Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 648 PM EDT Tue Oct 20 2020 Valid 12Z Sat Oct 24 2020 - 12Z Wed Oct 28 2020 ...Heavy precipitation possible for the Panhandle and coastal Southcentral areas late weekend/early next week... ...Guidance/Predictability Assessment... Latest models and ensemble means are similar for the large scale pattern evolution during the days 4-8 Sat-Wed period. The upper low initially associated with the system over the Bering Sea should drift toward the western mainland and then open up early next week as shortwave energy over the Arctic and northern/eastern Russia drops southeastward, ultimately leading to a trough extending from the mainland into the southeastern Bering/eastern Aleutians by next Wed. Meanwhile energy streaming across the North Pacific/Aleutians may feed into a central/east-central Pacific trough. Guidance agrees well that the Bering Sea low should reach close to the western coast of the mainland by 12Z Mon. There has been more spread and variability for a Pacific frontal wave expected to reach the Gulf of Alaska or vicinity by late Sun/Mon. ECMWF runs before the new 12Z version had been consistent in tracking the wave to the Panhandle but the ensemble means and 12Z CMC/UKMET offer support for the new run's adjustment closer to the Kenai Peninsula. This is also close to WPC continuity. The 12Z GFS is on the deep/western side of the spread. Operational runs show a small scale wave tracking along or south of the Aleutians after the weekend. Later in the period the ensemble means support the idea of a farther north track than seen in the 12Z ECMWF. Both of the North Pacific waves are fairly small in scale and thus have lower predictability than the Bering system. Finally, progressive Arctic flow will likely bring a front into the extreme northern mainland during the weekend and then more amplified upstream energy should push another cold front southeastward into the mainland and Bering Sea/Aleutians by early-mid week. Based on guidance comparisons today's forecast started with a 12Z operational model blend for most of the weekend. Then the forecast trended to a mix of models and 12Z GEFS/00Z ECMWF means, with lower GFS weight to downplay its Mon system over/near the Gulf and less 12Z ECMWF weight late in the period to emphasize consensus for the North Pacific/Aleutians wave. For the latter the old 00Z ECMWF was closer to the majority. ...Weather/Threats Highlights.. Expect the heaviest precipitation during the period to be over the far southern mainland and Panhandle with the developing frontal wave currently expected to reach the Gulf of Alaska or vicinity around late Sun/Mon. Meanwhile the southwestern mainland/Alaska Peninsula may see continued precip through the weekend ahead of the Bering Sea system drifting toward the western coast of the mainland. There may be localized areas of focused activity. Weakening/progression of the Bering system should promote a drier trend over the west after the weekend. Lingering low pressure over the Gulf should keep some precip chances along the southern coast/Panhandle after Mon but with lesser totals than with the Sun-Mon system. Above to much above normal temperatures should prevail over a majority of the mainland during the period. The North Slope and western portions of the mainland should see the greatest positive anomalies with min temperatures tending to be more above normal than the daytime highs. The upper trough/cold front dropping into the state from the northwest should lead to a cooling trend by midweek. The Panhandle and parts of the far southeastern mainland are likely to see below normal readings, moderating gradually with time. Rausch Hazards: - Heavy precipitation across portions of the Alaska Peninsula and southwestern Alaska, Fri, Oct 23. - Heavy precipitation across portions of the Alaska Panhandle and coastal Southcentral Alaska, Sun-Mon, Oct 25-Oct 26. - High winds across portions of the Alaska Peninsula, Fri, Oct 23. 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