Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Sun Sep 10 2023 Valid 12Z Thu Sep 14 2023 - 12Z Mon Sep 18 2023 ***Long duration heavy rainfall likely over portions of Southcentral to Southeast Alaska into next week*** ...Synoptic Overview... Stormy, Fall-like pattern will be in place late week as multiple strong storm systems traverse the Gulf of Alaska. At least a couple waves of moisture sourced from the Pacific look to impact Southcentral to Southeast Alaska bringing a prolonged period of moderate to heavy rain through next weekend. ...Guidance Evaluation/Preferences... A consensus of the model guidance indicates that the upper-level pattern should remain in a relatively stable state with an upper-low in place over the western Mainland/Bering Straight and multiple shortwaves traversing around the low from the Bering Sea into the Gulf of Alaska. However, disagreement in the timing/phasing of the individual waves and subsequent surface system movement leads to some uncertainty and a preference towards favored guidance clustering/persistence in the WPC forecast. For the early forecast period late week, the 12Z GFS and ECMWF are in general agreement taking a shortwave and associated surface low pressure system through the northern Gulf of Alaska while the CMC has a much deeper trough and develops a secondary system that tracks northeastward from the north Pacific towards Southeast Alaska. Into the weekend, the GFS is faster and more aggressive with the next system dropping southward into the Gulf of Alaska as the ECMWF as well as the GEFS/ECens means remain slower and take a bit more northerly track. However, the trend has been for the system to drop a bit further south which follows a track into the northern Gulf of Alaska rather than up the Alaska Peninsula in prior forecasts. Uncertainty increases by the end of the period into next week with deeper mean troughing in the GEFS mean approaching the Bearing Sea from upstream over eastern Russia while both the ECens and CMC ensemble means remain with the persistent upper low/troughing oriented further east over the western Mainland/Bering Straight. The updated WPC forecast thus favors a blend of the ECMWF/GFS early in the period with a shift to the ECMWF/CMC and ECens/CMC ensemble means into the mid- to late period. ...Weather/Hazards Highlights... A cool and very wet pattern will persist from Southcentral around Prince William Sound southeast into Southeast Alaska from late week into the weekend. Onshore flow associated with low pressure systems traversing the northern Gulf of Alaska will advect multiple rounds of enhanced moisture inland sourced from deeper over the Pacific. Some locations could receive upwards of 3 to 6 inches of rain over the entire forecast period, with the greatest totals expected across the windward terrain of the southern coastal mountain ranges. Some gusty winds will also be possible, particularly as the storm systems have trended stronger and more Fall-like in recent model runs. Otherwise, much of the remainder of the state looks to remain mostly dry through the period outside of the Aleutians/Alaska Peninsula where the storm systems tracking through should spread some rain chances into the area as well. Much uncertainty remains but there is a chance for increased precipitation chances over the western Mainland related to the potential for a stronger system upstream over eastern Russia taking a more northerly track into the mainland early to mid- next week. In terms of temperatures, expect readings to generally be a bit below average across southern portions of the state owing to both persistent upper troughing/waves of cooler air and increased clouds and rainfall coverage. Temperatures will be around to above average further north into the Interior and North Slope. Putnam 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