Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 630 PM EST Sat Dec 12 2020 Valid 12Z Wed Dec 16 2020 - 12Z Sun Dec 20 2020 ...Guidance and Uncertainty Assessment... Today's guidance is reasonably consistent with the forecast of the large scale pattern while also maintaining the idea that embedded features will be more difficult to resolve several days ahead of time. Continue to expect a broad weakness aloft across the Bering Sea and Mainland Alaska into northern Canada, south of a strong ridge over the Arctic. The best consensus still suggests that multiple pieces of energy from various directions (especially drifting eastward into the Bering and retrograding across the mainland) may converge into an upper low over the northern Bering by late week/weekend. Meanwhile shortwaves embedded within progressive North Pacific flow will provide some complexity, in light of typically low predictability of details in fast flow and uncertain interaction with the broad weakness immediately to the north. From Wed into early Fri, recent model runs have started to show a signal for what could be fairly vigorous surface development reaching within an area encompassing the eastern Aleutians/Alaska Peninsula, southeastern Bering Sea, and Gulf of Alaska. Overall a blend of the 12Z/00Z ECMWF and 12Z/06Z GFS runs provided the best intermediate solution for this system while balancing the desire for good detail versus accounting for continued uncertainty. By early Thu the UKMET was a bit faster and the CMC slower. After early Fri confidence in specifics further decreases. The best model/ensemble clustering shows separate surface lows persisting through the weekend over the western coast of the mainland/eastern Bering and over the Gulf of Alaska. Trending the aforementioned operational model blend to a model-mean blend (12Z GEFS/00Z ECens for the means) best represented the most common themes of guidance at the surface and aloft for next weekend. ...Weather/Hazards Highlights... During Wed-Fri a potential storm system may bring an episode of enhanced precipitation and brisk/strong winds to some areas from the eastern Aleutians into the Alaska Peninsula/southwest mainland and continuing along the southern coast into the Panhandle. Uncertainty over the storm's evolution/track keeps confidence in the lower half of the spectrum for specifics of coverage and intensity of precipitation. Low pressure may linger over the Gulf of Alaska through the weekend but moisture in its vicinity will likely decrease. Expect dry conditions or light/scattered precipitation over most other parts of the mainland while the Aleutians/Peninsula likely see periods of mostly light activity behind the aforementioned system. Winds over the mainland should be strongest mid-late week, corresponding to the tightest gradient between high pressure over the Arctic and low pressure reaching into the southeastern Bering/Gulf. Below normal temperatures should prevail over a majority of the mainland, especially for highs. Parts of the south may be modestly above normal and other areas may see pockets of above normal readings for lows. Most of the Panhandle should see below normal temperatures for highs but again with a more mixed picture for low temperature anomalies. 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