Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Thu Jul 20 2023 Valid 12Z Mon Jul 24 2023 - 12Z Fri Jul 28 2023 ...Very warm temperatures expected over the eastern interior and North Slope... ...Pattern overview with Guidance Evaluation/Preferences and Predictability Assessment... A Rex block pattern is forecast to persist over the Alaskan domain during the medium-range period as three upper lows--one centered southeast of the Gulf of Alaska, another over the Bering Sea, and still another near the North Pole--anchor an upper ridge across mainland Alaska into northwestern Canada. Model guidance remains in decent agreement with this slow-to-evolve pattern over the Alaskan domain, including a fourth upper trough forecast to move across the Kamchatka Peninsula and 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 have resulted in some spread in the model solutions in this vicinity. The GFS offers the fastest solution by rotating disturbances up across the eastern Bering Sea early next week, thereby pushing moisture into western Alaska much earlier than the rest of the guidance. In contrast, the GEFS has been slower with the incoming moisture, which is in better agreement with the slower ECMWF and EC mean solutions. The WPC forecasts are in favor of the slower ensemble mean solutions, with the bulk of the moisture arriving into western Alaska later next week. Elsewhere, the placement of the upper low over the Arctic Ocean appears to have trended southwestward closer to the Anzhu Islands off the northern coast of Siberia as lead by the ECMWF. This is in response to another notable model trend for a northward expansion of the upper ridge from northern Alaska into the Arctic Ocean. Meanwhile, model agreement remains good for the upper low southeast of the Gulf of Alaska to begin moving onshore into the Alaska Panhandle early next week, before opening up into a wave over western Canada. Therefore, 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. ...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, perhaps setting the stage for the beginning of a wetter period for the western mainland. 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 mainland Alaska, Sun-Mon, Jul 23-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