Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 753 PM EDT Tue Oct 19 2021 Valid 12Z Sat Oct 23 2021 - 12Z Wed Oct 27 2021 ...Overview and Guidance/Predictability Assessment... Today's models and ensembles agree rather well for the large scale upper pattern that should feature a mean trough whose axis extends from eastern Siberia into the northeastern Pacific. To start the period, a couple low pressure centers south of the mainland will likely consolidate under an upper low which ultimately weakens. Then expect North Pacific/Aleutians flow to carry a developing system just south of the Aleutians around Sunday. Some combination of this system and/or eastern Pacific low pressure may eventually lift northward closer to the Gulf of Alaska by next Wednesday. A weaker wave could cross the Bering Sea around Tuesday. On the other side of the upper trough axis at least a couple shortwaves should progress northward from the mainland/northwestern Canada into the Arctic. The first appears more likely to produce a defined surface reflection. As of the start of the forecast 12Z Saturday, the 12Z models have come into better agreement for resolving two separate surface lows south of the mainland and their relative position--one to the south/southeast of Kodiak Island and the other to the west of the southern Panhandle. These lows should consolidate and weaken in favor of low pressure near Haida Gwaii by early Monday. Farther west the models and ensemble means have latched onto the Sunday system tracking near the Aleutians only in the past day or so and are still showing some spread and variability for depth/timing/track. Sensible weather effects over the Aleutians will be very sensitive to the exact character of the storm. Based on data through the 12Z cycle an intermediate solution provided a reasonable starting point, with the 12Z GFS straying to the slow/deep/north side and the 12Z ECMWF adjusting noticeably faster/weaker/south versus its 00Z run. The new 18Z GFS has come in closer to the preferred blend that used a composite of the 12Z models from early Saturday into early Monday. The forecast quickly transitioned to a model/ensemble mean mix for the latter half of the period, reaching 50 percent total ensemble mean (12Z GEFS/00Z ECens) weight by day 8 Wednesday. This blend yielded an upper low near the Bering Strait by next Wednesday, reflecting the general model/mean average. Details become increasingly uncertain for specifics of energy aloft that reaches the northeastern Pacific along with corresponding surface low pressure, with fairly low predictability given their scale. Meanwhile there is a moderate signal for a wave to cross the Bering Sea around Tuesday into Wednesday. Detail confidence is not great here either given progressive supporting flow aloft and ongoing spread/variability in the models. ...Weather/Hazard Highlights... Consolidating low pressure to the south of the mainland may contribute to lingering precipitation along the southern coast/Panhandle into Saturday but with amounts trending lighter. The system expected to track just south of the Aleutians around Sunday would bring a brief period of enhanced rainfall and brisk to strong winds. Effects on the Aleutians will be very sensitive to the exact strength/track/timing of the storm so confidence is currently in the lower half of the spectrum for how much rain and wind the Aleutians will see. Flow around a potential Bering Sea wave may increase precipitation somewhat over areas between the eastern Aleutians and southwestern mainland Tuesday-Wednesday. Shortwaves on the east side of the large scale mean trough aloft may produce occasional episodes of precipitation over parts of the mainland. The southern coast/Panhandle could see a modest increase of rain/snow toward midweek depending on the proximity of low pressure by then. Expect above to much above normal temperatures over the northern two-thirds or so of the mainland through the weekend and into early next week followed by some moderation as heights aloft slowly decline. The southern mainland and Alaska Peninsula plus the Panhandle will likely see a mix of above/below normal readings (more above for min temperatures) with a trend toward greater coverage of below normal anomalies by midweek. 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 Hazards: - High winds across the western portion of the Aleutians, Fri, Oct 22 and Sun-Mon, Oct 24-Oct 25. 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