Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 719 PM EDT Fri Aug 25 2023 Valid 12Z Tue Aug 29 2023 - 12Z Sat Sep 02 2023 ...Multi-system Heavy Rainfall Threat for South-central Alaska next week... ...Guidance Evaluation/Preferences... The 12 UTC GFS/ECMWF seem to offer a seemingly reasonable forecast basis for early-mid next week in an active pattern with increasing predictability and favorable ensemble support. Forecast spread increases later next week prompting a transition toward increased GEFS/NAEFS/ECMWF ensemble means input to maintain max product continuity in a pattern with near average predictability, albeit with plenty of uncertainties in the details and timing of individual systems over time. Manual adjustments were applied as applicable to maintain/deepen offshore low pressure systems given favorable support. This seems to provide a reasonable solution best in line with the amplified flow and also with the expected stormy ejection toward Alaska of an uncertain but potentially deepened extratropical low associated with current west Pacific tropical system Damrey. ...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. A multi-day heavy rain event will repeatedly focus over the South-central Alaska and vicinity all next week, 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 upper ridging downstream from Southeast Alaska to the Interior. Several leading and organized surface lows will rotate/lift out from the Aleutians and also the northeast Pacific through the Gulf of Alaska and inland into early-mid next week as fueled by long fetch moisture plumes/atmospheric rivers in advance of an amplifying Bering Sea centered upper through. The combination of moist storms and cumulative affect of multiple systems/rainfall offers a threat for runoff problems. There is then an upstream signal that still uncertain system energies from Tropical Storm Damrey currently in the West Pacific may approach the Alaska domain next week, potentially as a deepened associated extratropical surface low to work towards/underneath the central-eastern Aleutians through by next midweek, then potentially onward to the mainland into later next week to fuel an additional heavy rain along with widespread maritime to coastal wind threats. Otherwise in terms of temperatures, values across much of the state will moderate closer to normal and above next week as upper level ridging builds into the region. 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 Hazards: - Heavy rain across portions of the Alaska Panhandle and southern mainland Alaska, Mon-Fri, Aug 28-Sep 1. 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