Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 PM EDT Sun Sep 15 2019 Valid 12Z Thu Sep 19 2019 - 12Z Mon Sep 23 2019 Guidance continues to show an active northern Pacific stream that will feature two dominant storm systems tracking along or just south of the Aleutians and then into or near the Gulf of Alaska. These systems will likely bring some enhanced rainfall to the Aleutians/Alaska Peninsula and support much heavier precipitation at times over the southeastern coast/Panhandle. Over at least the past couple days or so the consensus of model and ensemble guidance has been consistent in highlighting the southeastern coast/Panhandle region for highest five-day precip totals. Meanwhile an upper low (consisting of one or more individual impulses) should meander somewhere between the northwestern mainland coast and eastern Siberia. The first Pacific system should track from just south of the central Aleutians into the Gulf of Alaska Thu-Sat. It originates from an area of disturbed weather south of Japan (98W) and quickly becomes extratropical under the influence of mid-latitude dynamics. An operational model consensus represents the storm well into day 5 Fri with a stronger depiction than the means. Thereafter latest CMC runs stray northwest of consensus (the 12Z run less so than the prior 00Z version) while the 12Z GFS tracks south of consensus. The next system will likely reach near the Aleutians by day 6 Sat and then continue tracking east/northeast. There are more detail uncertainties that persist for this storm, including the relative emphasis of parent low pressure associated with the upper dynamics that emerge from eastern Asia versus leading waviness that lifts up from the lower latitudes of the western Pacific. A blend/ensemble mean approach best depicts this system given the questions with specifics, while holding onto some operational guidance makes up for the recent tendency for ensemble means to lose definition for a time between the central Aleutians and western Gulf. Through the arrival of the 12Z GFS, the 00Z GFS and 12Z/14 ECMWF provide the best overall comparison to the ensemble means. The 12Z/06Z GFS are generally slower than established consensus while the 00Z ECMWF eventually tracks the system well south of preference. The new 12Z ECMWF is much closer to the preferred track. For the upper low near the northwestern mainland, the latest guidance average as well as the ensemble means provide a position fairly close to yesterday's forecast--after yesterday's 12Z model runs had suggested potential for retrogression. There is still plenty of room for changes in the forecast given that the overall low may consist of multiple individual impulses. The early part of the forecast incorporates parts of the 00-06Z GFS and 00Z ECMWF/UKMET/CMC. The mid-late portion of the period adds the 06Z GEFS/00Z ECMWF means while adjusting the remaining operational model components more toward the 00Z GFS and 12Z/14 ECMWF. The resulting blend and minor manual adjustments yield fairly modest changes/refinement versus continuity. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html