MDT CREW MISSET ALTIMETER AT DEP STATION.

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WE DEPARTED HPN ON SEP/XA/98. DURING TAXI AND PRIOR; IT WAS INDICATED THAT THE SEVERE WX AVOIDANCE PROGRAM WOULD BE IN EFFECT AND WE WERE ADVISED TO EXPECT REROUTING AFTER DEP VIA THE WESTCHESTER 9 DEP. ATC GAVE US RADAR VECTORS (CONSISTING OF A BOX PATTERN) IN ORDER TO COORDINATE THE RERTE. DURING THE INITIAL CLB; WHICH CONSTITUTED INTERMEDIATE LEVELOFFS; WE WERE ADVISED THAT OUR MODE C INDICATED 16700 FT WHEN LEVEL AT 17000 FT. I IMMEDIATELY ASKED FOR CURRENT ALTIMETER SETTING; WHICH ATC GAVE AS 29.69 (JFK). THE ACFT'S ALTIMETERS WERE SET 29.97. THE ERROR WAS OBVIOUS. REFERRING BACK TO THE HPN ATIS; ALTIMETER WAS 29.97. THE XXZ SA AT HPN WAS 29.79. I CAN'T HONESTLY SAY IF THE ARPT ATIS WAS 29.79 OR 29.97. THE ORIGINAL ERROR WAS NOT NOTICED AT HPN PROBABLY BECAUSE OF NUMEROUS DISTRS ARISING FROM THE ENCROACHING SEVERE WX. WE HAD WRITTEN THE HPN ATIS AS 29.97 ON THE SCRATCH PAD BUT FAILED TO OBSERVE THE INDICATED ALT ON THE GND AT HPN. FACTORS I BELIEVE CONTRIBUTED TO THIS ERROR WERE THE ARRIVING ON PAX 20 MINS PRIOR TO THE PLANNED DEP TIME. GATE HOLD PROCS AND NUMEROUS ATC COMS ALL OCCURRING AT CRITICAL TIMES IN THE CHKLIST PROC. NONE OF WHICH EXCUSE SUCH AN OVERSIGHT. BEFUDDLING IS THE FACT THAT NO OTHER ATC FACILITIES RPTED THE VARIANCE EVEN THOUGH LEVELOFFS AT VARIOUS ALTS WERE MADE DURING THE CLB TO 17000 FT. ATC ADVISED US OF THE ERROR WHEN LEVEL AT 17000 FT. PROB WAS RESOLVED WITH THE CORRECT ALTIMETER SETTING ENTRY. ATC DID NOT INDICATE IF THERE HAD BEEN ANY CONFLICTS WITH OTHER ACFT.