Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 657 PM EST Fri Nov 10 2023 Valid 12Z Tue Nov 14 2023 - 12Z Sat Nov 18 2023 ...Bering Sea into Mainland Alaska storm likely to bring high winds and heavy precipitation to southern parts of the state by midweek... ...Overview... Latest guidance continues to advertise the transition of the initial energetic and progressive pattern to a fairly deep large scale upper trough becoming aligned over approximately the western half of the mainland. This change will lead to a significant colder trend with time, after passage of a strong storm system on the leading side of the mean trough--likely tracking from the Bering Sea into the southwestern mainland--brings a variety of hazardous weather conditions to areas from the Bering Sea/Aleutians into the southern mainland. Leading moisture should also focus at least a brief period of heavy precipitation over areas east of the Kenai Peninsula into the Panhandle. ...Guidance Evaluation and Preferences... Latest model/ensemble guidance still shows a fair amount of spread for specifics of the Bering Sea into mainland storm, which should be fairly well defined from the Bering Sea into the southwestern mainland and then quickly broaden into multiple centers as the overall system continues into the eastern mainland and beyond while also supporting a Gulf wave. At least in the past day the relative clustering has improved for the system's track as it reaches the mainland, close to the recent ensemble mean average a little south of Norton Sound. Interestingly the 12Z CMC and CMCens are the two extremes, the former leaning south and the latter somewhat north. Timing spread remains more problematic with the 12Z UKMET being the fast extreme and the new 18Z GFS trending slow (due to weakening the leading shortwave and incorporating upstream energy to form the dynamic support for the system). Preference today sided with the 12Z ECMWF/12Z GFS, with the ECMWF leaning a bit closer to continuity but both providing a reasonable starting point given the overall spread and GEFS/ECens support. Current model runs and the ensemble means are showing a stronger hint today that some combination of a North Pacific wave reaching near or south of the Aleutians as of early Thursday and/or leading frontal wave will develop over the northeastern Pacific and reach into/near the Gulf of Alaska by late Friday-Saturday, replacing the wave forecast to evolve near the southern coast late Wednesday-Thursday. A blend of model and ensemble mean ideas would be a good starting point to nudge the forecast toward such an evolution, with specifics still having fairly low predictability that far out in time. Recent trends have generally been toward a deeper upper trough that settles over the state after passage of the Bering Sea storm, so today's forecast reflects that trend. This adjustment has helped to push along the leading moisture surge a bit faster over the eastern part of the forecast area. On the back side of the upper trough, there is some degree of clustering for a shortwave that would support a wave/frontal system nearing the western mainland toward Friday or Saturday. Based on guidance considerations for the Bering Sea into mainland storm and then decreasing confidence in details of northeastern Pacific/Gulf of Alaska evolution later in the period, the forecast started with a 60/40 weight of the 12Z ECMWF/GFS for the first half of the period and then quickly transitioned to 50-60 percent total input of the 12Z GEFS/00Z ECens means by days 7-8 Friday-Saturday. Also note that this blend represents consensus for a leading wave offshore the Panhandle as of early Tuesday, with being the 12Z UKMET being a southern misfit versus the consistent majority. ...Weather/Hazards Highlights... A leading wave offshore the Panhandle as of early Tuesday should bring light to moderate precipitation to that part of the state as it tracks southeastward. Then the dominant focus of the forecast will be on the strong storm system expected to track from the Bering Sea into the southwestern mainland into midweek and broaden as the system continues onward across the state. With confidence improving compared to yesterday for track of this system but not yet for timing, there continues to be potential for areas of high winds over the Bering Sea/Aleutians and into the southwestern mainland, bringing rough seas and coastal erosion/flooding issues. Elevated tides will be a concern as well. Anomalously high moisture being carried by leading flow may support heavy snow over parts of the south-central mainland, with rain mostly confined to areas near the southwestern coast. Then this leading moisture surge will likely promote a brief episode of heavy precipitation along the southern coast east of the Kenai Peninsula and into the Panhandle during the mid-late week time frame. Within this area, expect rain over coastal/southern Panhandle locations and snow elsewhere. Some precipitation should continue along the southeastern coast/Panhandle into late week and the weekend but be somewhat lighter. Meanwhile the northern two-thirds of the mainland should see areas of snow but with less extreme totals than what will be possible to the south. A majority of the mainland should see a drier trend by Friday-Saturday as weak surface high pressure builds in. Aside from some below normal readings over parts of Southcentral and the Panhandle especially on Tuesday, much of the state will see above to well above normal temperatures through at least midweek. Then the approach and arrival of deep upper troughing will lead to a colder trend late week into the weekend, with below normal temperatures first reaching southwestern areas and then expanding eastward. This trend should generally leave only the North Slope and Panhandle above normal by next Saturday. Once the colder air settles in, the best potential for greatest negative anomalies will be over western-central parts of the southern half of the mainland. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Wed, Nov 15. - Heavy snow across portions of mainland Alaska, Wed, Nov 15. - High winds across portions of mainland Alaska and the Aleutians, Wed, Nov 15. - High winds across portions of mainland Alaska, Mon, Nov 13. 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