Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 753 PM EDT Mon Oct 04 2021 Valid 12Z Fri Oct 08 2021 - 12Z Tue Oct 12 2021 ...Strong Storm to Reach the Southwest Mainland Late This Week... ...Pattern Overview and Guidance/Predictability Assessment... Most of today's models and ensembles maintain the theme of progressive and energetic flow that will carry a series of storm systems across the Bering Sea then into the southwestern mainland and Gulf of Alaska. Areas of enhanced precipitation and/or strong winds will likely accompany each system from the Aleutians/Bering Sea into parts of the southern mainland and Panhandle. The leading storm expected to reach the southwestern mainland late this week should be the strongest of the forecast period. The next one may reach the eastern Bering Sea by Monday. Expect the associated dynamics from these systems to support Gulf of Alaska waves during the weekend and again around Tuesday. A third system could emerge over the Bering Sea by next Tuesday. Models show a gradual convergence for details of the Friday-Saturday storm. The 12Z ECMWF/UKMET/CMC have generally nudged slightly weaker/southward from yesterday in a partial nod to recent GFS runs. Meanwhile by early Saturday the 12Z/18Z GFS runs have trended toward maintaining the parent low for a longer time over the southwestern mainland as the other models have been showing. Continuity looks good for low pressure along the southeastern coast during the weekend. Into early Sunday the model/ensemble mean consensus has held up fairly well over the past couple days for the system tracking into the Bering Sea during the weekend. However after that time guidance begins to go astray for various important details. GFS runs have been more eager than other guidance to lift the initial low well north into the eastern tip of Siberia or to weaken it while increasing emphasis on North Pacific low pressure. The ECMWF/CMC along with the ensemble means generally favor having the main system continue into the eastern Bering Monday and then possibly the southwestern mainland followed by Gulf development, in a manner similar to the previous system. The end result by Tuesday would be a Gulf system farther north than in the GFS. There are still significant differences among the non-GFS majority, as the CMC leans a bit to the slower side (along with some low-confidence embedded details) but the 12Z ECMWF adjusted noticeably slower than the 00Z run. The new 12Z ECMWF mean trended a bit slower by the end of the period as well. Thus far guidance has displayed better than average agreement in principle relative to typical late-period forecasts for the next system currently expected to emerge into the western Bering Sea by next Tuesday. A blend of 12Z operational models represented latest consensus well into early Sunday. After that time the starting point transitioned toward a combination of half models (12Z/00Z ECMWF and 06Z GFS) and half means (12Z GEFS/00Z ECens) by day 8 Tuesday. The 06Z GFS was somewhat less disagreeable relative to the preferred majority versus the 12Z run after Sunday, while the model-mean approach helped to resolve the gradually widening timing/detail differences late in the period. ...Weather/Hazard Highlights... Additional fine-tuning will be likely in the coming days but the general forecast is consistent for a vigorous storm to reach the eastern Bering Sea and southwestern mainland late this week. This storm along with subsequent Gulf wave should produce a period of strong winds across the Aleutians/Bering Sea and Alaska Peninsula into the Northeast Pacific, with some brisk winds possibly extending into the southern mainland. Expect the broad moisture shield to bring highest rain/snow totals to areas along the southern coast and Panhandle, aided by the Gulf wave. Meaningful amounts are also likely over the southwestern mainland/Peninsula. The next system in the series may spread precipitation and enhanced winds across the Aleutians and Bering Sea during the weekend. The majority scenario would have some moderate to locally heavy precipitation extending into the southwestern mainland and then the southern coast/Panhandle around Monday-Tuesday, while a lower-probability suppressed evolution would be considerably drier. Yet another system with precipitation and brisk winds may reach the western Bering Sea by next Tuesday. The southern mainland and Panhandle should see below normal high temperatures through the period. The northern two-thirds of the mainland will see mostly above normal highs on Friday followed by a modest cooling trend to yield a mix of above/below normal readings from the weekend into early next week. Most of the state will see above normal low temperatures with warmest anomalies over northern areas. The main exception will be the southern Panhandle, with below normal lows possibly extending through the rest of the Panhandle into parts of Southcentral by next Monday. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Fri, Oct 8. - High winds across portions of mainland Alaska and the Aleutians, Thu-Sat, Oct 7-Oct 9. 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