ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 418 PM EDT MON APR 17 2017 VALID 12Z FRI APR 21 2017 - 12Z TUE APR 25 2017 A BLOCKING UPPER RIDGE WHICH HAS BEEN PERSISTENT ACROSS THE ARCTIC FOR SOME TIME SHOULD GRADUALLY RETROGRADE TOWARD RUSSIA THROUGH THE MEDIUM RANGE, OPENING THE DOOR FOR A NUMBER OF LOW-AMPLITUDE SHORTWAVES TO MOVE SOUTH FROM THE ARCTIC INTO ALASKA BY FRI ONWARD. THIS FLOW SEEMS TO SHOW SOMEWHAT LOW PREDICTABILITY AS IT DEVELOPS, WITH MODELS SHOWING DIFFERENCES WITH RESPECT TO THE INTENSITY OF THESE WAVES AS WELL AS THE DEGREE OF PHASING THAT OCCURS WITH THE BROADER AREA OR NEGATIVE HEIGHT ANOMALIES FARTHER SOUTH, ACROSS THE NORTH PACIFIC. ONE SUCH SHORTWAVE IS FORECAST BY ALL MODELS TO MOVE INTO THE NORTH COAST OF ALASKA ON FRI, WITH THE ECMWF TAKING THE WAVE SOUTHWESTWARD ACROSS CENTRAL AK MORE QUICKLY THAN THE GFS, WHICH PHASES THE ENERGY INTO AN ELONGATED TROUGH EXTENDING INTO NORTHWESTERN CANADA. ADDITIONAL SHORTWAVE ENERGY OR ARCTIC ORIGIN IS FORECAST TO AMPLIFY ACROSS NORTHWESTERN ALASKA BY MON-TUE. AGREEMENT BETWEEN THE GFS AND ECMWF IS SOMEWHAT BETTER WITH THIS FEATURE, WITH THE GEM GLOBAL BEING THE MOST SIGNIFICANT OUTLIER, SHOWING STRONGER RIDGING PERSISTING AND A LESS AMPLIFIED SYSTEM BY DAY 8. FARTHER SOUTH, THE FLOW ACROSS THE NORTH PACIFIC SEEMS TO HAVE SOMEWHAT MORE PREDICTABILITY. THE PATTERN WILL REMAIN ACTIVE WITH A FEW LARGER AMPLITUDE TROUGHS MOVING SOUTH OF THE ALEUTIANS THROUGH THE FORECAST PERIOD. MODELS SHOW GENERAL AGREEMENT NO THIS LARGE-SCALE PATTERN, IN WHICH THE AREA OF MEAN NEGATIVE HEIGHT ANOMALIES SHIFTS A BIT WESTWARD. PERSISTENT RIDGING ALONG THE CONUS/CANADIAN WEST COAST WILL TEND TO WEAKEN THESE SHORTWAVES AS THEY REACH THE NORTH AMERICAN COAST. THE GFS AND ECMWF, ALONG WITH THE ECMWF ENSEMBLE/GEFS/NAEFS MEANS SHOW GENERALLY GOOD AGREEMENT ON THE TIMING AND TRACK OF THE SYSTEMS TRAVERSING THIS FLOW, WITH THE GEM GLOBAL BEING THE MOST SIGNIFICANT OUTLIER, SHOWING A MUCH DEEPER AND FARTHER NORTH SURFACE LOW SOUTH OF THE ALEUTIANS BY DAY 7. THE WPC FORECAST LEANED HEAVILY ON THE DETERMINISTIC GFS/ECMWF EARLY IN THE FORECAST PERIOD, WITH A GRADUAL TOWARD MORE WEIGHTING OF THE ENSEMBLE MEANS (ECENS, GEFS, NAEFS) TOWARD THE END, ESPECIALLY ON DAYS 7-8. RYAN