B757-200 FLT CREW IS DISPATCHED WITH RIGHT ENG BLEED ON MEL REQUIRING AVOIDANCE OF ICING CONDITIONS ENROUTE BUT THE WEATHER DOES NOT COOPERATE.

Narrative

R ENG BLEED DEFERRED PRIOR TO DEP; DUE TO HPSOV FLUCTUATING. ASSURED BY DISPATCH THAT NORTHERN ROUTING WOULD KEEP US CLR OF WX; AND ALLOW DSCNT E OF THE WX SYS WITHOUT NEED OF ENG ANTI-ICE (NOT AVAILABLE DUE TO DEFERRAL). THE UNDERCAST DID NOT CLR AS FORECAST AND LEFT ONLY 1 OPTION: DECLARING AN EMER AND TURNING ON THE ENG BLEED IN ORDER TO MAKE THE ENG ANTI-ICE AVAILABLE. AFTER CONSULTING WITH DISPATCH AND MAINT AND USING OUR KNOWLEDGE OF THE SYS; WE DECLARED THE EMER; TURNED ON THE BLEED AND ANTI-ICE; AND COMMENCED DSCNT. BLEED PRESSURE REMAINED AT A CONSTANT 25 PSI AND THE ANTI-ICE VALVE OPERATED NORMALLY. AT 17000 FT WE TURNED THE BLEED BACK OFF; WE DECLARED THE EMER OVER; AND CONTINUED A NORMAL ARR AND LNDG AT ZZZ. ALL REMAINING SYS OPERATED NORMAL AT ALL TIMES. DECLARING THE EMER WAS IN OUR OPINION; THE ONLY VALID OPTION THAT WE HAD. THE DECISION TO GO WAS BASED ON THE FORECAST PROVIDED AND THE RADAR AND SATELLITE PICTURES VIEWED PRIOR TO DEP. DISPATCH CONCURRED ON THE ROUTING AND DSCNT PLAN BASED ON THE WX AND RESTRS ON THE MEL.