ALTDEV WHEN AN APCHING A320'S GPWS SENSES A CLOSURE RATE WARNING ON AN UNOBSERVED MOUNTAIN TOP NEAR THE APCH RTE WHILE AT MVA ALT OF 5100 FT; 8 MI NE OF LAS.

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WE WERE BEING VECTORED FOR A VISUAL APCH TO RWY 19L AT LAS. WX WAS RPTED AS 3000 FT OVCST; BUT CEILINGS WERE RAGGED. WE WERE LEVEL AT 5100 FT MSL PER APCH CTL AND APPROX 8 NM NE OF THE ARPT. SUDDENLY; WE GOT A GPWS WARNING OF 'PULL UP; PULL UP; TERRAIN.' I HAD BRIEFLY NOTICED A LARGE MOUNTAIN AT THIS APPROX LOCATION BTWN CLOUDS EARLIER. WE WERE IMC; SO WE IMMEDIATELY CLBED IN RESPONSE; APPROX 900 FT. WE TOLD THE CTLR WE WERE RESPONDING TO A GPWS WARNING AND TO VERIFY WE WERE AT THE PROPER AT. SHE RESPONDED THAT YES; HER MVA FOR THAT AREA WAS 5100 FT; BUT SHE WOULD HAVE LOWER FOR US SOON. WE RETURNED TO OUR PREVIOUS ALT OF 5100 FT; AND THEN DSNDED FURTHER WHEN CLRED AND MADE A NORMAL VISUAL APCH WHEN WE WERE CLR OF CLOUDS. WE ASSUMED THAT WHEN WE WENT OVER THE MOUNTAIN; WE GOT A CLOSURE RATE GPWS WARNING. I CALLED ATC ON THE GND AND TOLD THEM OUR STORY. WE RECOMMENDED THAT ALTHOUGH THE 5100 FT MVA MAY BE A LEGAL ALT FOR GND CLRNC; IT MAY NOT PROVIDE ADEQUATE CLRNC TO AVOID THESE CLOSURE RATE GPWS WARNINGS.