Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Sun Jul 23 2023 Valid 12Z Thu Jul 27 2023 - 12Z Mon Jul 31 2023 ...A wet pattern is expected to work its way across mainland Alaska late this week and into the weekend... ...Pattern overview with Guidance Evaluation/Preferences and Predictability Assessment... An omega block that has set up across the Alaskan domain will likely show signs of breaking down as the medium-range forecast period begins on Thursday when the Bering Sea low/upper trough begins to edge closer toward western Alaska. The ensemble means remain in good agreement with this evolution as the trough axis is forecast to approach the western coastline by early on Friday. The deterministic solutions are also in relative agreement with each other through the weekend as they unanimously push the entire system eastward across mainland Alaska steadily. Thereafter, there is general agreement for a low to form over the Gulf of Alaska early next week as the omega block retreats to the east and de-amplifies. The 12Z GFS appears to be at odds with the rest of the model guidance by over-amplifying a couple pieces of vorticity early next week over the Bering Sea as well as near/south of the Gulf of Alaska. In contrast, the 06Z GFS offers a much more compatible solution in comparison with the rest of the model guidance regarding both of these systems. The caveat of the above synopsis is that the omega block may break down later than what the models are currently predicting. 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. ...Weather/Hazards Highlights... As the Bering Sea low begins to edge toward the east against the amplified ridge midweek, increasingly deep and strong southerly flow is forecast to set up across western Alaska as the week progresses. Meanwhile, subsidence associated with the amplified ridge could prove to be harder to break than predicted, allowing the very warm afternoons to linger a bit longer over the interior section of eastern Alaska. Meanwhile, rainfall associated with antecedent moisture over interior western and northern Alaska 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, the omega block should gradually retreat to the east as deeper moisture ahead of the Bering Sea low reaches into western Alaska late this week. This will likely set the stage for a wet period for much of mainland Alaska from Thursday onward, with terrain-enhanced heavy rain possible into the weekend. Meanwhile, the Alaska Panhandle will likely be rather quiet through the medium-range period as western and southwestern Alaska may have a chance to dry out on Sunday. However, moisture from the next system is forecast to move from west to east across the Aleutians by next Monday given decent model agreement. 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