Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 456 PM EDT Mon Sep 27 2021 Valid 12Z Fri Oct 01 2021 - 12Z Tue Oct 05 2021 ...Powerhouse Storms to affect Alaska... ...Pattern and Guidance/Predictability Assessment... A stormy pattern for Alaska and offshore waters will persist and pattern forecast predictability remains above average overall. However, vorticity interactions and phasing issues aloft are still leading to pronounced main offshore surface low differences. A composite of the compatible 12 UTC GFS/ECMWF/Canadian/UKMET seems reasonable day 4/Fri, but I'd still lean more on ensemble means (especially the ECMWF ensemble mean) days 5-8 given embedded system development variance, albeit keeping in the blend smaller input from the 12 UTC GFS/ECMWF for more detail consistent with forecast spread.ÂThe ECMWF ensemble mean seems a closer match with the GFS/ECMWF at longer time frames. WPC product continuity is well maintained with this strategy and again planned manual edits to ensure 2 main deep storms given upper support and history.ÂIn particular, phasing aspects inherent in the GFS seems near worst case for the weekend Gulf of AK storm given forecast spread in both the other models and all the ensembles. I'd consider it a less likely but plausible outlier.ÂThat said, as consolidated in the Gulf it should prove to be a vigorous system that WPC progs will continue to show. The same can be said for the day 7/8 low slated to reach the southern Bering and Aleutians in about a week that is currently Typhoon Mindulle. Overall WPC product continuity was well maintained with these blends in conjuncture with manual surface system modifications to ensure sufficient offshore storm depth/intensity consistent with a highly supportive and amplified upper flow evolution as a powerful closed low/trough works from the eastern Bering Sea through the Gulf of Alaska late week/weekend well in advance of approach from the west-Pacific by early next week of a strong extratropical low associated with current west Pacific Typhoon Mindulle. ...Weather/Hazard Highlights... A vigorous storm system digging through the eastern Bering Sea this week will slam into a highly unsettled Gulf of Alaska Friday and the weekend to present a substantial maritime threat for high waves/winds along with heavy rain/snow for especially coastal areas of south-central Alaska through the Southeast Panhandle with moistening onshore flow. Meanwhile, A deep extratropical low associated with current west Pacific Typhoon Mindulle is slated to track toward the southern Bering Sea and Aleutians by early next week to present another significant heavy seas/wind and wrapping rainfall threat. Schichtel Hazards: - Heavy precipitation across portions of mainland Alaska, Thu-Fri, Sep 30-Oct 1. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Thu-Sat, Sep 30-Oct 2. - High winds across portions of the Alaska Panhandle, mainland Alaska, and the Aleutians, Thu, Sep 30. 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