Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Sun Apr 20 2025 Valid 12Z Thu 24 Apr 2025 - 12Z Mon 28 Apr 2025 ...Overview... Deep-layer Arctic ridging to the northwest of the mainland looks to have the best predictability during the period. Southeast of the ridge, confidence is lower for specifics of one or more bundles of energy that may retrograde within a general weakness aloft over the northern half of the mainland. The best guidance consensus shows the weakness eventually lifting toward the northern coast as ridging builds in from the southeast by next Sunday or Monday. Meanwhile the models and ensembles show a lot of spread and variability across much of the North Pacific into Bering Sea. This includes forecasts of initial northeastern Pacific low pressure that could maintain precipitation over the Panhandle through Thursday, as well as for upstream waves. Details will influence how much moisture reaches the Alaska Peninsula/southern coast by the weekend and early next week. ...Guidance/Predictability Assessment... Guidance clustering has shifted considerably over the past day for the system initially over the northeastern Pacific. The relatively greater proportion of guidance has adjusted toward the upper low lingering fairly far northward, most likely elongating/opening up as it reaches the Panhandle around Friday or Saturday. Prior to today this scenario had been very much in the minority (but existing in one or more ECMWF or UKMET runs). Latest GFS/GEFS runs plus one 00Z machine learning (ML) model and 00Z AIFS mean still pulled the system well south/southeast, though at times the GFS has had a history of being overdone with the southeastward progress. A large enough cluster favors the 12Z ECMWF/UKMET/ICON (but not as much the more persistent 12Z CMC) to adjust to that idea today, with new 12Z ML models and ECens mean further supporting this general solution. The forecast has become increasingly messy for details between the North Pacific and Bering Sea. As of the start of the period early Thursday there has been a trend toward parent low pressure hanging back farther west near the western Aleutians with an increasingly elongated frontal system extending well to the east. Thereafter some embedded waviness may continue eastward while a combination of the parent low and a separate wave tracking south of the Aleutians may reach a position within a general area from the southeastern Bering Sea into the Pacific by next Sunday, with some translation or tilting by Monday. There is very poor agreement among individual dynamical and ML models over the relative emphasis of low pressure centers. Mid-latitude Pacific emphasis would keep nearly all moisture south of Alaska while Aleutians or northward emphasis would support greater northward moisture transport. At the time of forecast preparation, even with all the spread among individual members and to some extent means leading into Sunday, by early that day there is better than average agreement among the means for low pressure to be somewhere near the eastern Aleutians or western Alaska Peninsula with a gradual eastward drift into Monday. This ensemble mean agreement offers a good placeholder and a moderate degree of continuity in the forecast while waiting for more significant potential changes in future runs. Although GFS runs do not compare well to the majority over the North Pacific early in the period, they do come closer to the ensemble mean themes later on. Dynamical and ML guidance agrees fairly well in principle for a system to reach south of the western half of the Aleutians by next Monday, albeit with a considerable degree of north-south spread. Latest ML models AIFS mean average near or at most a bit south of the ensemble mean track near 50N latitude as of early Monday. Dynamical models are generally somewhat farther south. Farther north, the primary extreme solution of note is the 12Z ECMWF that brings a deep upper low to a position near the northwestern coast by next Monday, lowering mainland surface pressures to a greater degree than other solutions. A more common idea is for a weaker upper low to reach eastern Siberia, allowing for more upper ridging to build over the mainland. The 12Z ECens mean reflects this evolution and the somewhat deeper 00Z ECMWF is similar for position. Today's preferences led to starting the Thursday-Friday part of the forecast with relatively more 12Z ECMWF/UKMET weight compared to some minority elements of the GFS/GEFS and 12Z CMCens/00Z ECens. The UKMET started to stray north of most other guidance over the Bering Sea by early Saturday, favoring a reduction of its input relative to the GFS. Then the forecast quickly shifted toward majority ensemble mean input and phased out the 12Z ECMWF, by next Monday reaching 80 percent total means to go along with 20 percent 12Z GFS. ...Weather/Hazards Highlights... The latest shift in the majority guidance cluster for the system initially over the northeastern Pacific would maintain some rain and higher elevation snow over the Panhandle and perhaps to a lesser extent back along the Southcentral coast through Thursday. The prior drier forecast still has a lingering low probability so continue to monitor future forecasts. Meanwhile an elongated low/frontal system should spread mostly light rain across the Aleutians on Thursday. Embedded low pressure and possibly upstream waviness tracking near the Aleutians may bring increasing precipitation to the Alaska Peninsula and Kenai Peninsula by the weekend into Monday. Some of this moisture could extend farther north into the southwestern mainland. However there is also a lesser potential for farther south low pressure emphasis that would lead to less precipitation reaching the southern mainland. Needless to say, forecast confidence is well below average at this time. The northern periphery of another Pacific system may affect the western/central Aleutians next Monday. Meanwhile, deep layer ridging over higher latitudes of the Arctic will likely support northeasterly winds of varying strength through the period. Mostly below normal temperatures should prevail over the northern half to third of the mainland due to flow around the Arctic ridge. Most southern areas will see above normal readings, though clouds and precipitation should support below average daytime highs along the Southcentral coast and Panhandle into late this week. Anomalies should generally be warmer for morning lows relative to those for daytime highs, while most of the state should see a very gradual warmer trend over the course of the period. 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