Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 745 PM EDT Wed Sep 14 2022 Valid 12Z Sun Sep 18 2022 - 12Z Thu Sep 22 2022 ***The eventual extratropical low from Typhoon Merbok is forecast to remain impactful across western Alaska this weekend*** ...Guidance/Predictability Assessment... The 12Z model guidance suite has good synoptic scale agreement across the Alaska domain for the weekend, however the GFS begins diverging from the consensus by Monday with the next low approaching the eastern Aleutians, with a position well to the southwest of the ensemble means and the CMC/ECMWF. Going into Tuesday, the models still agree quite well on a strong ridge axis centered over the eastern Gulf and western Alaska, but meaningful differences remain with the next low affecting western Alaska, with the ECMWF portraying a stronger storm and the GFS/CMC weaker. By next Wednesday, the guidance has the general idea of another strong low entering the western Bering, although there are significant placement differences, with the GFS well ahead of the better clustered CMC/ECMWF solutions by this time. Taking these factors into account, the WPC fronts and pressures forecast is primarily derived from a multi-deterministic blend through early Monday, and steadily reducing the GFS by Monday night and beyond. A gradual increase in the GEFS and ECENS are incorporated for the latter half of the forecast period. ...Pattern Overview and Weather/Hazards Highlights... The extratropical low from what is currently Typhoon Merbok is expected to be quite impactful as it crosses the Bering on Friday and then over the Bering Strait and Chukchi Sea by Sunday. Widespread gale to storm force winds are expected across the open waters and for coastal areas, and tidal flooding will remain a concern on Sunday with strong southwesterly onshore flow from the Seward Peninsula to northwestern mainland Alaska. Deep southerly flow across western Alaska is expected to result in periods of moderate inland rain across much of the Interior and the Brooks Range. This low is then forecast to weaken quickly as it lifts farther north over the Arctic by Monday, with some lingering light precipitation and diminishing winds. Farther south, a new surface low is forecast to develop along a front just south of the Aleutians Sunday night, and this is expected to intensify and track northeast towards the southwestern mainland region. A big increase in moisture with a potential atmospheric river event directed between the Kenai Peninsula and the St. Elias Mountains, with several inches of rainfall expected early next week, before the axis of heavier moisture shifts east towards the Southeast Panhandle region. Looking ahead to Tuesday night and into Wednesday-Thursday, another big storm system is looking more likely to affect portions of the Bering and Aleutians, although there is still quite a bit of uncertainty on specifics, with the 12Z GFS indicating a much more progressive solution. Temperatures are expected to generally be in the 40s and 50s for highs on most days across most of the state, with readings generally cooling down going into next week in the wake of the Bering Sea storm. Hamrick/Wegman Hazards: - Heavy rain across portions of the Alaska Panhandle and southern mainland Alaska, Mon-Wed, Sep 19-Sep 21. - Heavy rain across portions of mainland western Alaska, Sat, Sep 17. - Heavy rain across portions of northern Alaska, Sat-Sun, Sep 17-Sep 18. - High winds across portions of mainland western Alaska, Sat, Sep 17. 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