Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Sat Jul 22 2023 Valid 12Z Wed Jul 26 2023 - 12Z Sun Jul 30 2023 ...Very warm temperatures expected over the eastern interior early next week... ...A wet pattern appears to work its way across mainland Alaska late next week into the weekend... ...Pattern overview with Guidance Evaluation/Preferences and Predictability Assessment... The medium-range forecast period will likely commence with an ongoing highly blocky synoptic pattern across the Alaskan domain as an upper high centered near/over northeastern mainland Alaska will be sandwiched between three upper lows--one centered southeast of the Gulf of Alaska, another over the Bering Sea, and still another closer to the North Pole. The ensemble means remain in good agreement with this slow-to-evolve pattern over the Alaskan domain, including a fourth upper low/trough coming off the Kamchatka Peninsula will likely be in the process of merging with the Bering Sea low early next week. The complex interaction of the two upper lows over the Bering Sea continues to introduce higher uncertainty in the model solutions over this vicinity. The 12Z GFS appears to be at odds with the rest of the model guidance by over-amplifying a piece of vorticity on the southern portion of the Bering Sea cyclone merger and the swinging it toward the Alaska Peninsula late next week. A similar over-amplification of an upstream shortwave as it gets ready to move off northeastern Siberia on Day 8. In contrast, the 06Z GFS is much more compatible with the rest of the guidance regarding both of these systems. Meanwhile, model guidance, especially the EC/EC mean, continues to indicate a tendency for the upper high over northeastern Alaska to expand northward into the Arctic Ocean as next week progresses. This will force a piece of the upper low near the North Pole into northeastern Siberia and toward the Bering Sea low, allowing the entire synoptic pattern to become more amplified across the Alaskan domain with a stronger and deeper southerly flow through western Alaska by late week. By next weekend, there appears to be rather good model agreement on pushing the entire upper trough onshore into mainland Alaska. The WPC Alaskan forecast package was based on a composite blend of the deterministic and ensemble guidance starting with 40% from the 12Z ECMWF/00Z EC mean, 40% from the 06Z GFS/GEFS mean, and 20% from the 12Z CMC/CMC mean, with increased proportion from the ensemble means toward the end of the forecast period, along with subjective manual adjustments. This solution maintains good continuity from yesterday's forecast package with a slightly faster progression of the moisture into mainland Alaska late next week into the weekend. ...Weather/Hazards Highlights... As the cyclone merger rotates over the Bering Sea while the upper ridge expands northward into the Arctic Ocean early next week, increasingly deep and strong southerly flow is forecast to set up across western Alaska as next week progresses. Meanwhile, subsidence associated with the upper ridge, which could prove to maintain itself longer than predicted, is expected to provide very warm afternoons from the North Slope to the interior section of eastern Alaska (where highs could reach well into the 80s on Tuesday and perhaps on Wednesday). Meanwhile, rainfall associated with antecedent moisture over interior western and northern Alaska early next week should generally be light but can be enhanced by wind-facing terrain and localized convection/thunderstorms due to daytime heating on the southern periphery of the upper high where the air will be more unstable. However, as the Bering Sea begins to swing toward the west coast of Alaska by midweek, the upper blocking high well inland is forecast to retreat, allowing deeper moisture ahead of the Bering Sea low to reach into western Alaska late next week. This will likely set the stage for wet weather to spread from west to east into much of mainland Alaska, with terrain-enhanced heavy rain possible by next weekend. Meanwhile, the Alaska Panhandle will likely be rather quiet once moisture from an upper trough exits into western Canada on Tuesday. Kong 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