Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 623 PM EDT Wed Sep 09 2020 Valid 12Z Sun Sep 13 2020 - 12Z Thu Sep 17 2020 ...Potent Weekend Storm for the Aleutians then Southwest to Southern Alaska early next week... ...Weather/Threats Highlights and Guidance/Predictability Assessment... A potent storm is slated to reach the southern Bering Sea/Aleutians by this weekend with ample energy from a northern stream closed upper trough/low as well as tropical influences. There is a growing signal for deep low development as enhanced by an infusion of energy/moisture with the extratropical evolution of former Super Typhoon Haishen. This leads to a threat of high winds/waves and heavy rains for the Bering Sea/Aleutians this weekend to then spread into southwest/AKpen and southern Alaska into early next week. Some accumulating snows are possible in the higher elevations of southern Alaska as well. In addition, a separate Arctic stream flow to the north should bring a cooling frontal push into an unsettled North Slope/Interior this week. Models and ensembles have come into better agreement for the first few days of the period across the board, though some of the more typical smaller scale embedded flow detail differences remain. This leads well to a majority deterministic model blend (between the 12z ECMWF/GFS/CMC) for days 4 and into day 5. After that time, the forecast begins to get increasingly chaotic, especially at the surface, with regards to individual pieces of energy rounding the base of the large upper low sending various surface lows towards the Aleutians/southern Alaska early to mid next week. While the 12z ECMWF is less of an outlier than in previous days, and more closely resembles that of the GFS, it seems safe at this point to lean more on the ensemble means days 6-8 to mitigate the uncertainties. This approach provides good continuity with yesterday's WPC forecast, with manual adjustments made to ensure sufficient storm depth which is consistent with the tropical history of the system and favorable upper level support. Santorelli 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 Hazards: - Heavy rain across portions of mainland Alaska and the Aleutians, Sun-Mon, Sep 13-Sep 14. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Sep 15-Sep 16. 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