ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 330 PM EDT WED AUG 17 2016 VALID 12Z SUN AUG 21 2016 - 12Z THU AUG 25 2016 A COUPLE SHORTWAVE TROUGHS SWING EASTWARD EXTENDING FROM EXTREME NORTHEASTERN SIBERIA AND ACROSS WESTERN ALASKA EARLY ON DAY 4/SUNDAY. THE LEAD SHORTWAVE APPEARS TO BE LESS SIGNIFICANT THAN THE NEXT SHORTWAVE SYSTEM THAT AMPLIFIES OVER THE BERING SEA ON DAY 5/MONDAY AND CONTINUES TO DEEPEN OVER THE EASTERN ALEUTIANS ON DAY 6/MONDAY. THIS AMPLIFICATION HAS GOOD SUPPORT FROM MANY OF THE OPERATIONAL MODELS...INCLUDING THE ECMWF AND GFS ALTHOUGH THE ECMWF HAS MORE CONSISTENCY OVER THE PAST 24 HOURS OF RUNS. WHILE THE LONGWAVE TROUGH APPEARS TO BE FORECAST WELL...IT IS COMPOSED OF MULTIPLE SHORTWAVE FEATURES WHERE SIGNIFICANT VARIABILITY EXISTS WITH THE TREND TOWARDS SLOWING THE SYSTEM AS A DISTINCT SHORTER WAVELENGTH TROUGH IS FOUND SOUTH OF THE EASTERN ALEUTIANS BUT ALSO A NORTHERN TROUGH AS WELL OVER THE NORTHERN BERING SEA. THERE ARE SIGNIFICANT MODEL TO MODEL DIFFERENCES THAT WERE SMOOTHED OVER BY USE OF THE ECMWF/NAEFS ENSEMBLE MEANS. AS THE FORECAST VEERS INTO DAYS 7 AND 8...COMPARISONS OF MODEL FIELDS FROM VARIOUS OPERATIONAL RUNS BEGIN TO RUN AMOK AS PREDICTABILITY SEEMS TO DROP PRECIPITOUSLY. EVEN THE ENSEMBLE MEANS QUICKLY EVOLVE INTO A BROADLY WAVY ZONAL PATTERN...EVIDENCE OF RAPIDLY INCREASING SPREAD. SO WHILE DAY 7/TUESDAY SEEMS TO BE A REASONABLE COMPROMISE BETWEEN THE OPERATIONAL AND ENSEMBLE MEANS WITH A LITTLE MORE 12Z GFS AND A LITTLE LESS ECMWF...DAY 8/WEDNESDAY DOES NOT PRESENT ANY MEANINGFUL CHOICES FOR THE OPERATIONAL RUNS AND THE FORECAST TRENDED MOSTLY OF THE ECMWF ENSEMBLE MEAN AND THE NAEFS IN A SUPPORTING ROLE. IT APPEARS WITH JUST SOME HINTS THAT THE TROUGH OVER WESTERN ALASKA BEGINS TO SHIFT EASTWARD AS A POSSIBLE SIGNIFICANT SHORTWAVE TROUGH MOVES EASTWARD FROM NORTHEASTERN SIBERIA. HOWEVER...PREDICTABILITY APPEARS POORLY AT THE END OF THE FORECAST. KOCIN