Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 505 PM EDT Sat Aug 26 2023 Valid 12Z Wed Aug 30 2023 - 12Z Sun Sep 03 2023 ...Multi-system Heavy Rain Threat for Southern/Southwest Alaska next week... ...Guidance Evaluation/Preferences... 12 UTC GFS/ECMWF/UKMET/Canadian solutions have become better clustered Tuesday to Thursday in an active pattern with above normal predictability and good ensemble support. Forecast spread increases later next week led to a switch to still compatible GEFS/NAEFS/ECMWF ensemble means to mitigate error and maintain max product continuity as predictability lowers toward average levels with pattern transition to less amplified and more progressive flow. ...Weather/Hazards Highlights... There is a strong signal showing a quite wet and stormy/windy pattern from the Alaska Peninsula and Kodiak Island through the Cook Inlet, Kenai Peninsula and Prince William Sound as well as inland across South-central Alaska into the Alaska Range leading into and through early-mid next week. A multi-day heavy rain event will repeatedly focus over the South-central Alaska and vicinity, extending quite a wet and unsettled pattern as several deepened lows and deep moisture plumes feed into the region as channeled between a highly amplified/closed upper trough position over the Bering Sea/Aleutians and ample/shielding/warming upper ridging downstream from Southeast Alaska to the Interior. In this period, tandem deepened surface systems will rotate/lift inland out from the Aleutians and also the northwest Gulf of Alaska as fueled by long fetch moisture plumes/atmospheric rivers. The combination of moist storms and cumulative affect of multiple system rainfall offers a threat for local runoff/mudslide concerns. There is then a growing upstream signal that system energies and deep moisture from Tropical Storm Damrey currently in the West Pacific are slated to breach Alaska mid-later next week, potentially to include a deepened associated extratropical surface system forecast to work through the Aleutians then onward across much of the mainland to fuel and spread steadily eastward over time heavy rains and unsettled conditions. All these systems are also expected to enhance maritime to coastal winds and waves. Expected system/upper trough progression robustly across the mainland by next weekend would allow significant height falls to work downstream through aforementioned lead/ambient upper ridging leading to a less amplified pattern. Schichtel 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