AN ACR FLC FLYING B757 INTO LAS BECOMES DISTRACTED WHEN DEVIATING FOR WX AND LANDS ON THE WRONG RWY.

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ON PHX/LAS SCHEDULED ACR FLT. NUMEROUS TSTM CELLS OVER ENTIRE RTE AS WAS FORECASTED AND NOTED ON RADAR BEFORE DEP. ONLY SLIGHT DEVS REQUIRED TO AVOID. ON DSCNT IN PGS AREA RECEIVED NUMEROUS SPD CHANGES AND DELAY VECTORS FROM ZLA AND LAS APCH. THROUGH 17000 FT PENETRATED SOME LIGHT PRECIPITATION (ON OUR AIRBORNE RADAR) WHICH ACTUALLY WAS MODERATE TO HVY WITH ACCOMPANYING MODERATE TURB. LAS ATIS WAS GIVING RWY 25L AS THE APCH WITH WINDS VARIABLE LESS THAN 10000 FT GOOD CEILING AND VISIBILITY. WHEN WE WERE ABOUT 20 MI OUT APCH RPTED MODERATE TO HVY RAIN OVER THE ARPT AND LLWS. THERE WAS EXTENSIVE LIGHTNING IN AREA. ARPT WAS VISIBLE THROUGH 11000 FT. APCH THEN STARTED VECTORING TFC FOR RWYS 1; WE WERE STILL QUITE HIGH ON A SW HEADING WHEN CLRED FOR A VISUAL TO RWY 1R. BECAUSE I WAS USING SPD BRAKES FOR DSCNT I KEPT BANK ANGLE MINIMAL; FLEW THROUGH THE EXTENDED CTRLINES AND INTERCEPTED ON A NE HEADING. AT THIS POINT THE TWR ASKED 'WILL YOU BE ABLE TO GET DOWN FOR RWY 1L?' WE ACKNOWLEDGED IN THE AFFIRMATIVE; AND LANDED RWY 1L WITH NO PROB. ON ROLLOUT TWR ADVISED 'YOU WERE CLRED TO LAND RWY 1R.' WE BOTH ASSUMED A 'CLRNC TO LAND' FROM THE PREVIOUS QUERY BUT I WAS REMISS FOR NOT CONFIRMING A LNDG CLRNC CHANGE. WE BOTH THOUGHT WE'D UNDERSTOOD RWY 1L WAS OUR RWY. WE HAD A GOOD SIGHT PICTURE FOR RWY 1L. THE TWR WAS GIVING MULTIPLE SLOWING AND DELAY VECTORS. ACFT WERE REQUESTING HOLDING; TWR REPLIED 'WE ARE UNABLE; THERE IS NO PLACE TO HOLD BECAUSE OF THE RAPIDLY DETERIORATING WX.' OUR OWN RADAR WAS APPARENTLY MALFUNCTIONING AS WE NOW HAD A 'WX RADAR WEAK' EICAS MESSAGE SO WE HAD A CLRNC TO LAND ON RWY 1L (WE THOUGHT). RAPIDLY CHANGING WX CONDITIONS AND DEGRADED RADAR CAPABILITY; WE LANDED; AS CLRED. AT NO TIME WAS THIS FLT IN JEOPARDY OF ANY KIND. AT NO TIME WAS SAFETY COMPROMISED IN ANY WAY. AFTER TOUCHDOWN MODERATE TO HVY RAIN COMMENCED. A COUPLE OF POINTS: RWY 1L WAS HIRL; RWY 1R ONLY MIRL. NO PRECISION ELECTRONIC GUIDANCE FOR EITHER RWY. ATC DID A COMMENDABLE JOB UNDER VERY RAPIDLY CHANGING CONDITIONS.