ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 318 PM EDT WED SEP 13 2017 VALID 12Z SUN SEP 17 2017 - 12Z THU SEP 21 2017 A HIGHLY AMPLIFIED FLOW PATTERN IS EXPECTED ACROSS ALASKA THROUGH THE MEDIUM RANGE. THE DEGREE OF POTENTIAL INTERACTIONS BETWEEN ARCTIC SHORTWAVE ENERGY AND NORTHERN PACIFIC ENERGY INTRODUCES SOME UNCERTAINTY AND SPREAD INTO THE FORECAST EVEN EARLY IN THE PERIOD, BUT SPREAD REALLY BEGINS TO AMPLIFY LATER IN THE FORECAST PERIOD (FROM TUE ONWARD). INITIALLY ON DAY 4 (SUN), MODELS SHOW GENERAL AGREEMENT ON AN UPPER LOW CENTERED OVER THE GULF OF AK, WITH ARCTIC SHORTWAVE ENERGY DIGGING ACROSS THE KAMCHATKA PENINSULA. THE DEGREE OF INTERACTION BETWEEN THESE TWO FEATURES RESULTS IN SOME SPREAD AMONG THE GUIDANCE, AND THE GFS HAS SHOWN A LOW DEGREE OF RUN-TO-RUN CONSISTENCY, WITH SOME RUNS KEEPING THE FEATURES MORE SEPARATE AND OTHER (LIKE THE 06Z/12Z RUNS) SHOWING A HIGH DEGREE OF INTERACTION, RESULTING IN A CLOSED UPPER LOW MUCH FARTHER NORTH, ACROSS SOUTHERN AK. THE ECMWF, ON THE OTHER HAND, HAS MUCH MORE CONSISTENTLY KEPT THE FEATURES/STREAMS SEPARATE, WHICH LIKELY SUGGESTS BETTER HANDLING OF THE ARCTIC ENERGY BY THE MODEL. THUS, THE ECMWF IS THE PREFERRED DETERMINISTIC SOLUTION ACROSS ALASKA EARLY IN THE FORECAST PERIOD. THIS SOLUTION WOULD AMPLIFY THE ARCTIC SHORTWAVE ACROSS WESTERN AK BY DAY 5-6 (MON-TUE). MEANWHILE, THE FLOW ACROSS THE NORTH PACIFIC WILL BECOME INCREASINGLY AMPLIFIED, WITH A LARGE AND ANOMALOUS UPPER RIDGE FORECAST BY ALL GUIDANCE TO DEVELOP, CENTERED NEAR 40 DEG NORTH/ WEST OF THE RIDGE, A LONG WAVE TROUGH IS FORECAST BY ALL GUIDANCE TO DEVELOP, CROSSING THE ALEUTIANS SUN-TUE. WHILE ALL GUIDANCE AGREE ON THE PRESENCE AND GENERAL TIMING OF THIS FEATURE, LARGE DIFFERENCES EXIST WITH RESPECT TO THE EXACT DETAILS AND STRUCTURE OF THE FEATURE. THE ECMWF HAS BEEN FAIRLY CONSISTENT WITH SHOWING AN ELONGATED, POSITIVELY-TILTED UPPER TROUGH, WHILE THE GFS HAS OSCILLATED BETWEEN A EC-LIKE SOLUTION, AND A DEEP CLOSED UPPER LOW, AND EVEN A RIDGE. THUS, THE ECMWF REMAINS THE PREFERRED DETERMINISTIC SOLUTION THROUGH DAY 6. DURING DAYS 6-7, EVEN THE ECMWF BEGINS TO STRUGGLE WITH HOW TO HANDLE ADDITIONAL ARCTIC ENERGY THAT MOVES FROM EASTERN RUSSIAN INTO THE BERING SEA/WESTERN AK. ADDITIONAL INTERACTIONS BETWEEN THIS ARCTIC SHORTWAVE AND NORTH PACIFIC ENERGY ARE ALSO HIGHLY UNCERTAIN. THUS, THE WPC FORECAST PREFERENCE SHIFTS HEAVILY TOWARD ENSEMBLE MEANS (ESPECIALLY THE ECENS MEAN) BY THIS TIME FRAME. RYAN