Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 701 PM EDT Sun Aug 27 2023 Valid 12Z Thu Aug 31 2023 - 12Z Mon Sep 04 2023 .....Major Storm to focus into Southern/Southwest Alaska into later week with deep fetch moisture and threats of heavy rain, coastal flooding and high winds/waves... ...Guidance Evaluation/Preferences... The 12 UTC GFS/ECMWF seem best clustered late week in a stormy pattern with above normal predictability and max ensemble support, but have reasonably trended slower and deeper with the approach of the extratropical low associated with current WPac Typhoon Damrey. This blended solution is slower to erode the downstream upper ridge than the 12 UTC Canadian/UKMET, respecting amplitude. Growing forecast spread by next weekend with transition to less amplified flow favors a broader GFS/ECMWF and GEFS/ECMWF ensemble means composite to mitigate error and maintain max product continuity. Trends show a growing signal for more energetic Bering Sea to mainland flow and high latitude uncertainties. ...Weather/Hazards Highlights... There is a strong signal that system energies and deep moisture linked to Typhoon Damrey currently in the West Pacific will likely slam into Alaska into later this week. This is slated to include a deepened associated extratropical surface system forecast to work/lift northeastward through the Aleutians to western Alaska before ejecting inland as a weakening system across the interior. This will spread widespread and highly unsettled/stormy weather along and downstream of the system across a broad region as fueled by a deep plume of moisture with low latitude/tropical origins. The main focus for threat impacts may occur from the Aleutians to Western/Southwestern Alaska and into the Interior as well as with lead inflow through the Gulf of Alaska 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 and then the Southeast as an initially shielding upper ridge breaksdown next weekend as height falls press robustly eastward and flatten the upper flow pattern. The main threats in this pattern may be for heavy rainfall, coastal flooding, high/gusty winds and dangerous maritime conditions. With a pattern transition away from high amplitude, systems embedded within the more progressive upstream flow become increasingly uncertain, especially north of the Arctic Circle. Increased reliance on ensemble mean information at these longer time frames may be needed until the pattern solidifies, but there is a growing signal that there will be several system opportunities to hone into both across the Arctic and underneath out across the Bering Sea to the mainland given ample energies in both of these main flows. The WPC Alaska surface progs attempt to depict systems, but suspect future renditions may prove different. 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