ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 306 PM EDT FRI SEP 29 2017 VALID 12Z TUE OCT 03 2017 - 12Z SAT OCT 07 2017 UPPER RIDGING IN PLACE ACROSS EASTERN/SOUTHEASTERN AK DURING THE LATE SHORT RANGE IS FORECAST BY ALL GUIDANCE TO BREAK DOWN QUICKLY BY DAY 4 (TUE), AS THE STATE IS OPENED UP TO AN ACTIVE FLOW REGIME ORIGINATING ACROSS EASTERN ASIA. THE RESULT WILL BE A STORMY AND WET PATTERN, ESPECIALLY FOR SOUTHERN/SOUTHEASTERN AK WHERE DEEP-LAYER FLOW WILL BECOME INCREASINGLY SOUTHWESTERLY AHEAD OF EACH INCOMING TROUGH. DETERMINISTIC MODEL SOLUTIONS HAVE COME INTO BETTER AGREEMENT RELATIVE TO THIS TIME YESTERDAY DURING THE EARLY PORTION OF THE MEDIUM RANGE (DAY 4 THROUGH LATE DAY 5). A SERIES OF RELATIVELY WEAK SHORTWAVES ARE FORECAST TO CROSS AK AND THE BERING SEA THROUGH WED WHILE A LARGER TROUGH DIGS ACROSS THE GULF OF AK. MODELS SHOW SOME TIMING/AMPLITUDE DIFFERENCES WITH THESE FEATURES BUT THE DIFFERENCES ARE RELATIVELY SMALL AND A BLEND OF DETERMINISTIC SOLUTIONS SHOULD HANDLE THIS PART OF THE FORECAST WELL. BY LATE WED INTO EARLY THU, A SHORTWAVE IS FORECAST BY ALL GUIDANCE TO QUICKLY AMPLIFY OVER THE BERING SEA WITH MOST SOLUTIONS SHOWING A CUTOFF UPPER LOW AND DEEP SURFACE CYCLONE DEVELOPING. THE GFS, WHICH YESTERDAY AT THIS TIME WAS SHOWING A MUCH WEAKER SHORTWAVE AND CYCLONE, HAS NOW TRENDED SUBSTANTIALLY TOWARD THE STRONGER ECMWF OVER THE PAST FEW RUNS. SOME TIMING DIFFERENCES REMAIN, WITH THE GFS FASTER THAN THE ECMWF WITH THIS SYSTEM INITIALLY. THE ECMWF HAS CONTINUED TO SHOW RELATIVELY HIGH RUN-TO-RUN CONTINUITY WITH THIS SYSTEM, AND REMAINS WELL-CENTERED AMONG THE VARIOUS ENSEMBLE MEANS. THUS THE ECMWF CONTINUES TO BE THE PREFERRED DETERMINISTIC MODEL FOR HANDLING THIS SYSTEM AT THIS TIME. GIVEN THE TRENDS SHOWN IN THE GFS, HOWEVER, THIS MODEL IS NO LONGER EXCLUDED FROM THE FORECAST BLEND. REGARDLESS OF THE EXACT TIMING/MODEL SOLUTION, THE LIKELIHOOD IS INCREASING OF AN INTENSE CYCLONE ACROSS THE BERING SEA BY THE MIDDLE OF NEXT WEEK, WITH RAIN AND WIND IMPACTS POSSIBLE ACROSS THE ALEUTIANS MIDWEEK AND INTO WESTERN MAINLAND AK BY WED-FRI. BY LATE IN THE MEDIUM RANGE (THU-SAT), THE BERING CYCLONE SHOULD MATURE AND BEGIN TO FILL AS IT MOVES INTO AK. WHILE SPREAD BEGINS TO INCREASE, MODELS AND ENSEMBLES SHOW GENERAL AGREEMENT THAT THE OVERALL PROGRESSIVE FLOW PATTERN WILL REMAIN THE SAME IN THE WAKE OF THIS SYSTEM, WITH A SERIES OF SHORTWAVES CONTINUING TO TRAVERSE FROM EAST ASIA TOWARD AK. SPREAD INCREASES A BIT BY THIS TIME DUE TO THE RELATIVELY LOW PREDICTABILITY ASSOCIATED WITH SMALL-SCALE WAVES AT THESE TIME SCALES. HOWEVER, THE GFS AND ECMWF ALONG WITH THE ECENS/GEFS/NAEFS MEANS ARE IN GENERAL AGREEMENT THAT ANOTHER SIGNIFICANT TROUGH MAY BEGIN TO AMPLIFY ACROSS THE BERING SEA/WESTERN ALEUTIANS BY NEXT WEEKEND. CONFIDENCE IS LOW FOR THE SPECIFIC DETAILS OF THIS SYSTEM OR ITS EVOLUTION, HOWEVER. THE WPC FORECAST WAS INITIALLY BASED HEAVILY ON A BLEND OF DETERMINISTIC GUIDANCE DURING DAYS 3-4, ESPECIALLY THE 00Z ECMWF, BUT ALSO INCLUDING LESSER COMPONENTS OF THE GFS/00Z CMC. FROM DAY 5 ONWARD, A GRADUAL TREND TOWARD HEAVIER ECENS/GEFS/NAEFS ENSEMBLE MEANS WAS SHOWN. RYAN