ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 247 PM EDT TUE SEP 26 2017 VALID 12Z SAT SEP 30 2017 - 12Z WED OCT 04 2017 BROAD SPREAD AND RUN TO RUN INCONSISTENCY IN GUIDANCE HAS BEEN PROVIDING A SIGNIFICANT CHALLENGE IN DEVISING A SPECIFIC DETERMINISTIC FORECAST WITH RESPECT TO DETAILS ACROSS THE NORTH/NORTHEAST PACIFIC AND BERING SEA. ONE AREA OF DIFFICULTY INVOLVES POTENTIAL INTERACTION OF BERING SEA/WESTERN MAINLAND ENERGY WITH A CENTRAL PACIFIC TROUGH IN THE FRI-SAT TIME FRAME, FOLLOWED BY QUESTIONS OVER WHAT PROPORTION OF OVERALL TROUGH ENERGY THEN PROGRESSES SOUTHEAST TOWARD THE LOWER 48 VERSUS REMAINING OVER THE PACIFIC-- WITH ANY OF THE LATTER POTENTIALLY SUPPORTING LOW PRESSURE TRACKING INTO THE NORTHEAST PACIFIC/GULF OF ALASKA. RECENT ECMWF/ECMWF MEAN RUNS AND THE 00Z CMC REPRESENT ONE CLUSTER THAT PROGRESSES A LARGER PART OF THE ENERGY THUS YIELDING A SUPPRESSED NORTHEAST PACIFIC SURFACE PATTERN. THE 00Z GFS/GEFS MEAN AND 06Z GFS (PLUS 00Z UKMET/NAVGEM) REPRESENT THE OPPOSING CLUSTER, KEEPING MORE ENERGY OVER THE NORTH PACIFIC AND ULTIMATELY BRINGING LOW PRESSURE NORTH/NORTHEAST. THE 06Z GEFS MEAN MADE A SIGNIFICANT ADJUSTMENT TOWARD THE ECMWF CLUSTER AND THE 12Z GFS HAS FOLLOWED SUIT. TELECONNECTIONS RELATIVE TO A CORE OF POSITIVE HEIGHT ANOMALIES OVER THE EXTREME EASTERN BERING SEA IN THE D+4/D+5 MULTI-DAY MEANS SUGGEST MORE OF A SPLIT FLOW PATTERN ALOFT WITH A TENDENCY TOWARD NORTHEAST PACIFIC TROUGHING, WHICH SHOULD SUPPORT THE 06Z GEFS MEAN/12Z GFS AND 00Z ECMWF/CMC CLUSTER. THE 12Z UKMET IS ONE HOLDOUT THAT STILL BRINGS LOW PRESSURE NORTHWARD TOWARD THE SOUTHWESTERN PENINSULA. UPSTREAM THERE IS GOOD AGREEMENT THAT A FRONT SHOULD ENTER THE WESTERN BERING SEA AROUND THE START OF THE PERIOD SAT. HOWEVER IN ANOTHER DAY OR SO MODELS/ENSEMBLE MEMBERS DIVERGE REGARDING POTENTIAL WAVE DEVELOPMENT ALONG THIS FRONT. THERE HAD BEEN A NUMBER OF OPERATIONAL MODEL RUNS THAT IN ONE WAY OR ANOTHER SUGGESTED POTENTIAL FOR A VIGOROUS SYSTEM TRACKING NEAR THE ALEUTIANS BUT 00Z ECMWF/CMC AND 06Z GFS TRENDS SEEM TO BE PROVIDING GREATER SUPPORT FOR THE ENSEMBLE MEANS THAT KEEP STRONGEST LOW PRESSURE OVER THE NORTHWEST BERING WHILE EXTENDING A MERE FRONT ACROSS THE ALEUTIANS. THE 12Z GFS/GEFS MEAN HAVE TRENDED TO THE IDEA OF LOW PRESSURE TRACKING ACROSS THE CENTRAL BERING INTO THE NORTHEAST PACIFIC/GULF OF ALASKA. BROADLY SPEAKING THERE IS SUPPORT FOR LOW PRESSURE OVER THE GULF LATE IN THE PERIOD, WHETHER FROM A PARENT LOW OR FROM A TRIPLE POINT WAVE AS HINTED AT BY FRONTAL PROGRESSION IN THE 06Z GEFS/00Z ECMWF MEANS. THE 06Z GFS/00Z ECMWF-CMC INDICATE ANOTHER FAST MOVING SYSTEM REACHING THE WESTERN BERING BY 00Z-12Z TUE (DAY 7), ACCOUNTED FOR IN THE MEANS BY PERSISTENCE OF WESTERN BERING LOW PRESSURE. FINALLY THERE IS BETTER THAN AVERAGE CLUSTERING RELATIVE TO TYPICAL DAY 8 FORECASTS FOR A LARGE SYSTEM EXPECTED TO BE OVER KAMCHATKA/WESTERN BERING SEA BY NEXT WED. BASED ON INFORMATION UP TO ARRIVAL OF THE 12Z GFS, THE 06Z GEFS/00Z ECMWF MEANS SEEMED TO PROVIDE THE BEST WAY OF RESOLVING FORECAST UNCERTAINTY MID-LATE PERIOD. AT LEAST INTO DAY 5 SUN A CONSENSUS APPROACH LOOKS REASONABLE FOR THE UPPER LOW EXPECTED TO TRACK FROM JUST NORTH OF THE MAINLAND INTO THE CANADIAN ARCHIPELAGO. THE MANNER IN WHICH THE 00Z ECMWF PULLS SOME OF THE ENERGY BACK TO FORM ANOTHER UPPER LOW NORTH OF THE MAINLAND BY EARLY NEXT WEEK IS QUESTIONABLE THOUGH. OTHER 12Z SOLUTIONS THUS FAR DO NOT SUGGEST THAT SCENARIO EITHER. OVER THE MAINLAND THERE IS FAIR CONSENSUS TOWARD ABOVE NORMAL HEIGHTS/MEAN RIDGING WITH A COUPLE INTRUSIONS OF SHORTWAVE ENERGY. BASED ON THE ABOVE CONSIDERATIONS TODAY'S FORECAST STARTED WITH BLEND OF 12Z GFS/00Z ECMWF AND 06Z GEFS/00Z ECMWF MEAN FOR THE FIRST PART OF THE PERIOD, WEIGHTED MORE TO THE OPERATIONAL RUNS INTO EARLY DAY 5 SUN. THEN REFLECTING THE RAPID DECLINE OF CONFIDENCE AMONG INDIVIDUAL SOLUTIONS, THE BLEND TRANSITIONED TO THE ENSEMBLE MEANS EXCLUSIVELY BY DAYS 7-8 TUE-WED. RAUSCH