Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Mon Jul 24 2023 Valid 12Z Fri Jul 28 2023 - 12Z Tue Aug 01 2023 ...Wet weather 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 currently across the Alaskan domain is forecast to remain intact as the medium-range forecast period begins on Friday. Meanwhile, the low/upper trough over the Bering Sea is forecast to edge closer toward western Alaska with continued good agreement among the ensemble means. The deterministic solutions are also in relatively good agreement with each other as they unanimously push the entire system eastward into western and southwestern Alaska by Saturday. Thereafter, a piece of the Bering Sea trough is forecast to break off into a low over the Gulf of Alaska early next week. On the other hand, recent models, particularly the ECMWF, have shown a trend of keeping a piece of the upper high in place near northeastern Alaska into early next week instead of retreating it farther east as in previous forecast cycles. This will have implications on extending the very warm conditions over eastern and northern Alaska into early next week. If a more compact upper low develops over the Gulf of Alaska, the upper high to the northeast could be positively reinforced, thereby locking in the omega block pattern further out in time than previously predicted. The lower QPFs along the Brooks Range in this forecast cycle may be attributed to the stronger upper high in the vicinity. Also by early next week, ensemble means are in relatively good agreement for the next trough to move into the Bering Sea. The eastward progress of the system appears to be slowed by the omega block over mainland Alaska as the leading edge of this next system will tend to slide under the block toward the low over the Gulf of Alaska. If the omega block re-establishes itself and strengthens over mainland Alaska, the eastward progress of this next wave would be further retarded. 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, 20% from the 06Z/12Z GFS, 20% from the 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 upper ridge midweek, increasingly deep and strong southerly flow is forecast to set up across western Alaska through mid to late week. 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. Moisture ahead of the Bering Sea low is expected to reach into western Alaska late this week, setting the stage for a period of wet weather for much of mainland Alaska from Thursday onward, with terrain-enhanced rainfall possible into the weekend. The rain chances may be lessened over northern to eastern Alaska if the upper high gets stronger than expected. Meanwhile, the Alaska Panhandle will likely be rather quiet through the medium-range period as western and southwestern Alaska will have a chance to dry out on Sunday. Farther west, moisture from the next system is forecast to move across the Aleutians from west to east by next Monday given decent model agreement. While rain is forecast to overspread much of the Aleutians early next week, the eastern edge of the rain shield may not reach the western shoreline of mainland Alaska especially if the omega block re-establishes itself and strengthens more than expected over mainland Alaska. 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 - Hazardous heat across portions of mainland Alaska, Thu-Mon, Jul 27-Jul 31. 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