ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 307 PM EDT THU NOV 02 2017 VALID 12Z MON NOV 06 2017 - 12Z FRI NOV 10 2017 THE EXTENDED FORECAST FOR ALASKA HAS BECOME MORE CHALLENGING COMPARED TO THE PAST COUPLE DAYS, FOR BOTH THE SYSTEM OVER THE GULF OF ALASKA/NORTHEAST PACIFIC AND THE TRAILING UPPER RIDGE THAT BUILDS OVER THE MAINLAND. THE GULF SYSTEM THAT FORMS LATE IN THE SHORT RANGE IS PLAGUED BY SIGNIFICANT MODEL SPREAD AND RUN TO RUN VARIABILITY. ENSEMBLES ARE ALSO DIVERSE, AND WHILE THE MEANS HAVE BEEN SOMEWHAT MORE STABLE/AGREEABLE THEY HAVE TENDED TO LAG THE OPERATIONAL GUIDANCE IN TERMS OF DEPICTING THE STRENGTH OF THE SYSTEM. AS OF 12Z DAY 4 MON THE 12Z GFS ADJUSTED TO THE 00Z ECMWF, AND WITH DECENT ENSEMBLE SUPPORT AT THAT TIME THE FORECAST LEANS TOWARD THOSE RUNS. AFTER THAT TIME THE MAJORITY OF MODELS/ENSEMBLES WOULD HAVE THE SYSTEM TRACKING FARTHER SOUTHEAST THAN DEPICTED IN THE 00Z ECMWF. A COMPROMISE AMONG THE 06Z GEFS/00Z ECMWF MEANS AND CONTINUITY SEEMS TO BE THE BEST OPTION AT THIS TIME. MOST REMAINING NEW 12Z GUIDANCE AGREES WITH GREATER SOUTHWARD PROGRESSION BUT IS STILL VERY DIVERSE FOR LONGITUDE. MEANWHILE THE 12Z/01-00Z/02 ECMWF RUNS AND 00Z CMC MEAN SHOW THE MAINLAND RIDGE CLOSING OFF A NORTHERN COAST MAINLAND UPPER HIGH AROUND TUE, WHILE RECENT GFS RUNS HAVE TRENDED AWAY FROM THE ARCTIC UPPER HIGH SEEN IN 06Z-12Z/01 RUNS. THE ECMWF MEAN SHOWS A HINT AT THIS IDEA BUT WITHOUT A FULLY CLOSED CONTOUR AND FARTHER SOUTH IMPLIED UPPER HIGH CENTER. OPERATIONAL CMC/UKMET/NAVGEM RUNS MAINTAIN A MORE PROGRESSIVE/OPEN RIDGE. RELATIVE MAJORITY OF GUIDANCE PLUS COORDINATION WITH WFO AFG LEADS TO A PREFERENCE THAT LEANS MORE TOWARD THE PROGRESSIVE SCENARIO. CONSIDERATIONS OVER THE NORTHEAST PACIFIC/MAINLAND YIELD A STARTING BLEND THAT INCORPORATES 70 PCT TOTAL 12Z GFS/00Z ECMWF WITH THE REST 06Z GEFS/00Z ECMWF MEANS FOR 12Z MON, FOLLOWED BY ABOUT 1/3 EACH WPC CONTINUITY/06Z GEFS MEAN/00Z ECMWF MEAN FOR DAYS 5-7 TUE-THU. THIS SOLUTION RESOLVES SOME LINGERING TIMING/DETAIL DIFFERENCES FOR A LEADING FRONT THAT WEAKENS AS IT CROSSES THE BERING SEA AND NEXT FEATURE THAT FOLLOWS ABOUT A DAY LATER. LATE IN THE PERIOD GUIDANCE SHOWS GOOD AGREEMENT/CONSISTENCY WITH THE IDEA OF A STRONG RIDGE AT THE SURFACE AND ALOFT BUILDING ACROSS THE NORTH PACIFIC/ALEUTIANS/BERING SEA. TELECONNECTIONS RELATIVE TO THE ASSOCIATED POSITIVE HEIGHT ANOMALY CENTER FAVOR NORTHWESTERLY FLOW ALOFT OVER THE MAINLAND AND MEAN TROUGH OVER THE NORTHEAST PACIFIC. THE ENSEMBLE MEANS DEPICT THE PREFERRED PATTERN WELL SO BY DAY 8 THE FORECAST STARTS WITH A BLEND OF THE ENSEMBLE MEANS. THERE IS STILL FAIRLY HIGH UNCERTAINTY REGARDING THE GULF/NORTHEAST PACIFIC SURFACE REFLECTION IN RESPONSE TO MIDWEEK BERING SEA ENERGY FEEDING INTO THE UPPER TROUGH. THE ENSEMBLE MEAN APPROACH HAS THE BEST DEFINED CIRCULATION/LOWEST PRESSURES LINGERING NEAR HAIDA GWAII WITH A SURFACE TROUGH EXTENDING BACK THROUGH THE GULF OF ALASKA, BUT DEPENDING ON STRENGTH/SHAPE OF UPPER SUPPORT THERE WILL BE A WINDOW OF OPPORTUNITY FOR A SEPARATE SURFACE LOW TO DEVELOP OVER THE GULF AS SUGGESTED BY SOME OPERATIONAL RUNS. RAUSCH