Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 643 PM EDT Wed Sep 27 2023 Valid 12Z Sun Oct 01 2023 - 12Z Thu Oct 05 2023 ...Storm track from south of the Aleutians to the northern Gulf of Alaska to focus impacts from southern to Southeast Alaska this weekend... ...Pattern Overview and Guidance Evaluation/Preferences... The focus during the first part of the period will be on a deepened surface low pressure system as it moves into the Gulf on Sunday. The associated cold front should move into western Canada early next week but low pressure will be maintained over the Gulf as additional shortwaves cross western Alaska and the Alaska Peninsula into the Gulf Monday-Wednesday. Behind this, upper ridging should build into the Bering Sea and eventually parts of Mainland Alaska as another upper low moves into the Western Aleutians by the middle of next week. Recent guidance continues to show good enough agreement on the overall upper pattern through the middle of next week, with some typical spread in the details. Details and timing of individual shortwaves rounding the Arctic upper low next week remain in question but are smoothed out by a general model blend days 4-6. There are some more noticeable timing/placement differences into the Western Bering next week with the next system. GFS is by far the strongest/deepest with this system, while the ECMWF is much weaker/quicker. There is also a lot of uncertainty surrounding an upper low meandering in the north Pacific through the period, which some of the guidance wants to lift northward towards the central/eastern Aleutians next Thursday. The GFS and CMC are most pronounced with this, which seems related to a much stronger/blockier upper ridge downstream over Alaska. The WPC blend for today used a general model blend of the latest 12z guidance through day 6. Transitioned towards a majority ensemble mean blend by day 8, with some smaller contributions still from the deterministic guidance just for some added detail. This maintained good continuity with yesterdays WPC forecast as well. ...Pattern Overview and Weather/Hazards Highlights... The deepening Pacific Storm will track south of the Aleutians into the northern Gulf this weekend, offering possibly significant maritime and coastal high wind/wave threats along with heavy rains. The main impacts will spread from the Alaska Peninsula/Kodiak Island through Southcentral to Southeast Alaska, with models continuing to show a possible atmospheric river type set-up. A cold frontal surge in the wake of the main system passage, as aided by northern stream flow, may also then set the stage for enhanced gap wind flow through Southcentral Alaska and vicinity. The rest of Mainland Alaska should stay relatively quiet/showery but there is potential for possibly more enhanced precip across Western Alaska but highly dependent on a still uncertain upper flow pattern. Precipitation chances will increase with time and from west to east across the Aleutians as the next system approaches from the West, and possibly even more so if models trend towards a separate system lifting northward from the north Pacific. Temperatures across Alaska should trend cooler with time, with the greatest anomalies across Southern/Southeast Alaska and the Brooks Range. The North Slope, especially closer to the coast, should stay near or above normal during the period, with far western Alaska trending warmer as well by the middle of next week. Santorelli Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Mon, Sep 30-Oct 2. 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