ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 257 PM EDT THU MAY 11 2017 VALID 12Z MON MAY 15 2017 - 12Z FRI MAY 19 2017 BEHIND A SHORT RANGE UPR LOW EXPECTED TO TRAVEL FROM THE ALEUTIANS/SRN BERING SEA THROUGH THE GULF OF ALASKA TO A POSN NEAR THE SRN PANHANDLE AS OF THE START OF THE EXTENDED PERIOD 12Z MON, MOST GUIDANCE INDICATES AN EVOLUTION TOWARD A MORE STAGNANT PATTERN FEATURING AN UPR LOW OVER THE SERN-SRN BERING SEA AND A MEAN RIDGE EXTENDING ACROSS THE MAINLAND FROM NWRN CANADA. OPERATIONAL MODELS PROVIDE BETTER DETAIL FOR INDIVIDUAL FEATURES AROUND THE START OF THE PERIOD BUT THEN DIVERGE WITH SPECIFICS, THUS FAVORING A TYPICAL TRANSITION OF EMPHASIS FROM OPERATIONAL MODELS TO ENSEMBLE MEANS WITH TIME. FOR THE UPR LOW NEAR THE SRN PANHANDLE AT THE START OF THE PERIOD, GUIDANCE THROUGH 06Z STILL OFFERED SOME SPREAD FOR TIMING WHILE THE CMC CONTINUES TO HOLD THE FEATURE WELL NWD. FOR THE SECOND DAY IN A ROW THE 06Z GFS/GEFS MEAN TRENDED SLOWER THAN THE 00Z GFS/ECMWF AS PART OF AN OVERALL MULTI-DAY SLOWER TREND IN THE PRIMARY GUIDANCE CLUSTER. HOWEVER TODAY THE 12Z GFS ADJUSTED TO A 06Z GFS/00Z ECMWF COMPROMISE, SO THIS INTERMEDIATE SOLN APPEARED BEST GIVEN THE ARRAY OF GUIDANCE. IN THE NEW 12Z RUNS, THE CMC HAS FINALLY ADJUSTED TOWARD CONSENSUS ALBEIT SLOWER LIKE THE 06Z GFS WHILE THE UKMET IS A LITTLE FASTER THAN THE 00Z ECMWF. ECMWF/ECMWF ENSEMBLE CONTINUITY AND GFS/GEFS TRENDS OVER THE PAST DAY SUGGEST INCREASING POTENTIAL THAT ERN SIBERIA SHRTWV ENERGY MAY DROP S/SE ACROSS THE BERING SEA AND INTERACT WITH THE NRN PAC SYSTEM TRACKING TOWARD THE ERN ALEUTIANS. THIS INTERACTION WOULD LEAD TO A FARTHER NEWD TRACK INTO THE SERN BERING SEA. THE OVERALL EVOLUTION STILL HAS A FAIR AMOUNT OF UNCERTAINTY, AS HIGHLIGHTED BY DETAIL DIFFS IN OTHER 12Z SOLNS, BUT CLUSTERING AMONG THE MODELS AND MEANS THROUGH 06Z WAS BETTER THAN AVG FOR A FCST 5-7 DAYS OUT IN TIME. THERE IS GENERAL AGREEMENT THAT THIS SYSTEM MAY DISSIPATE BY DAY 8 THU. AS THIS SYSTEM WEAKENS AN EMERGING WRN PAC MID-LATITUDE SYSTEM MAY COME INTO THE PICTURE TO THE S OF THE ALEUTIANS. THERE IS A DECENT SIGNAL FROM MODELS/ENSEMBLE MEMBERS FOR THE EXISTENCE OF THIS LATE PERIOD SYSTEM BUT POOR AGREEMENT ON TIMING AND LATITUDE, WITH THE 06Z GEFS/00Z ECMWF MEANS THE BEST OPTION AT THIS TIME. OPERATIONAL MODELS STILL SHOW A LOT OF MEDIUM TO SMALLER SCALE DIFFS AND RUN TO RUN VARIABILITY OVER THE MAINLAND AS FLOW ALOFT TRANSITIONS TOWARD MEAN RIDGING FROM A GENERAL WEAKNESS EARLY DAY 4 MON. OVER RECENT DAYS THE ENSEMBLE MEANS HAVE PROVIDED THE BEST AGREEMENT/CONTINUITY FOR THE MEAN RIDGE EXPECTED TO DEVELOP OVER THE MAINLAND. BASED ON THE ABOVE CONSIDERATIONS TODAY'S BLEND STARTED WITH THE 06Z-12Z GFS/00Z ECMWF RUNS FOR 12Z MON AND THEN INCORPORATED 06Z GEFS/00Z ECMWF ENSEMBLE MEAN INPUT AT AN INCREASING RATE SO THAT DAYS 7-8 THU-FRI USED 80-100 PCT OF THE ENSEMBLE MEANS. RAUSCH