ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 309 PM EST MON DEC 18 2017 VALID 12Z FRI DEC 22 2017 - 12Z TUE DEC 26 2017 DETERMINISTIC AND ENSEMBLE GUIDANCE CONTINUE TO SHOW A STRONG SIGNAL OF A HIGH AMPLITUDE RIDGE BUILDING NORTHWARD AND EVOLVING INTO A HIGH AMPLITUDE BLOCKING PATTERN SPREADING FROM THE NORTHEAST PACIFIC AND ACROSS ALL OF ALASKA DURING THE WEEKEND. MODEL ENSEMBLE MEANS HAVE CONTINUED TO TREND TOWARD A HIGHER AND HIGHER AMPLITUDE SOLUTION. TO THE WEST OF THE RIDGE AXIS...THERE HAS BEEN A DEFINITE CHANGE IN THE MODEL SOLUTIONS SINCE YESTERDAY...AS THEY BEGIN TO INDICATE CYCLONE CENTERS MOVING NORTH-NORTHWESTWARD ACROSS THE EASTERN ALEUTIAN ISLANDS DURING THE WEEKEND IN RESPONSE TO CLOSER PROXIMITY OF BAROCLINIC INTERACTIONS WITH THE UPSTREAM HIGH AMPLITUDE TROUGH NOSING INTO THE BERING SEA. THE 06Z GFS WAS SLOW WITH RESPECT TO THE FORWARD MOTION OF THIS CYCLONE COMPARED WITH THE 00Z ECMWF. HOWEVER...THE 12Z GFS SPED UP THIS LOW AND ITS FORECAST POSITIONS ARE MORE COMPARABLE WITH THE 00Z ECMWF/EC MEAN (ALTHOUGH FASTER AT 00Z SUN DEC 24). A BLEND OF THE 00Z ECMWF/EC MEAN TOGETHER WITH THE 12Z GFS WAS USED TO HANDLE THIS SYSTEM. THEREAFTER...MODELS AGREE THAT THE HIGH AMPLITUDE BLOCKING PATTERN OVER ALASKA WILL SLOWLY BREAK DOWN EARLY NEXT WEEK...GIVING WAY TO WEAKER PRESSURE GRADIENT. THE WPC GRIDS PUT INCREASINGLY MORE WEIGHTS TOWARDS THE END OF THE MEDIUM-RANGE PERIOD BASED ON THE 00Z EC MEAN AND THE 06Z GEFS MEAN. KONG