Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 604 PM EDT Thu Sep 10 2020 Valid 12Z Mon Sep 14 2020 - 12Z Fri Sep 18 2020 ...Potent storm with impacts for the Aleutians and into Southwest/parts of southern Alaska this weekend into early next week... ...Weather/Threats Highlights and Guidance/Predictability Assessment... By the beginning of the Alaska medium range period (Monday, Sept 14), a potent storm is slated to reach the Bering Sea as the attendant cold front swings through the Aleutians. This low will have ample energy from a northern stream closed upper trough/low as well as tropical influences from former Super Typhoon Haishen. Expect a threat for high winds/waves and heavy rainfall impacting the Bering Sea/Aleutians this weekend, moving into southwest AK by early next week. There is more uncertainty regarding the evolution of this low past about mid week and thus severity of impacts across Southern Alaska and the Gulf Coast. Regardless, some precipitation and gusty winds are likely as the front weakens as it moves eastward next week. Elsewhere, a separate Arctic stream flow to the north should bring a cooling frontal push into the North Slope region next week, while models trend stronger with ridging over much of the Interior/Eastern Alaska. The models and ensembles continue to come into better agreement at least for the first day or two of the period with regards to the large low entering the Bering Sea. The 12z GFS is quicker to erode the ridge across mainland Alaska through next week, also with a quicker progression of the main upper low. The trend seems to be for a slower progression of the low, lingering across the Bering longer and with stronger ridging inland. This is best represented by the 12z deterministic ECMWF and the ensemble means. The WPC forecast blend for today used a majority deterministic blend, more weighted towards the ECMWF than the GFS, for days 4-5. After that, differences become much more apparent especially with respect to additional energies undercutting the main upper low/trough sending various surface lows towards the Aleutians/Gulf region. To help mitigate these differences, the WPC forecast leaned heavily towards the ensemble means (ECENS/GEFS) for days 6-8. 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, Mon-Wed, Sep 14-Sep 16. - Heavy rain across portions of 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