ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 301 PM EDT THU JUL 20 2017 VALID 12Z MON JUL 24 2017 - 12Z FRI JUL 28 2017 THE LARGE SCALE FLOW PATTERN ACROSS ALASKA SHOWS A RELATIVELY HIGH DEGREE OF PREDICTABILITY FOR DAYS 4-8, WITH MODELS AND ESPECIALLY ENSEMBLES SHOWING A RELATIVELY LOW AMOUNT OF SPREAD. A PERSISTENT UPPER LEVEL RIDGE OVER THE NORTH PACIFIC WILL WORK ITS WAY EASTWARD INTO THE ALEUTIANS BY THE MIDDLE TO END OF NEXT WEEK. MEANWHILE, A TROUGH WILL BECOME REESTABLISHED ACROSS THE NORTHEAST PACIFIC OFF THE SOUTHEAST ALASKA/BRITISH COLUMBIA COAST. FARTHER NORTH, AN AREA OF NEGATIVE HEIGHT ANOMALIES WILL PERSIST ACROSS THE ARCTIC OCEAN. THE COMBINATION OF THESE THREE LARGE SCALE FEATURES WILL KEEP MUCH OF MAINLAND ALASKA IN FAIRLY PROGRESSIVE WEST-NORTHWESTERLY FLOW AT THE MID/UPPER LEVELS, WITH A SERIES OF SHORTWAVES TRAVERSING THE STATE THROUGH THE FORECAST PERIOD. THESE FEATURES ARE WHERE A MODERATE DEGREE OF SPREAD/UNCERTAINTY IS INTRODUCED INTO THE FORECAST, PRIMARILY DUE TO THE SMALLER SCALE OF THE SHORTWAVES AND THEIR RELATIVELY LOW LEVEL OF PREDICTABILITY IN THE MEDIUM RANGE. ONE SUCH SHORTWAVE IS FORECAST BY THE GFS/ECMWF TO CROSS THE STATE ON DAY 4 (MON). THE GEM GLOBAL IS A BIT SLOWER WITH THIS WAVE, BUT GIVEN THE RELATIVELY GOOD AGREEMENT BETWEEN THE GFS/ECMWF, THESE SOLUTIONS WERE WEIGHTED FAIRLY HEAVILY INITIALLY IN THE FORECAST PERIOD. A SERIES OF WEAKER SHORTWAVES WILL FOLLOW THE SAME PATH IN THE WAKE OF THE ONE ON DAY 4. GRADUALLY RISING HEIGHTS ACROSS THE ALEUTIANS AND SOUTHERN ALASKA AS THE NORTH PACIFIC RIDGE BUILDS EAST SHOULD SERVE TO SHIFT THE MOST ACTIVE MID/UPPER FLOW SLOWLY NORTH THROUGH THE PERIOD. BY DAYS 7-8 (THU-FRI) ANOTHER MORE SIGNIFICANT SHORTWAVE IS FORECAST BY MOST GUIDANCE TO CROSS THE KAMCHATKA PENINSULA AND PERHAPS REACH WESTERN AK. SPREAD IS A BIT HIGHER BY THIS TIME, WITH THE ECMWF SHOWING A STRONGER AND SOMEWHAT FASTER WAVE THAN THE GFS/GEM. GIVEN THE INCREASING SPREAD AND UNCERTAINTY WITH RESPECT TO THE SMALLER SCALE ASPECTS OF THE FLOW, THE FORECAST WAS TRENDED GRADUALLY TOWARD HEAVIER ENSEMBLE WEIGHTING DURING THE DAY 6-8 TIME PERIOD. RYAN