TO BOI FROM NW. SOME SCATTERED HIGH CLOUDS AND NO MOON ILLUMINATION OF TERRAIN. VISIBILITY NO PROB. ON THE DSCNT ABOUT 20 MI OUT; APCH PAINTED AN INITIAL TFC PICTURE AND OUTLINED THEIR PLAN FOR THE APCH. THERE WAS ANOTHER CARRIER ON ABOUT A 10 MI FINAL TO RWY 28 WHO WAS IN SIGHT BUT WOULD NOT BE A FACTOR. WE WERE APCHING FROM THE NW AND WERE TOLD WE WOULD BE A R BASE TO RWY 28R. THERE WAS A DIFFERENT CARRIER FLT ABOUT 2 O'CLOCK POS WHO WAS TO ACCOMPLISH A L BASE TO RWY 28L. WE WERE OVERTAKING THIS ACFT AND IT APPEARED TO ALL INVOLVED THAT WE WOULD ALL ARRIVE AT BASE AT THE SAME TIME TO THE PARALLEL RWYS. CAPT BRIEFED THE SIT. HE WAS CONCERNED ABOUT THE R BASE AND I THINK HE SAID HE HADN'T DONE IT BEFORE. I SAID THAT I HAD BUT IN DAY VMC. WE WERE BOTH IN TERRAIN MODE AND DISCUSSED THE RISING TERRAIN N OF THE FIELD. WE WERE GIVEN A DSCNT TO 7000 FT WHILE WE TALKED. OUR FLT PATH TOOK US ABOUT 3 MI N OF BOI ON A HDG OF ABOUT 120 DEGS (CONVERGING TO FINAL SLIGHTLY -- MAKES GOOD SENSE WITH TERRAIN AS IT LAYS). ABOUT 3 MI N OF THE FIELD WE WERE CLRED FOR A VISUAL APCH TO RWY 28R AND INSTRUCTED TO MAINTAIN 4500 FT OR ABOVE UNTIL ON FINAL. WE BOTH REACTED THE SAME WAY SAYING THAT 4500 FT WAS TOO LOW FOR WHERE WE WOULD BE FLYING. WE STARTED A GRADUAL DSCNT AND SLOWED TO CONFIGURE. CAPT CUT THE DSCNT RATE AT ABOUT 5500 FT AND WE CONTINUED TO SLOW. I WOULD PUT OUR POS AT ABOUT 030/5 WHEN WE GOT A TERRAIN WARNING PULL UP. CAPT ADDED PWR AND STARTED A CLB. THE TERRAIN DISPLAY WAS YELLOW ONLY AND ALL OF IT WAS L OF THE FLT PATH. OUR ALT AT THE START WAS ABOUT 5300 FT. THE WARNING NEVER REPEATED AND WE LEVELED OFF JUST ABOVE 6000 FT. I ADVISED TWR THAT WE HAD A TERRAIN WARNING AND FINAL WOULD BE EXTENDED. OTHER ACR WAS SWITCHED TO RWY 28R AND LANDED. WE CONFIGURED AND ACCOMPLISHED A NORMAL VISUAL TO RWY 28R BEHIND OTHER ACR.