Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 602 PM EDT Tue Sep 08 2020 Valid 12Z Sat Sep 12 2020 - 12Z Wed Sep 16 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 to then spread into southwest/AKpen and southern Alaska into early next week. The models and ensembles continue to offer a reasonably similar larger scale pattern evolution aloft, with the typical smaller scale embedded system detail differences, especially after day 4. To the north, separate Arctic stream flow should bring a cooling frontal push to an unsettled North Slope/Interior this week, underneath an amplified Arctic closed low/trough. The ensemble means along with the GFS/CMC support this overall scenario better than that of the 12z ECMWF. The 12z ECMWF stayed consistent with it's 00z run showing additional energy undercutting the bigger upper low in the Bering Sea day 5-6, which brings a surface low towards the Eastern Aleutians, a solution not shown by the other deterministic models or means. It's also farther south/west with the Arctic upper low which supports stronger northerly flow for longer over the mainland late in the period. The WPC Alaskan medium range product suite was primarily derived from a composite blend of the 12z GFS/CMC/ECMWF for day 4, transitioning to heavier weighting of the ensemble means (00z ECENS/12z NAEFS) beyond that time, with decreasing contributions from the 12z GFS/CMC days 5-8. Given the explanation above, the ECMWF was not included in the blend past day 5. This approach provides good continuity with yesterday's WPC forecast as well. Manual adjustments to the blend were also applied to the big weekend storm to ensure sufficient storm depth consistent with the tropical history of the system and expected 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. - High winds across portions of mainland Alaska and the Aleutians, Sun-Mon, Sep 13-Sep 14. 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