Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 649 PM EST Tue Nov 23 2021 Valid 12Z Sat Nov 27 2021 - 12Z Wed Dec 01 2021 ...Arctic cold over much of the Interior through the weekend followed by moderation over the east... ...Overview and Guidance Evaluation/Preferences... The models and ensemble means generally agree on the large scale evolution, with flow around an initial mainland upper low curling around to the west and then south over or near the Bering Sea, leading to some interaction with dynamics/low pressure tracking across the North Pacific. This would ultimately draw what could be a fairly strong system toward the Alaska Peninsula or Gulf of Alaska by the early to middle part of next week with a corresponding increase of precipitation over the Panhandle and possibly parts of the southern coast depending on the low track. A downstream ridge aloft should extend from western Canada into the eastern mainland early-mid week as well. The forecast pattern will initially favor well below normal temperatures over many areas, followed by some moderation over eastern areas while cold anomalies persist over the west. Although guidance is similar for the broad pattern, there are still various embedded detail uncertainties that significantly lower confidence in important specifics. These include issues with North Pacific/Aleutians upper dynamics and associated low pressure, possible development farther east, precise evolution of Bering Sea-mainland upper low/troughing, and the interaction of the two primary streams. Guidance comparisons through the 12Z cycle recommended starting with a blend of the 00Z/12Z ECMWF runs and the 12Z GFS/CMC from day 4 Saturday through early day 6 Monday, followed by reducing GFS input and incorporating some 12Z GEFS/00Z ECMWF means (up to 40 percent total) while maintaining some weight of the CMC and ECMWF runs. Main considerations were to lean away from the 12Z UKMET through its run (farther north North Pacific/Aleutians evolution during the weekend; but the 18Z GFS trended north as well) and to minimize GFS influence after early Monday when 06Z/12Z runs brought low pressure into Bristol Bay--farther northwest than most other guidance. This northwest scenario is in the minority among ensemble members but within the spread. On a percentage basis more CMC members offered that possibility relative to the GEFS/ECMWF members. The 18Z GFS trended more suppressed for a while before eventually looping its surface low into Bristol Bay from the east. The 12Z ECMWF mean provided reasonable continuity. Farther north, consensus shows upper ridging to the north of eastern Siberia while an upper low/trough prevail to the northeast of the mainland. Guidance is undecided over how much flow around the western side of this trough could reach farther southwestward. For most of the period the mainland should be in the surface gradient between high pressure over the Arctic/northern coast of the mainland and the combination of an initial Gulf system and then stronger low pressure approaching from the North Pacific. ...Weather/Hazard Highlights... Through the weekend expect many areas to see very cold temperatures, especially over the south/southwest where readings may be 20F or more below normal. The Panhandle into far southeast corner of the mainland as well as parts of the north could see slightly above normal lows though. Transition of the upper pattern toward a trough near the west coast and ridge extending from Canada into the eastern mainland should promote a moderating trend over eastern areas while the west remains well below normal. Expect light to locally moderate precipitation to persist along the Panhandle and southern coast during the weekend with low pressure initially over the Gulf of Alaska. From the weekend into the start of next week North Pacific low pressure should spread an area of precipitation as well as the potential for strong easterly winds across the Aleutians. Winds will be sensitive to exact low track, with a farther north Aleutians track (and lighter winds over the islands) lower in probability but still possible. As this low or separate leading development extend into the Northeast Pacific/Gulf of Alaska next week, rain/snow should trend heavier at least over the Panhandle. Uncertainty over specifics of flow aloft and track of low pressure leads to lower confidence in the westward extent and intensity of precipitation (ranging between heavy and fairly light) along the southern coast/Peninsula. The consensus mean pattern should favor at least some moisture reaching the southern coast. Rausch Hazards: - Heavy precipitation across portions of mainland Alaska, Mon, Nov 29. - Heavy precipitation across portions of the Alaska Panhandle, Tue, Nov 30. - Much below normal temperatures across portions of mainland Alaska, Fri-Mon, Nov 26-Nov 29. 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