Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 600 PM EST Sun Dec 06 2020 Valid 12Z Thu Dec 10 2020 - 12Z Mon Dec 14 2020 ...Guidance and Uncertainty Assessment... Today's guidance continues to suggest a challenging forecast regarding specifics within a complicated and blocky evolution aloft. Models and means are similar to start the period early Thu--a deep upper low/storm near the Aleutians, an upper low over the northwest mainland, a southeast-northwest ridge between the two, and a separate ridge over the Arctic. As time proceeds solutions go astray for how these features evolve/interact. By the latter half of the period the low predictability of shortwaves embedded within progressive Pacific flow also lowers confidence in forecast specifics over the Aleutians/North Pacific. The 12Z models as a whole are trending toward some weakening/shearing of the initial northwest mainland upper low during the first half of the period but then diverge for how much upper ridging moves in from the west. Over the past day the ECMWF and GFS have essentially flipped with the 12Z ECMWF now building a ridge into the southwest half of the mainland by Fri/Sat--similar to what yesterday's 12Z GFS showed before the model's later 00/06/12Z runs more closely reflected the 00Z ECMWF. Ensemble means have maintained a better defined mainland upper low, similar to yesterday but with a moderately weaker trend. The new 12Z ECMWF mean accelerates this weakening trend. At the same time guidance has been split over what proportion of the Aleutians storm's upper low energy may progress east/northeast versus retrograde beneath northern Bering/Siberia ridging--or even just elongate and drift northward per the 12Z CMC if the ridge to the north retreats enough. Thus far the means have favored the eastern scenario while the GFS/ECMWF have been more split and inconsistent. Ahead of this system there is a decent signal for development of a triple point wave (and/or a frontal wave just to the east) to the south of the Alaska Peninsula with an eventual track close to though remaining somewhat south/east of the Peninsula. By the latter half of the period the best model/ensemble mean consensus has adjusted this general area of low pressure a little southwest from yesterday. One other part of the forecast still up for debate is the surface pattern from the Gulf of Alaska southeastward as of 12Z Thu. Operational models show the best defined low pressure to the south of the Panhandle, albeit still with wide variation. Ensemble means have been farther north over the Gulf but the 12Z CMC mean has become more elongated and the new ECMWF mean has done likewise. The initial forecast blend started with a 12Z operational blend (with a little 00Z ECMWF included as well) early in the period to reflect majority trends for the mainland upper low, a non-mean scenario over/southeast of the Gulf, and detail for the storm near the Aleutians (which has adjusted north some but not to the degree of the 12Z ECMWF). Preference to maintain some degree of continuity in the latter half of the period led to trending the blend toward the 12Z GEFS/00Z ECMWF means and 12Z GFS/00Z ECMWF by Sun-Mon. Even with the uncertainty in the upper pattern there continues to be better agreement at the surface that the mainland should stay in the gradient between strong high pressure over the Arctic and low pressure over the North Pacific. ...Weather/Hazards Highlights... During Thu-Fri ahead of the strong Aleutians storm and its leading occluded front, expect an area of strong winds along with focused precipitation over the eastern Aleutians and possibly southwestern Alaska Peninsula. The winds and precip will also extend west-northwest across the Bering Sea. Already by Fri some uncertainty develops over specifics of Pacific wave development, with precipitation details from the Peninsula through the southern coast and Panhandle continuing to have lower than desired confidence for the rest of the period. As a whole today's model/ensemble guidance has trended a little lighter with five-day totals but individual solutions vary considerably for amounts/coverage. Most precipitation farther north over the mainland should be light and scattered. Temperatures are most likely to be above normal over the northern and southeastern mainland plus parts of the Panhandle. There is better potential for below normal readings over the central/southwestern mainland and other parts of the Panhandle. 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