ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 222 PM EST THU DEC 21 2017 VALID 12Z MON DEC 25 2017 - 12Z FRI DEC 29 2017 SIGNIFICANT MODEL SPREAD AND RUN TO RUN VARIABILITY FOR INDIVIDUAL SYSTEMS MAKE IT DIFFICULT TO INCORPORATE PARTICULAR OPERATIONAL SOLUTIONS INTO THE FULL-DOMAIN FORECAST. THUS TODAY'S MANUAL PROGS STARTED WITH A BLEND OF THE 06Z GEFS/00Z ECMWF MEANS WHILE DERIVED WIND SPEEDS WERE INCREASED SOMEWHAT TO ACCOUNT FOR THE LIKELIHOOD THAT THEY WILL BE HIGHER THAN THE INITIAL ENSEMBLE MEAN BLEND OVER SOME AREAS, ESPECIALLY OPEN WATERS. ON THE POSITIVE SIDE, GUIDANCE DOES SHOW SOME SIMILAR IDEAS FOR THE LARGE SCALE EVOLUTION. A RELATIVE MAJORITY OF MODELS AND TO A GREATER EXTENT ENSEMBLE MEANS INDICATE A SHIFT IN EMPHASIS FROM AN INITIAL RIDGE ALOFT OVER/NORTH OF THE EASTERN MAINLAND BORDER TO A MORE WESTERN HIGH ASSOCIATED WITH A RIDGE BUILDING INTO THE BERING SEA DURING THE FIRST HALF OF THE WEEK. THIS LATTER HIGH CLOSES OFF OVER/NORTH OF EXTREME EASTERN SIBERIA TOWARD DAYS 7-8 THU-FRI AND AT THAT TIME THE ENSEMBLE MEANS ARE REMARKABLY SIMILAR FOR THAT FAR OUT IN TIME. MEANWHILE DEEP LOW PRESSURE NEAR KAMCHATKA SHOULD GRADUALLY EXTEND EASTWARD (THROUGH A SERIES OF ONE OR MORE REDEVELOPMENTS) WITH AN AXIS OF SURFACE TROUGHING EVENTUALLY EXTENDING EASTWARD ACROSS MUCH OF THE NORTH PACIFIC. WITHIN THIS PRESSURE TROUGH THERE IS POTENTIAL FOR ONE OR MORE WELL DEVELOPED SYSTEMS. GUIDANCE SHOWS MEAN TROUGHING ALOFT FROM THE BERING SEA THROUGH THE EAST-CENTRAL PACIFIC BUT WITH POOR CLUSTERING FOR SPECIFICS OF INDIVIDUAL SYSTEMS THAT MAY OR MAY NOT AFFECT LOCATIONS OVER/NEAR THE MAINLAND. AT THE START OF THE PERIOD THERE ARE ALREADY DIFFERENCES OVER THE BERING SEA/WEST COAST OF THE MAINLAND WITH THE 12Z GFS/GEFS MEAN LEANING TO THE SLOWER SIDE OF THE SPREAD WITH LOW PRESSURE CROSSING THE ALASKA PENINSULA IN THE SHORT RANGE. TIMING HAS AN INFLUENCE ON A SEPARATE COMPACT CENTRAL BERING SEA SYSTEM THAT SHOULD WEAKEN AFTER EARLY MON, AND TRACK MAY BE A LITTLE NORTH OF THE GFS GIVEN MAJORITY IDEAS FOR THE SYSTEM TO THE EAST. SOLUTIONS VARY WIDELY FOR ONE OR MORE EAST-CENTRAL PACIFIC SYSTEMS. ECMWF/CMC ENSEMBLE MEANS ARE NOTABLY SUPPRESSED THROUGH THE PERIOD WHILE THE 06Z GEFS MEAN REFLECTS SOME OF THE OTHER OPERATIONAL MODEL SIGNALS THAT LOW PRESSURE COULD REACH FARTHER NORTHWARD AS PER CONTINUITY, ALBEIT PERHAPS NOT AS FAR NORTHWARD AND/OR AS QUICKLY AS SOME GFS RUNS. CONFIDENCE IS BELOW AVERAGE REGARDING FLOW DETAILS ALOFT OVER THE MAINLAND/GULF OF ALASKA BY LATE NEXT WEEK GIVEN THE POTENTIAL SHIFT IN UPPER RIDGING. SOME DEGREE OF WEAKNESS IS LIKELY TO EXIST TO THE SOUTHEAST OF ANY RIDGE THAT BUILDS OVER/NORTH OF EASTERN SIBERIA BUT MODELS HAVE BEEN VERY DIVERSE AND INCONSISTENT WITH WHERE THIS WEAKNESS WILL EXIST. THE GREATER PROPORTION OF GUIDANCE HAS BEEN SUGGESTING THIS WEAKNESS WILL LIE OVER THE MAINLAND BUT TELECONNECTIONS RELATIVE TO THE CORE OF POSITIVE HEIGHT ANOMALIES ASSOCIATED WITH THE SIBERIA/ARCTIC RIDGE IN D+8 CHARTS COULD SUPPORT A FARTHER SOUTH/SOUTHEAST AXIS EVEN IF NOT TO THE EXTENT/DEPTH OF THE 00Z ECMWF. FINALLY, GEFS MEANS HAVE BEEN NOTABLY DEEPER THAN ECMWF MEANS FOR NORTHWEST PACIFIC LOW PRESSURE TOWARD LATER IN THE PERIOD, MERITING GEFS MEAN INCLUSION, WHILE THE 06Z GEFS MEAN HAS ADJUSTED A BIT SOUTHWARD WITH ITS OVERALL POSITION OF THE SURFACE LOW AND FRONTAL ZONE TO THE EAST VERSUS THE 00Z RUN, A NOD TO THE ECMWF MEAN AXIS. TOWARD DAYS 7-8 WED-THU RECENT GFS RUNS HAVE BEEN ON THE MORE ENTHUSIASTIC SIDE OF GUIDANCE IN SHOWING POTENTIAL DEVELOPMENT NEAR THE ALEUTIANS VERSUS A MORE SUPPRESSED SCENARIO IN MOST OTHER SOLUTIONS. RAUSCH