Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 748 PM EDT Mon Aug 28 2023 Valid 12Z Fri Sep 01 2023 - 12Z Tue Sep 05 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/UKMET/Canadian have converged upon a better clustered solution days 4-6 (Fri/Weekend) in a pattern with above normal predictability and ensemble support, bolstering forecast confidence despite a transition to a less amplified flow regime. A blend maintains good continuity. Growing forecast spread into early next week increasingly prompted a switch to broad GEFS/ECMWF ensemble means guidance amid growing system variance. ...Weather/Hazards Highlights... There remains a strong signal that system energies and deep moisture linked to Tropical Storm 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 Thursday before ejecting inland as a weakening system across the interior on Friday. Widespread and highly unsettled/stormy weather is expected heading 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 breaks down next weekend as height falls press robustly eastward and flatten the upper flow pattern. The more significant impacts from this pattern, including heavy rainfall, coastal flooding, high/gusty winds and dangerous maritime conditions, have begun moving into the short-range. However, rainfall will linger into the weekend particularly into the interior as the system lifts northward, especially along terrain-favored areas including the Alaska Range and Brooks Range. With a pattern transition away from high amplitude, systems are excepted to be embedded within the more progressive upstream flows both across the Arctic and underneath out across the Bering Sea, with some uncertainty regarding their eventual evolution. However, there is a growing signal that energies from one or both of these flows will continue the unsettled pattern with enhanced precipitation and winds through the interior later this weekend. Recent concerns over high stream flows may be somewhat abated by lowering snow levels and reduced runoff. Rapidly increasing forecast spread lowers confidence in the pattern by early next week. Putnam/Schichtel Hazards: - Heavy rain across portions of southern mainland Alaska, Thu-Fri, Aug 31-Sep 1. - Heavy rain across portions of southern interior mainland Alaska, Fri-Mon, Sep 1-Sep 4. - High winds across portions of the Alaska Peninsula and the Aleutians, Thu, Aug 31 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