Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 742 PM EDT Sat Jun 24 2023 Valid 12Z Wed Jun 28 2023 - 12Z Sun Jul 02 2023 ...Overview... The most stable part of the forecast is the likely mean ridge expected to extend from western Canada into the northern mainland and Arctic, though the western part of the ridge could get eroded for a time by the leading edge an Arctic trough just to the west. Meanwhile guidance has varied considerably for the details of an initial upper low near the Alaska Peninsula as of Wednesday along with a couple surges of digging upstream energy. At least the general theme of a fairly cloudy and wet regime over much of the state persists, even if the day-to-day details continue to be in doubt. ...Guidance Evaluation/Preferences... Over the past day the models and ensemble means have made a dramatic adjustment for the ultimate evolution/path of the upper low forecast to be near the Alaska Peninsula as of Wednesday. The prior consensus had anticipated eastward progression across the Gulf of Alaska while upstream energy would carve out a new Bering Sea low, but most of the latest model/mean runs now keep the initial low nearly in place while incorporating upstream energy into the overall circulation. Another surge of digging Siberia/Bering Sea energy may come into play next weekend, leading to additional uncertainty over how the overall circulation will evolve at that time. There is also a lot of variability for how associated surface pressure may evolve, with potential input from North Pacific/Aleutians waviness. The end result of guidance changes since yesterday is the potential for a more persistent period of rainfall focus along the southern coast, while farther north a less easterly component of the upper flow may favor a little less moisture reaching the northern-central mainland. Early in the period an average of the 12Z models agrees fairly well with the new ensemble means, favoring a 12Z model composite as the basis for the forecast during days 4-5 Wednesday-Thursday. Initially this blend yields just a northward adjustment for the initial upper low/surface system versus yesterday but then reflects latest trends more clearly. After Thursday the models stray in varying ways so increasing weight of the more compatible ensemble means would provide a more coherent and intermediate forecast. Ultimately the forecast blend reaches 60-70 percent total input from the 12Z GEFS-CMCens/00Z ECens means by days 7-8 Saturday-Sunday. The 12Z/18Z GFS runs pull the overall upper low south of most other guidance by late week and the weekend while the CMC does so later in the forecast. Recent CMC runs have also been showing a greater eastward extent of Arctic troughing at times after Thursday. Next weekend there is a general theme in the guidance for additional energy to dig over Siberia and the Bering Sea, but as is typical for 7-8 days out, with considerable spread for how this energy will evolve. ...Weather/Hazards Highlights... Latest guidance trends for the general pattern favor greater persistence of rainfall along the southern coast and Panhandle, though there is a lingering signal from before for slightly greater emphasis from near the Kenai Peninsula into the Panhandle at the start of the period on Wednesday. Otherwise there is still enough detail uncertainty for upper level/surface features to keep confidence low for resolving the precise distribution/intensity of precipitation along the southern coast on individual days. The rest of the mainland should be fairly cloudy with decent coverage of precipitation though with a few drier pockets possible. Guidance still signals the potential for enhanced moisture to lift northward into the northern mainland by the late week/weekend time frame but latest guidance adjustments in the orientation of flow aloft (somewhat less easterly/more southerly component) have led to this moisture being not as anomalous or more transient. Some scattered moderate to heavy rain may still be possible at times though. In general the forecast for temperatures is more consistent than for some details of the upper pattern. The widespread cloud cover should keep the diurnal range more narrow than usual, with lows tending to be near or above normal and highs generally below normal. Expect the most extreme anomalies to be with cool highs along and just north of the southern coast, including an increasing proportion of the Panhandle with time. Some pockets of above normal highs will be possible over eastern parts of the Interior around midweek and mid-late week over portions of the northwestern coast and Seward Peninsula. 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