ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 258 PM EDT MON JUL 24 2017 VALID 12Z FRI JUL 28 2017 - 12Z TUE AUG 01 2017 A BROAD UPPER LOW WILL REMAIN DOMINANT OVER THE GULF OF ALASKA THROUGH THE MEDIUM RANGE, WITH A PERSISTENT UPPER RIDGE ANCHORED OVER THE NORTH PAC. MUCH OF AK WILL REMAIN IN RELATIVELY PROGRESSIVE WESTERLIES WITH A SERIES OF SHORTWAVES OF VARIOUS AMPLITUDES MOVING EAST FROM NERN RUSSIA ACROSS THE BERING STRAIT INTO AK BEFORE DIVING SOUTHEAST AND REINFORCING THE UPPER LOW ACROSS THE GULF. MODELS/ENSEMBLES GENERALLY SHOW GOOD AGREEMENT ON THIS LARGE SCALE PATTERN PERSISTING THROUGH THE MEDIUM RANGE. THE FIRST SIGNIFICANT TROUGH DURING THE FCST PERIOD SHOULD REACH WESTERN AK ON FRI. GFS/ECMWF/GEM SHOW GENERALLY GOOD AGREEMENT ON THE TIMING OF THIS WAVE. THE GFS/GEM SHOW THIS WAVE LINGERING ACROSS S-CNTRL AK THROUGH SUN AS IT GETS CAUGHT IN A ZONE OF FLOW DEFORMATION, WHILE THE ECMWF NOW TAKES IT SOUTHWARD AND MERGES IT INTO THE LARGE SCALE CIRCULATION OVER THE GULF. THE NEXT SHORTWAVE IS SHOWN BY THE GFS/ECMWF CROSSING THE BERING STRAIT ON DAY 7 (MON). ADDITIONALLY, THE GFS/ECMWF/GEM ALL SHOW A BLOCKING MID/UPPER LEVEL RIDGE EXPANDING ACROSS EASTERN/CENTRAL AK FROM NWRN CANADA BY DAYS 6-8 (SAT-TUE). THE GEM IS SUBSTANTIALLY STRONGER THAN THE GFS/ECMWF WITH THIS RIDGE, WHILE THE ECMWF IS THE WEAKEST. THE WPC FORECAST WAS BASED HEAVILY ON THE DETERMINISTIC GFS/ECMWF EARLY IN THE MEDIUM RANGE, WITH A GRADUAL TRANSITION TOWARD HEAVIER ENSEMBLE WEIGHTING (ECMWF ENS, GEFS, NAEFS) IN THE LATTER HALF OF THE FORECAST PERIOD. GIVEN THE ACTIVE FLOW PATTERN THROUGH MUCH OF THE PERIOD, MOST OF THE MEDIUM RANGE SHOULD REMAIN RATHER UNSETTLED ACROSS MUCH OF THE STATE, WITH NUMEROUS UPPER DISTURBANCES RESULTING IN FREQUENT PRECIP. MEANWHILE, THE UPPER LOW IN THE GULF WILL KEEP MOIST SOUTHEASTERLY FLOW IN PLACE ACROSS SE AK, KEEPING OCCASIONAL SHOWERS AND PREVALENT. BY EARLY NEXT WEEK, DEPENDING ON THE STRENGTH OF THE DEVELOPING RIDGE FROM EASTERN AK INTO YUKON TERRITORY, DRIER CONDITIONS COULD DEVELOP ACROSS MUCH OF EAST CENTRAL AK. RYAN