Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 747 PM EDT Wed Jul 19 2023 Valid 12Z Sun Jul 23 2023 - 12Z Thu Jul 27 2023 ...Very warm temperatures expected over the eastern interior and North Slope as rain lingers across portions of southern to southeastern Alaska... ...Pattern overview with Guidance Evaluation/Preferences and Predictability Assessment... A Rex block pattern is forecast to set up 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 the merger of two upper troughs in the short-range period into the aforementioned upper low over the Bering Sea late this week. Farther south across the northern Pacific, disagreement on the timing of the eastward-traveling shortwaves closer to the main jet stream appears to have somewhat resolved. Elsewhere, differences in the placement of the upper low over the Arctic Ocean remains appreciable, although the trend has been for the upper low to edge closer to the Anzhu Islands off the northern coast of Siberia. Another notable model trend since yesterday is the northward expansion of the upper ridge from the North Slope into the Arctic Ocean (which helps to nudge the polar low closer to Siberia). Toward the end of the medium-range period, models generally agree that an upper trough coming across the Kamchatka Peninsula early next week will merge with the Bering Sea low by midweek but with variations in timing and configuration. The merger will likely keep a Bering Sea low in place through midweek next week as the upper low southeast of the Gulf of Alaska begins to move onshore into the Alaska Panhandle and western Canada, perhaps opening up into a wave by then. Finally, the QPF axis over the eastern Aleutians in the GFS/GEFS is notably east of the rest of the guidance during the weekend. Therefore, the WPC Alaskan forecast package was based on a composite blend of the deterministic and ensemble guidance of about 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. ...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 may 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 approach 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 during the 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: No significant hazards are expected over Alaska during this forecast period. 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