Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 648 PM EST Tue Nov 28 2023 Valid 12Z Sat Dec 02 2023 - 12Z Wed Dec 06 2023 ...Overview... Over the course of the weekend through the middle of next week, most guidance suggests that a mean upper low over the Bering Sea along with surrounding flow (with a possible contribution from retrograding Arctic energy too) will transition to an upper trough aligned over the western mainland and extending south into the Pacific. Detail predictability at the surface and aloft still looks rather low during the weekend from the Aleutians/North Pacific into the northeastern Pacific, given the initially strong jet over the mid-latitude Pacific. Subsequent interaction of Bering Sea/Pacific dynamics and resulting potentially strong storm tracking north into the Gulf of Alaska during Monday-Wednesday also has typical detail uncertainty but the pattern evolution indicates greater confidence in the general idea of low pressure gravitating toward the Gulf in some fashion. Expect a wet/snowy regime over the Panhandle and back through portions of the southern coast while most locations farther north over the mainland should be fairly dry. Temperatures will start out above/much above normal and then should trend gradually colder as upper troughing moves in. ...Guidance Evaluation and Preferences... Models and individual ensemble members do not cluster as well from south of the Aleutians eastward/northeastward during the weekend, as solutions display no coherent theme for how individual systems may spin up just north of the strong Pacific jet. By early Sunday, the past couple ECMWF runs have tracked their strongest wave south of the Panhandle with 06Z/12Z GFS runs somewhat in that neighborhood--though with those GFS runs technically deeper with a separate wave south of the western Alaska Peninsula. This trailing GFS wave could be considered a slower version of the latest ensemble means that generally show a primary low track just south of Kodiak Island and would recommend some inclusion of the 12Z UKMET/CMC in weaker form. Some reflection of the UKMET/CMC and the means would yield just a slightly slower version of yesterday's continuity. Behind this area of uncertain surface details, the past day of guidance has started to show a more pronounced clustering toward strong Pacific low development by early next week as initial Bering Sea and Pacific dynamics interact. This low would then track northward into the Gulf of Alaska as the mean trough axis aloft settles over the western mainland. Latest GFS/ECMWF/CMC runs suggest the storm could get as deep as the 950s mb for a time, though with a fair amount of spread for exactly how far south of the mainland the storm achieves its greatest strength before trending weaker. There is also typical timing and east-west spread and run-to-run variability, with a multi-model/run blend and some ensemble mean input providing a reasonable starting point to reflect recent trends. Persistence of guidance in upcoming runs would allow for a deeper depiction closer to the models in upcoming forecasts. Elsewhere, 12Z guidance overall has trended farther north with upper level energy that may retrograde from northern Canada. This is generally a nod to the ensemble means, versus some individual solutions that had previously brought lowest heights into the North Slope region. Meanwhile there are decent signals in the models toward the idea of one or two waves/frontal systems affecting parts of the Bering Sea/Aleutians from Monday onward, but with typical spread and run-to-run variability for details. A blended approach reflects the potential existence of such features in fairly conservative fashion. Based on the available guidance through the 12Z cycle (before arrival of the 12Z ECMWF mean), the first half of the period started with an even blend of the 12Z GFS/ECMWF/UKMET/CMC and ensemble means (12Z GEFS/00Z ECens). Then the forecast increased ensemble weight to 40 percent while keeping some GFS/ECMWF/CMC influence. The GFS contribution was split between 12Z/06Z runs starting on day 6 Monday and the ECMWF split between 12Z/00Z runs starting on day 7 Tuesday, to account for track variance of the strong system tracking toward the Gulf. ...Weather/Hazards Highlights... The primary precipitation focus during the period will be along the southern coast and Panhandle, with most of the interior seeing only spotty light snow if any precipitation at all. Uncertainty with surface low details from south of the Aleutians into the northeastern Pacific/Gulf of Alaska during the weekend keeps confidence lower than desired for precipitation coverage and amounts along the coast/Panhandle. Potentially deep storm tracking northward toward/into the Gulf of Alaska during the first half of the week may spread an area of enhanced precipitation over the Panhandle and back to the west along the southern coast, with westward extent depending on the exact low track and depth. There will also be the potential for strong winds at least over the open waters. Given the fairly recent development of this evolution in the guidance and best precipitable water values tending to stay southeast of the state, today's 3-7 Day Hazard chart does not depict any defined areas associated with this system. However the storm's potential effects will be monitored closely in coming days. Farther west, the Aleutians will see unsettled conditions during much of the period with mostly light rainfall and winds varying with system progression. Temperatures will start out above to well above normal across the state during the weekend, with the greatest anomalies likely over the North Slope. Expect a gradual colder trend next week as upper troughing becomes established over the western mainland. Southwestern and some interior areas may see below normal highs by next Tuesday-Wednesday while other locations should still see above normal readings. Lows will tend to stay more above normal through the middle of next week. 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