Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 748 PM EDT Tue Jul 30 2024 Valid 12Z Sat Aug 03 2024 - 12Z Wed Aug 07 2024 ...Wet pattern likely to set up over the Alaska Peninsula and vicinity... ...Overview... Latest guidance agrees upon an evolution toward an amplified trough-ridge upper pattern, with a Bering Sea upper low embedded within the overall trough and the downstream ridge axis likely reaching the eastern mainland/northwestern Canada by the early to middle part of next week. A North Pacific upper low forming in the short range should also get absorbed into the flow by around Sunday-Monday. The large scale pattern and associated surface features will likely focus a multi-day period of enhanced rainfall into the Alaska Peninsula and vicinity, with some moisture possibly extending northward across the far western mainland. A majority of the state should see a pronounced warming trend after a cool start to the period on Saturday. ...Guidance Evaluation/Preferences... While guidance has been showing varying degrees of spread and shifting with various aspects of the forecast over recent days, latest solutions generally follow through on yesterday's emerging majority cluster with the CMC model being the primary misfit in a couple respects. Stronger trends of the GEFS/ECens means for both the Bering Sea into Pacific upper trough and mainland/northwestern Canada upper ridge by next week also add confidence to the definition offered by the majority of operational models. There are still the typical detail differences that have lower predictability and will take time to resolve. ECMWF/GFS-based machine learning (ML) models support the dynamical model/mean majority which depicts the Pacific upper low initially south of 50N lifting up toward the Alaska Peninsula by around Sunday-Monday and then getting absorbed into the larger scale flow. The past couple ECMWF runs try to cut the upper low's energy into the downstream ridge somewhat more, with the 12Z run also appearing a bit on the fast side. However the ECMWF is still closer to consensus than the 12Z CMC which takes the upper low across the northern Panhandle (with corresponding greater eastward extent of moisture). The 12Z CMCens mean made a notable trend toward the majority after the 00Z run had a more open North Pacific trough tracking toward the Panhandle. By the latter part of the period, the 12Z CMC's Bering Sea upper low strays to the southeast of most other guidance that shows better than average agreement for a days 7-8 forecast. The 12Z ECMWF opens the Bering Sea upper low earlier than consensus by next Wednesday. Over the course of the period, yet-to-be-resolved finer details of the overall trough/ridge will influence the coverage and magnitude of moisture reaching locations to the north of the Alaska Peninsula. At the surface, most guidance agrees that the system lifting northward over the North Pacific will combine with the approaching Bering Sea/Aleutians frontal system (as the upper trough amplifies) to produce a fairly tight pressure gradient over the Alaska Peninsula and vicinity--especially in the Sunday-Tuesday time frame. Lower predictability details for individual waves will modulate the intensity and exact location of associated rainfall focus. Today's forecast utilized a blend of the 12Z GFS/UKMET and 12Z/00Z ECMWF runs from Saturday into early Monday and 30 percent total weight of the 12Z GEFS/00Z ECens means thereafter. This reflected the non-CMC consensus for the overall pattern evolution and downplayed the less confident details of one or both ECMWF runs, while maintaining most of the operational model definition as supported by ensemble mean trends. Continuity is fairly good aside from the consensus deeper trend for the Bering Sea upper trough/low. ...Weather/Hazards Highlights... On Saturday there may be some lingering light precipitation over the central/south-central mainland in association with the last vestiges of progressive flow before ridging builds over the mainland. Then expect the developing amplified trough-ridge pattern aloft and associated surface systems (a leading one lifting northward over the Pacific and another approaching from the Bering Sea/Aleutians) to support increasing rainfall and brisk winds over and near the Alaska Peninsula from Sunday onward. Some of this moisture and breezy conditions may also extend northward across portions of the western mainland. Currently the Days 3-7 Hazards Outlook depicts areas for heavy rain over parts of the eastern Alaska Peninsula, Kodiak Island, and Kenai Peninsula during Sunday-Tuesday. Meaningful rainfall along the southern coast should continue into Wednesday. It will take additional time to resolve finer-scale upper level/surface details that will determine specifics of location and magnitude of heaviest rainfall. Most areas will start on the cool side Saturday but then see a pronounced warming trend with a strong upper ridge building over the mainland and eventually into northwestern Canada. Typically warmest locations in the eastern Interior could see highs reach into the upper 70s/low 80s by early-middle part of next week. Portions of the southern quarter of the mainland and northern Panhandle may get left out of the warming trend at least for high temperatures which may remain below normal. Rausch 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