Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Thu Apr 18 2024 Valid 12Z Mon Apr 22 2024 - 12Z Fri Apr 26 2024 ...Stormy pattern expected over the Aleutians/Bering Sea next week... ...Overview... Latest guidance seems to be converging toward the idea of mean ridging aloft persisting over most of the mainland next week, with some lingering uncertainty over whether weak shortwave energy initially over/near the southwestern mainland may pass through or around the ridge. The southern part of the elongated upper trough/low should support a wave tracking toward or a little north of Haida Gwaii with some potential influence on the southern Panhandle. Meanwhile the models and ensembles are stabilizing for the general idea of a slow moving deep-layer low over the Aleutians into southern Bering Sea. The overall system should be strongest during the first half of the week while it is over or near the western Aleutians, with gradual weakening thereafter. Areas of strong winds will accompany the system while one or more leading fronts will help to enhance precipitation from the eastern Aleutians into the Alaska Peninsula. ...Guidance Evaluation and Preferences... Both the dynamical models/ensembles and ECMWF-initialized machine learning models (MLs) continue to show difficulty with the finer details of the initial elongated upper low trough extending over and southeastward from the southwestern corner of the mainland. There is still a mix of solutions for the southwestern mainland energy, ranging among progression across the mainland/weakening in place/lifting northward around the ridge. Meanwhile there is a lot of latitude run-to-run variability and spread for the remainder of the system that tracks across the Northeast Pacific. With the MLs not really favoring one portion of the dynamical guidance spread, preference sides with an intermediate track toward or a bit north of Haida Gwaii. This would be a compromise between the southern 12Z and northern 00Z ECMWF runs. GFS runs lean on the southern side while the 12Z CMC is close to the 00Z ECMWF. Guidance is making progress in stabilizing the solution for the strong storm likely to be near the western Aleutians early next week. Latest trends are gradually increasing confidence toward the deeper half of yesterday's spread, with recent GFS/ECMWF runs generally waffling among the mid 960s - low 970s mb for greatest depth around late Monday-Tuesday and 00Z MLs ranging between the upper 960s and 970s. The UKMET is in this range while the CMC is on the weak end. The manual forecast depicts a 972-975 mb system valid 12Z Tuesday to provide a deeper trend from continuity but accounting for ongoing variability. Beyond Tuesday, the ensemble means maintain good agreement and continuity that the overall area of low pressure should wobble very slowly eastward over the Aleutians/southern Bering Sea. Operational model runs lean a bit west of the means late next week, with the CMC tending to be on the western side of the envelope at most valid times. MLs span most of the model/ensemble mean spread but with an average closer to the means. The mid-late week forecast for specifics at the surface and aloft have lower predictability, given potential input of dynamics from Siberia and/or the western Pacific. There will be various possibilities for specifics, whether maintenance of the initial surface low or redevelopment in some form, along with multiple fronts whose character may change from day to day. Consensus suggests the most likely low track will be across the southern half of the Bering Sea by mid-late week. Aside from aforementioned issues with the ultimate path of initial southwestern mainland shortwave energy, model/ensemble guidance is generally better clustered today toward the mainland mean ridge aloft holding onto a southeast-northwest orientation through late next week. This is in contrast to yesterday's GEFS/CMCens that depicted broader southwesterly upper flow by next Thursday. Based on guidance preferences and data currently available locally, today's forecast started with a blend of the 12Z/00Z ECMWF, 12Z/06Z GFS, and a small weight of the 12Z CMC for the first half of the period. This approach, with a little editing, yielded good definition for the deep Aleutians/Bering Sea storm along with the desired intermediate track for the weak system over the Northeast Pacific plus a tempered depiction of initial southwestern mainland energy aloft. The multi-model blend already accounted for positional spread over the Aleutians/Bering Sea fairly well, so days 7-8 Thursday-Friday added only 20-30 percent total weight of the means (12Z GEFS/CMCens). ...Weather/Hazards Highlights... The strong storm near the western Aleutians early next week will be the dominant system of interest through the period, though it should gradually trend weaker after Tuesday. A leading front will likely focus an axis of anomalous moisture into the eastern Aleutians by Monday, with this area highlighted for heavy precipitation in the Days 3-7 Hazards Chart. As this and possibly additional fronts push eastward, the precipitation emphasis will likely shift into the Alaska Peninsula and surrounding areas by mid-late week. Some of this activity may also be heavy but confidence in specifics is lower at that time frame. Meanwhile this storm should also produce areas of strong winds across the Aleutians and Bering Sea. Forecast wind speeds are currently below hazard criteria and thus no high wind area is depicted. Highest wind speeds should be early in the week, ahead of the leading front and just to the south of the surface low center. Surface low/front details over the Aleutians/Bering Sea become more uncertain later in the week but there is a good signal for the general persistence of low pressure and potentially additional fronts. Some lingering precipitation is possible along the southern coast early in the week due to the initial shortwave over/near the southwestern corner of the mainland. The weak system tracking across the northeastern Pacific may bring a period of mostly light precipitation to the southern Panhandle. This part of the forecast has low confidence though, as the southern side of the guidance spread would be far enough south to keep the Panhandle mostly dry. Most of the state should see above normal temperatures during next week, with the highest anomalies tending to be over the Interior. To the north of a front settling over the central mainland, cold high pressure over the Arctic may keep temperatures near to somewhat below normal across the North Slope. Locations near the Southcentral coast and northern Panhandle may also see near or slightly below normal readings. 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