Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 618 PM EDT Wed Sep 13 2023 Valid 12Z Sun Sep 17 2023 - 12Z Thu Sep 21 2023 ...Powerful Gulf storm offers heavy rain/wind threat from Southcentral to Southeast Alaska this weekend... ...Synoptic Overview and Guidance Evaluation/Preferences... The 12 UTC GFS/ECMWF/UKMET/Canadian model solutions overall seem well clustered into Sunday/Monday in a stormy and progressive flow regime, albeit with a less certain West Pacific tropical roots from JTWC invest 93W along with an associated long fetched deep moisture plume and additional eastern Asian jet/system energy phasing. A favored composite seems to provide good detail. Growing forecast spread and uncertainty as well as worsening run to run continuity heading into next midweek prompted transition to primarily the compatible 12 UTC GEFS/NAEFS/ECMWF ensemble means at the cost of less defined individual features, despite a pattern still favoring more in a series of dynamic systems. Accordingly, opted to also include modest input from best matching ECMWF detail to the ensemble mean blend along with some manual adjustments to counter the weakening effect of the blending process on systems. ...Pattern Overview and Weather/Hazards Highlights... A series of deepened low pressure systems will track progressively from across the North Pacific and the southern Bering Sea/Aleutians and through the Gulf of Alaska with translation over the next week underneath a mean upper trough position centered over the Bering Sea/western Alaska. These systems will help direct long fetch moisture/atmospheric river/south-southwesterly flow into the Southern Coast/Southeast Alaska resulting in the potential for multiple days of very wet weather and heavy rainfall. Some locations could receive upwards of 4 to 8 inches of rain over the entire forecast period, with the greatest totals expected across the windward terrain of the southern and southeastern coastal mountain ranges. These storms should prove to be periodic significant maritime hazards. High gales with very gusty winds are also possible into the coast as well. This is all especially evident with a main powerful storm emerging from the western Pacific to the Gulf of Alaska this weekend. Otherwise, much of the remainder of the mainland looks to remain mostly dry/light side precipitation into early next week. However, enhanced moisture should work more earnestly into southwest and western Alaska early-mid next week with slow Bering Sea upper low progression and possible deepened low pressure frontal system approach, but there is still a lot of uncertainty in the details. Schichtel Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat, Sep 16. - Heavy rain across portions of the Alaska Panhandle, Sat-Sun, Sep 16-Sep 17. - High winds across portions of the Alaska Panhandle and mainland Alaska, Sun, 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