ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 339 PM EDT TUE APR 28 2015 VALID 12Z SAT MAY 02 2015 - 12Z WED MAY 06 2015 THE ALASKA MEDIUM RANGE FORECAST PACKAGE CONTINUED TO FOLLOW A 70/30 BLEND OF THE 28/00Z ECENS AND NAEFS MEAN AND FEATURES THE EASTWARD MIGRATION OF TWO MID-LEVEL CYCLONES ACROSS THE NORTH CENTRAL PACIFIC. AT THE START OF THE FORECAST PERIOD---(SATURDAY)---THESE TWO SYSTEMS ARE FORECAST TO BE INVOF 155W AND 175E LONGITUDE RESPECTIVELY. THE 'LEAD' SYSTEM 'SPLITS' WITH THE BULK OF THIS CYCLONE TRACKING INTO COASTAL WASHINGTON STATE AND A PORTION OF THE UPPER-LEVEL SUPPORT MIGRATING NORTHEASTWARD INTO THE SOUTH CENTRAL ALASKA COAST AND NORTHERN PANHANDLE. DURING THE WPC DAY 4-6 SURFACE GRAPHIC SEQUENCE---IT IS DEPICTED AS A 'DYING OCCLUSION'. MEANWHILE...THE SECOND SYSTEM IS MOVING EASTWARD AND IS IN A LOCATION VERY NEAR 175W LONGITUDE (AT DAY 6). BEYOND DAY 6...THE FORECAST WILL BE CHALLENGING---WITH MIGRATORY MID-LEVEL SHORTWAVE RIDGE POSITIONING (BETWEEN SYSTEMS) HOLDING THE KEY TO THE ACCURACY OF THE SECOND SYSTEM'S EVOLUTION AND TRAJECTORY. FOR NOW...THE MEANS SUGGEST THE BULK OF THIS UPPER-LEVEL CYCLONE WILL PROGRESS EASTWARD AT A CONSISTENT PACE---AND ARRIVE VERY NEAR 165W LONGITUDE AT DAY 8 (6/12Z) HOWEVER...THE AMPLITUDE OF THE SHORTWAVE RIDGE MIGRATING INTO THE WESTERN REACHES OF THE GULF OF ALASKA---APPEARS TO BE 'GAINING' AMPLITUDE (HEADING MORE NORTHWARD). IF THE ECENS/NAEFS MEANS CONTINUE TO TREND THIS WAY---THERE MAY BE AN ADDITIONAL ADJUSTMENT IN THE FUTURE---THE CURRENT WPC DAY 7-8 SURFACE GRAPHIC SEQUENCE. THE CURRENT SEQUENCE (VALID FOR 5/12Z AND 6/12Z) MIGRATES A SIGNIFICANT MID-LEVEL WARM ADVECTION PATTERN AND SHORTWAVE IMPULSE ACROSS THE GULF OF ALASKA AND INLAND (BETWEEN ANCHORAGE AND JUNEAU)---IN ADVANCE OF---THE UPPER PORTION OF THE TROUGH. VOJTESAK