Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Fri Jul 21 2023 Valid 12Z Tue Jul 25 2023 - 12Z Sat Jul 29 2023 ...Record/near record high temperatures expected over the eastern interior early next week... ...A wet pattern appears to overspread much of western Alaska late next week... ...Pattern overview with Guidance Evaluation/Preferences and Predictability Assessment... An ongoing highly blocky synoptic pattern is forecast to begin the medium-range forecast period across the Alaskan domain as an upper high centered near/over northeastern mainland Alaska is 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. Model guidance remains in decent agreement with this slow-to-evolve pattern over the Alaskan domain, including a fourth upper low/trough coming off the Kamchatka Peninsula begins to merge with the Bering Sea low early next week. The mutual interaction of the two upper lows over the Bering Sea along with shortwaves arriving from the North Pacific jet continues to introduce higher uncertainty in the model solutions over this vicinity. Since yesterday, a flip-flop has occurred in the timing of the arrival of moisture into southwestern Alaska late next week--the most recent runs from the ECMWF depict a faster arrival of the moisture relative to the GFS, which was the opposite of what was depicted from yesterday's runs. A blend of the ensemble means together with a lesser proportion from the deterministic guidance was used to handle the uncertainty across western Alaska late next week. Meanwhile, models, especially the EC/EC mean, continue to indicated 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 toward northeastern Siberia and 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 is decent model agreement to push the entire upper trough onshore. 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 12Z 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 but with decreasing proportion from the GFS/GEFS, along with subjective manual adjustments. This solution maintains good continuity from yesterday's forecast package with a slightly faster progression of the Bering Sea cyclone toward western Alaska late next week. ...Weather/Hazards Highlights... As the Rex block sets up over the Alaskan domain, persistent southeasterly flow around the upper low centered southeast of the Gulf of Alaska is forecast to transport antecedent moisture from southeastern Alaska over the weekend into southern Alaska early next week, and eventually into western and northern Alaska by midweek next week. Rainfall amounts should generally be light but can be enhanced by wind-facing terrain and localized convection/thunderstorms due to daytime heating over portions of southeastern/interior southern Alaska early next week. In the meantime, subsidence associated with the upper ridge farther north will provide very warm afternoons from the North Slope (where highs could reach 80 degrees) to the interior section of eastern Alaska (where highs could reach 90 degrees). Across eastern Aleutians, the moist south to southeasterly flow ahead of the Bering Sea low is forecast to keep a good chance of rain for the region late this weekend. By late next week, deeper moisture ahead of the Bering Sea low is forecast to reach western Alaska, which appears to set the stage for a wetter period to overspread the western mainland, with terrain-enhanced heavy rain possible by next weekend. 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 Hazards: - Hazardous heat across portions of central and eastern Interior Alaska, Mon, Jul 24. 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