ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 247 PM EDT FRI JUL 21 2017 VALID 12Z TUE JUL 25 2017 - 12Z SAT JUL 29 2017 THE LARGE SCALE FLOW PATTERN ACROSS ALASKA CONTINUES TO SHOW A RELATIVELY HIGH DEGREE OF PREDICTABILITY FOR DAYS 4-8. A PERSISTENT UPPER RIDGE OVER THE NORTH PACIFIC WILL BUILD EAST INTO THE ALEUTIANS BY THE MIDDLE TO END OF NEXT WEEK. MEANWHILE, A TROUGH WILL BECOME REESTABLISHED ACROSS THE GULF OF ALASKA AFTER AN UPPER LOW INITIALLY IN PLACE ON DAYS 4-5 MOVES EAST INTO THE COAST. FARTHER NORTH, AN AREA OF NEGATIVE HEIGHT ANOMALIES WILL PERSIST ACROSS THE ARCTIC OCEAN. THE COMBINATION OF THESE LARGE SCALE FEATURES WILL KEEP MUCH OF MAINLAND ALASKA IN FAIRLY PROGRESSIVE WESTERLY FLOW AT THE MID/UPPER LEVELS, WITH A SERIES OF SHORTWAVES TRAVERSING THE STATE THROUGH THE FORECAST PERIOD. A SHORTWAVE CROSSING THE STATE ON DAY 4 (TUE) WILL INFLUENCE THE DEVELOPMENT OF THE TROUGH/UPPER LOW IN THE GULF OF ALASKA BY DAY 6 (THU). MODELS SHOW SOME TIMING DIFFERENCES ON THIS SHORTWAVE, WITH THE GFS A BIT FASTER THAN THE ECMWF AND GEM GLOBAL. TIMING AND AMPLITUDE DIFFERENCES TRANSLATE INTO DIFFERENCES IN THE EVENTUAL UPPER LOW ACROSS THE GULF, WITH A RANGE OF SOLUTIONS EVIDENT BY LATE NEXT WEEK WITH RESPECT TO THE EVENTUAL PLACEMENT AND DEPTH OF THE UPPER LOW. ALSO BY LATE NEXT WEEK, A COUPLE ADDITIONAL SHORTWAVES ARE FORECAST BY ALL GUIDANCE TO MOVE FROM NORTHEASTERN RUSSIA INTO THE BERING STRAIT AND WESTERN AK. THE GFS AND ECMWF ARE RELATIVELY SIMILAR IN THEIR TIMING/INTENSITY WITH RESPECT TO THIS SYSTEM, WHICH SHOULD SUPPORT AT LEAST SOME USE OF THESE DETERMINISTIC SOLUTIONS IN THE FORECAST THROUGH DAY 8. RYAN