Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 746 PM EDT Sat Sep 18 2021 Valid 12Z Wed Sep 22 2021 - 12Z Sun Sep 26 2021 ...Stormy Pattern from the North Pacific into the Gulf of Alaska... ...Guidance and Predictability Assessment... The model and ensemble consensus shows a mean trough aloft extending from Mainland Alaska southward/southwestward while an Arctic upper low likely drifts north and then west. The overall pattern should support a storm track across the North Pacific into the Gulf of Alaska. The most confident systems will be a short-range storm in the process of weakening along the southeastern coast as of the start of the period early Wednesday and a trailing North Pacific storm whose triple point development likely tracks into the Gulf by Thursday-Friday. Then latest guidance is developing a more pronounced signal for another storm that could track south of the Aleutians and Alaska Peninsula late week into the weekend. Early in the period a blend of 12Z operational models represented the most common elements of guidance. Clustering was good for the weakening low initially along the southeastern coast while some detail differences remained for the trailing strong North Pacific storm. For locations along the southern coast/Panhandle, the main consideration was that the 12Z CMC tracked the triple point low (evident by late Wednesday-early Thursday) farther south than other guidance. Thus the blend under-weighted the CMC. The 12Z GFS was also a bit more consolidated than other solutions with a track closer to Kodiak Island, and the blend toned down this aspect as well. By late week through the weekend, recent model runs have been very inconsistent and ensemble means very ill-defined for a trailing North Pacific system. However latest 00Z-12Z model runs have been strengthening their signal for such a storm, supported by some combination of separate pieces of energy emerging from Kamchatka and dropping south/southeast from Siberia. Today's forecast transitioned the aforementioned early model blend toward about half models/half means for days 7-8 Saturday-Sunday. This provided an intermediate starting point between the deep depiction in the 12Z models and ill-defined pattern in the means. Among the means available at the time of forecast preparation, only the CMC mean offered some hint of a wave. The new 12Z ECMWF mean now has a weak wave while the 18Z GFS still has a strong storm. ...Sensible Weather... The southeastern coast/Panhandle will see only a very brief lull in significant precipitation on Wednesday, between a weakening low reaching the southeastern coast by the start of Wednesday and a trailing storm initially south of the Aleutians. This latter storm will generate a broad area of brisk to strong winds from the southern Bering Sea/Aleutians/North Pacific into the Gulf of Alaska, with organized precipitation to the north of the low track midweek and potentially moderate to heavy activity directed into the southeastern coast/Panhandle Thursday-Friday. Most guidance suggests the greatest precipitable water anomalies should be confined to the southern Panhandle. The remnants of West Pacific Tropical Cyclone Chanthu may provide some input for this system. The next storm in the series may bring another episode of strong winds and organized precipitation to the Aleutians and vicinity late week into the weekend. Effects on the Aleutians/Alaska Peninsula will be very sensitive to storm evolution/track which has moderate confidence at best given model inconsistency before clustering improved somewhat today. Away from far southern areas, most locations should see mostly light/scattered precipitation or dry conditions. Best potential for some periods of light activity would be over the North Slope. The mean trough aloft will keep temperatures below normal over much of the state. Coldest anomalies should be over Southcentral and near the Brooks Range. Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook 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