WRONG ALTIMETER SETTING CONFUSES PIC OF A B737 DURING AN APCH IN INCLEMENT WX TO STL; MO.

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APCHING STL; ABOUT 100 MI OUT; I TUNED AND COPIED THE CURRENT ATIS. I COPIED THE ALTIMETER AS 30.44 (HEARD IT TWICE AND REMEMBERED THINKING THAT IT SEEMED AWFULLY HIGH FOR SUCH LOUSY WX). ATIS SAID LNDGS -- ILS RWY 24 OR RWY 30R; DEPS RWY 30L. WE COMPLETED THE APCH/DSCNT CHKLIST AND BRIEFED THE RWY 30R ILS APCH. I DON'T REMEMBER IF WE EVER LEVELED OFF DURING THE DSCNT; BUT IF WE DID; WE WERE 400 FT LOW DUE TO THE INCORRECT ALTIMETER. WE PROBABLY STOPPED AT 10000 FT TO LOW; BUT WOULD HAVE ACTUALLY BEEN AT 9600 FT ABOUT 250 KTS. (AS WE FOUND OUT LATER AT THE GATE; ACTUAL ALTIMETER WAS 30.04.) APCH TURNED US ON BASE; THEN DOGLEG TO FINAL; AND CLRED US FOR ILS RWY 30L. I CONFIRMED THE CHANGE FROM RWY 30R TO RWY 30L AND WE SCRAMBLED TO RETURN AND REBRIEF FOR THE RWY 30L ILS. (THIS IS A VERY COMMON PROB AT STL -- INITIAL ASSIGNMENT IS TO ONE RWY; THEN CLRED FOR THE PARALLEL RWY TURNING ON FINAL.) VERY SOON THEREAFTER; WE GOT AN OM FLASH AND TONE; BUT THOUGHT IT WAS THE RWY 30R BEACON SINCE MY ALTIMETER READ ABOUT 2500 FT INSTEAD ABOUT 2150 FT. (WE WERE ON GS AT THIS POINT.) AT THE 1000 FT AGL CALL (ABOUT 1600 FT MSL) WE BEGAN TO REALIZE THERE WAS A FAIRLY LARGE DIFFERENCE BTWN THE BAR ALTIMETER (1600 FT = 1000 FT AGL) AND THE RADAR ALTIMETER READING (600 FT). SINCE WE WERE ON GS AND NOW HAD THE END OF THE RWY IN SIGHT; WE CONTINUED TO AN UNEVENTFUL LNDG. AT THE GATE; I CHKED THE ATIS AGAIN; BUT IT HAD BEEN UPDATED AND WAS GIVING 30.04. I DON'T HONESTLY KNOW IF I COPIED THE WRONG ALTIMETER OFF THE ATIS OR IF THE WRONG ALTIMETER WAS GIVEN BY THE ATIS. APCH NEVER QUESTIONED OUR ALT AND I DON'T EVER REMEMBER HEARING APCH GIVE OUT AN ALTIMETER. I MISSED THE OBVIOUS CLUE OVER THE OM; BUT WAS STILL A LITTLE BEHIND DUE TO THE LAST MIN RWY CHANGE AND HAD JUST RETURNED MY NAV FROM THE VOR FREQ NEEDED TO IDENT THE STEP-DOWN AT JOICE. I WAS TOO QUICK TO ASSUME WE RECEIVED THE RWY 30R MARKER; AND SHORTLY AFTER WONDERING WHY WE DIDN'T GET THE RWY 30L MARKER; WE DID SEE THE RWY AND DID THE 100 AGL CALL.