Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 654 PM EDT Sat Oct 31 2020 Valid 12Z Wed Nov 04 2020 - 12Z Sun Nov 08 2020 ...Guidance/Predictability Assessment... Models and ensemble means are agreeable and consistent in showing a Bering Sea upper ridge building into the mainland after midweek, leading to southeastward displacement of the mean trough initially over the southern mainland/Northeast Pacific--as well as associated low pressure. Also there has been good stability in the mid-late week forecast of a strong Kamchatka/Siberia storm pushing a broad area of precipitation and strong winds across parts of the Aleutians/most of the Bering Sea and into portions of the mainland. Broad ridging aloft will likely persist over the mainland through the weekend. Guidance expects upstream Bering Sea and/or Pacific shortwave energy to interact with a mid-lower latitude Pacific system but with very uncertain details by next weekend. Thus confidence is low regarding possible weather effects that may reach the western/southwestern mainland at that time. With typical spread for specifics, there is a general model/mean consensus toward another strong Kamchatka storm that would bring another episode of enhanced winds into the western Aleutians/Bering Sea by next Sun. The 12Z operational model blend that is favored from day 4 Wed into early day 6 Fri uses somewhat less than the usual weight of the GFS Wed-Thu, as that model is somewhat on the fast/suppressed side of the model envelope with the system forecast to track toward the southern Panhandle or Haida Gwaii. Guidance has had difficulty with individual low centers over this area in recent days but at least models are maintaining the definition of this system to follow up pronounced trends in that direction 24 hours ago. There are also lingering question marks over specifics of residual low pressure over the Gulf and the system supported by energy dropping south/southeast from the Alaska Peninsula early Wed onward. The front extending from the strong Kamchatka/Siberia storm has trended a little faster over the past couple days but otherwise there has been reasonable agreement and continuity with the moisture and strong wind field crossing the Aleutians/Bering Sea and into the mainland. By next weekend models/means diverge considerably over the details of Bering Sea through Pacific shortwave energy and interaction with a system that briefly cuts off over the mid/lower-latitude Pacific. ECMWF/ECMWF runs show more stream separation and thus keep low pressure to the south of the eastern Aleutians/Alaska Peninsula. Other solutions show greater degrees of low pressure extending into the eastern Bering, whether incorporating the Pacific system or in response to Bering Sea dynamics. Over the past couple days even the GEFS/ECMWF means have shown some variability with this evolution, keeping confidence in specifics below average. Forecast uncertainty favors a rapidly increased ensemble mean component after Fri, leading to 60 percent total weight by Sun. The blend yields a more phased solution than the ECMWF but a more conservative evolution than the 12Z GFS/CMC. ...Weather/Threats Highlights... Wed-Thu system tracking toward the southern Panhandle or Haida Gwaii may bring meaningful precipitation to the southern Panhandle. There is still considerable spread over precise track and timing, keeping confidence in precip coverage and amounts lower than desired. A drier trend will follow. The broad area of precipitation and strong winds spreading across the western Aleutians and Bering Sea on Wed will rapidly extend eastward and reach the western mainland by Thu-Fri. Highest precip totals will be over favored terrain in the west/northwest while lesser totals are possible over other portions of the mainland. The forecast becomes very uncertain from the eastern Aleutians into the southwestern mainland next weekend. Potential outcomes range from most precipitation staying along and south of the Alaska Peninsula to a significant amount of moisture reaching as far north as the southwestern mainland. Current deterministic preference would bring some moisture northward, between the two extremes. By late next weekend the wind/moisture ahead of an upstream storm may reach the western Aleutians/Bering Sea. The approach/arrival and then persistence of upper ridging will steadily reduce the coverage of below to much below normal temperatures initially over the southern and eastern mainland while bringing above normal readings across more of the mainland. By next weekend only the Panhandle should continue to see well below normal readings while modest negative anomalies may persist over parts of the extreme southeast. The North Slope will see above/much above normal temperatures through the period and a tendency toward more extreme anomalies with time. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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