ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 315 PM EDT MON SEP 11 2017 VALID 12Z FRI SEP 15 2017 - 12Z TUE SEP 19 2017 WITHIN A PATTERN FEATURING A MEAN TROUGH OVER THE BERING SEA AND VICINITY, AND PERIODIC MID-LATITUDE SHORTWAVES INTERACTING TO VARYING DEGREES, TWO PRIMARY SYSTEMS OF INTEREST REMAIN THE FOCUS OF THE FORECAST WITH SIGNIFICANT UNCERTAINTIES PERSISTING FOR BOTH. FOR THE FIRST SYSTEM, ESTABLISHED CONSENSUS HAS SHOWN A TRACK REACHING INTO THE SOUTHERN/EASTERN BERING SEA EARLY IN THE PERIOD WITH GREATEST STRENGTH IN THE LATE FRI/EARLY SAT TIME FRAME. UPPER DYNAMICS AND SURFACE LOW PRESSURE THEN TRANSLATE EAST/SOUTHEAST INTO THE GULF OF ALASKA/NORTHEAST PACIFIC. HOWEVER AWAY FROM THIS MAJORITY SCENARIO THERE HAS BEEN CONSIDERABLE SPREAD AMONG SOME MODEL RUNS AND INDIVIDUAL ENSEMBLE MEMBERS. SOLUTIONS DIVERGING FROM THIS SCENARIO FOR THE BERING SYSTEM (THOUGH STILL SUGGESTING SOME NORTHEAST PACIFIC LOW PRESSURE) HAVE INCLUDED THE 12Z/10 GFS THAT WAS MORE SUPPRESSED AND RECENT UKMET RUNS/00Z ECMWF-ECMWF MEAN THAT LIFT THE SYSTEM WELL NORTH TO NEAR THE BERING STRAIT. BASED ON GUIDANCE AVAILABLE THROUGH THE 06Z CYCLE AND 12Z GFS, THE 06Z-12Z GFS/06Z GEFS MEAN AND 00Z CMC/CMC MEAN AS WELL AS 00Z NAVGEM FAVOR MAINTAINING A SOLUTION CLOSER TO CONTINUITY THAN THE 00Z ECMWF CLUSTER, WITH THE PRIMARY ADJUSTMENT BEING TO NUDGE THE SYSTEM A BIT NORTHWARD EARLY IN THE PERIOD IN A COMPROMISE BETWEEN THE GFS/ECMWF (REACHING NEARLY AS FAR NORTH AS NUNIVAK ISLAND BY 12Z SAT) BEFORE TRENDING THE FORECAST CLOSER TO THE GFS AND GEFS/NAEFS MEANS. FOR THE TRAILING SYSTEM THAT COMES INTO THE PICTURE SOUTH OF THE ALEUTIANS DURING THE WEEKEND, MEANS FROM THE GEFS/ECMWF HAVE PERSISTENTLY DIFFERED WITH THE GEFS STRONGER/SOUTHWEST OF THE WEAKER ECMWF MEANS THAT HAVE BEEN TRACKING NEAR THE ALEUTIANS. OPERATIONAL GFS/ECMWF RUNS HAD SHOWN SOME SIMILARITY IN DEPICTING A SOMEWHAT ELONGATED SYSTEM BUT THE 00Z ECMWF JUMPED TO A STRONGER AND FASTER/NORTHEASTWARD SYSTEM DUE IN PART TO PHASING WITH NORTHERN BERING UPPER LOW ENERGY ASSOCIATED WITH THE LEADING SYSTEM. ON THE OTHER HAND LATEST OPERATIONAL GFS RUNS HAVE BEEN SUPPRESSED SOUTH OF THE ALEUTIANS. HIGH UNCERTAINTY REGARDING HOW NORTHERN STREAM ENERGY WILL INTERACT SEEMS TO FAVOR AN INTERMEDIATE SOLUTION AMONG THE ENSEMBLE MEANS, ESPECIALLY CONSIDERING THAT THE GEFS/CMC/ECMWF MEANS CLUSTER REMARKABLY WELL WITH A SURFACE LOW REACHING NEAR BRISTOL BAY BY DAY 8 TUE IN SPITE OF THE DIFFERENCES LEADING UP TO THAT TIME. BASED ON CONSIDERATIONS THROUGH ARRIVAL OF THE 12Z GFS, THE STARTING BLEND USED A COMPROMISE AMONG THE 12Z GFS/06Z GEFS MEAN AND 00Z ECMWF/ECMWF MEAN (TILTED SOMEWHAT MORE TO THE OPERATIONAL RUNS) DAY 4 FRI INTO EARLY DAY 5 SAT, QUICKLY TRANSITIONING MORE TOWARD THE GFS/GEFS CLUSTER THEREAFTER BEFORE GOING TO A NEARLY EVEN BLEND AMONG THE 06Z GEFS/00Z NAEFS/00Z ECMWF MEANS FOR THE LATTER HALF OF THE FORECAST. THE REST OF THE 12Z GUIDANCE SUITE DOES NOT ADD MUCH CLARITY TO THE FORECAST. THE CMC JUMPED COMPLETELY TO THE 00Z ECMWF FOR BOTH MAJOR SYSTEMS AND THE UKMET HOLDS ONTO ITS NORTHERN BERING TRACK FOR THE LEADING SYSTEM. HOWEVER FOR THAT FIRST SYSTEM THE 12Z ECMWF REVERTED BACK TO CONSENSUS/CONTINUITY. THE ECMWF IS A SLIGHTLY SLOWER VERSION OF ITS OLD RUN FOR THE TRAILING SYSTEM. RAUSCH