ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 257 PM EDT SAT SEP 09 2017 VALID 12Z WED SEP 13 2017 - 12Z SUN SEP 17 2017 DURING MID-LATE WEEK MOST GUIDANCE SHOWS LARGE SCALE FLOW BEING DOMINATED BY A MEAN TROUGH ALOFT COVERING EXTREME EASTERN SIBERIA AND THE BERING SEA/ALEUTIANS WHILE A MEAN RIDGE BECOMES ESTABLISHED OVER WESTERN CANADA. AN ONGOING SERIES OF MID-LATITUDE IMPULSES TRACKING OUT OF ASIA WILL BE INTERACTING WITH THIS MEAN TROUGH TO VARYING DEGREES, LEADING TO SOME DETAIL UNCERTAINTIES FOR ASSOCIATED SURFACE SYSTEMS. BY THE LATTER HALF OF THE FORECAST PERIOD THE OVERALL UPPER TROUGH SHOULD TAKE ON A NEGATIVE TILT AS IT BEGINS TO APPROACH THE EXTREME SOUTHWEST MAINLAND AND GULF OF ALASKA. THE LEADING SYSTEM THAT MAY HAVE A SOMEWHAT ELONGATED APPEARANCE SHOULD BE OVER THE EASTERN BERING SEA OF AS EARLY DAY 4 WED. DURING THE WED-THU PERIOD THE GENERAL TREND FROM 24 HOURS AGO IS TOWARD A SOMEWHAT FARTHER SOUTH TRACK WHILE THERE IS STILL A MODERATE AMOUNT OF MODEL/ENSEMBLE SPREAD. A WEAK FRONTAL WAVE MAY SPLIT OFF AND TRACK INTO THE GULF OF ALASKA BY LATE IN THE WEEK. A GUIDANCE BLEND PROVIDES A REASONABLE STARTING POINT TO RESOLVE PERSISTING DETAIL QUESTIONS. THE NEXT WESTERN PACIFIC SYSTEM SHOULD BE REACHING NEAR THE WESTERN ALEUTIANS BY DAY 5 THU. ONLY IN THE PAST 24-36 HOURS HAS GUIDANCE BEGUN TO SHOW REASONABLE AGREEMENT ON THE EXISTENCE OF THIS SYSTEM FROM THAT TIME ONWARD. EVEN MORE RECENTLY, THE 00Z-06Z CYCLE OF MODELS/ENSEMBLE MEANS HAS ADJUSTED THE TRACK NORTHWARD INTO THE BERING SEA VERSUS JUST SOUTH OF THE ALEUTIANS. TYPICAL UNCERTAINTY THAT EXISTS IN SUCH CASES OF POTENTIAL STREAM INTERACTION SEEMS TO ARGUE FOR A MODEL/ENSEMBLE MEAN COMPROMISE THAT YIELDS A PARTIAL TREND TOWARD THE DEEPER/NORTHWARD SCENARIO IN THE LATEST GUIDANCE. FOR A TIME THE 06Z GFS IS ON THE FASTER SIDE OF THE SPREAD FOR NORTHERN PACIFIC SHORTWAVE ENERGY. WITH THE 12Z GFS RETURNING CLOSER TO CONSENSUS FOR THIS PART OF THE FORECAST, GFS INPUT CONSISTS OF THE 00Z/12Z RUNS. REMAINING 12Z MODELS (ARRIVING AFTER FORECAST PREPARATION) HOLD ONTO A TRACK INTO THE BERING SEA. THE PREFERRED MODEL/MEAN BLEND WOULD ULTIMATELY BRING AN UPPER LOW ACROSS THE ALASKA PENINSULA INTO THE GULF OF ALASKA DAY 8 SUN, WITH SURFACE LOW PRESSURE LIKEWISE EMERGING INTO THE GULF LATE IN THE PERIOD. FOR THE TIME BEING THE ENSEMBLE MEANS AND GFS/ECMWF RUNS COMPARE FAIRLY WELL REGARDING ANOTHER NORTHWESTERN PACIFIC WAVE COMING INTO THE PICTURE TOWARD THE WESTERN ALEUTIANS BY DAY 8 SUN. UPPER SUPPORT FOR THIS SYSTEM COMES FROM A CLOSED LOW OVER EASTERN ASIA AT THE START OF THE FORECAST PERIOD. THE COMBINED CHALLENGES OF TIMING THE EJECTION OF THE UPPER LOW AND DETERMINING THE INFLUENCE FROM HIGHER LATITUDE FLOW MAY LEAD TO SPREAD/VARIABILITY OVER THE COMING DAYS. BASED ON THE CONSIDERATIONS FOR THE ABOVE SYSTEMS, TODAY'S FORECAST STARTS WITH MOSTLY OPERATIONAL MODELS (00Z-12Z GFS/00Z ECMWF) EARLY, TRANSITIONING TO ABOUT A 50/50 MODEL/MEAN WEIGHT BY DAYS 7-8 SAT-SUN. ENSEMBLE MEAN INPUT COMES FROM THE 06Z GEFS/00Z ECMWF MEANS WITH SOME 00Z NAEFS ALSO INCLUDED LATE. RAUSCH