B737-700 FLT WAS CLEARED FOR A VISUAL APCH. ATC ISSUED A LOW ALT ALERT AS THE FLT MANEUVERED FOR THE APCH.

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WE WERE ON THE GRANDPA-ONE RNAV ARR. WE WERE CLRED TO (IF I REMEMBER RIGHT) 5100 FT. WE HAD CONTINUOUS MODERATE TURB AND VERY GUSTY WINDS. AROUND 5 MI OUT BEFORE TRROP INTXN; WE BROKE OUT OF THE CLOUDS AND THE CAPT WANTED ME TO CALL THE ARPT. I DID; AND WE WERE 'CLRED THE VISUAL APCH TO RWY 19C; BUT KEEP TRACKING THE GRANDPA-ONE ARR.' CAPT IMMEDIATELY DIALED IN 4100 FT ON THE ALT WINDOW. (BY OUR COMPANY PROC; I; THE PNF; SHOULD HAVE DIALED ALL NEW ALTS. BUT THERE ARE NO IFR APCHS TO RWY 19L OR RWY 19R. BY DOING WHAT HE DID; AND STICKING IN AN ALT; HE SORT OF TOOK ME BY SURPRISE. IF HE HAD LET ME DO IT; I WOULD HAVE USED THE CLASS B CHART ALTS.) ANYWAY; HE SLOWLY DSNDED TO 4100 FT; AND APCH SAID 'LOW ALT ALERT; DO YOU HAVE THE TERRAIN IN SIGHT?' I SAID; 'WE DID;' AND IT WAS AT OUR 2:30-3 O'CLOCK POS; ABOUT 3 MI AWAY. I TOLD THE CAPT; YOU WENT BELOW THE CLASS B AIRSPACE. FROM HERE; WE MADE A NORMAL APCH AND LNDG. NO EGPWS WENT OFF. WHAT CAUSED THIS: 1) WE WERE TIRED. OUR 1ST DAY ENDED AS A VERY LATE PM TRIP; AND BY DAY 3 (THIS DAY); THIS DAY TURNED INTO AN EARLY AM TRIP. 2) CAPT SHOULD HAVE LET ME DIAL IN THE ALTS; AND SAY SOMETHING IF HE IS DOING SOMETHING NON-STANDARD. 3) I WISHED I HAD DOUBLE-CHKED THE ALT HE THREW IN; WITH CLASS B CHART. NORMALLY I RELIGIOUSLY TRY TO BE VERY CAREFUL ABOUT THE CLASS B AIRSPACE. I FORGOT TO CHALLENGE HIM ON THE ALT; AND VERIFY. WHY? TIRED. THE CONSTANT MODERATE TURB MADE ME FOCUS ON THE LAS ARPT. I WILL BE VERY CAREFUL FOR THIS IN THE FUTURE.