Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 657 PM EST Wed Dec 30 2020 Valid 12Z Sun Jan 03 2021 - 12Z Thu Jan 07 2021 ...Overview and Guidance/Uncertainty Assessment... Latest/recent models and ensemble means hold onto fairly similar ideas for the general mean pattern but indicate ongoing uncertainties for embedded specifics. Consensus still shows some combination of retrograding northern mainland energy, a southward-moving Arctic feature to the northwest, and lingering cyclonic flow over the Bering leading to a mean low whose center wobbles back to near eastern Siberia toward mid-late next week. Meanwhile guidance is consistent in showing that the northern fringe of progressive North Pacific flow will pull the initial deep low over the southern Bering Sea eastward on a track near the Aleutians early next week. The forecast also maintains mean troughing aloft over the northeastern Pacific/Gulf of Alaska with shortwaves carried by the Pacific stream supporting one or more surface lows whose details are yet to be confidently resolved. Primary emphasis on the 12Z/00Z ECMWF runs and 12Z GFS/CMC for a majority of the period represented the most common elements of guidance across the forecast domain, with somewhat more detail than the means while also downplaying less confident aspects of any particular model run. The blend included a small minority of 12Z GEFS/00Z ECMWF mean input by day 7 Wed, and then increased the means to 40-50 percent weight by day 7 Thu as some differences become more pronounced. After recent trends for the deep low initially over the southern Bering, the past day of guidance has held into the theme of eastward progression early-mid period before likely becoming absorbed within the broader cyclonic flow over the northeastern Pacific/Gulf. After being split for timing yesterday, the new 12Z ECMWF is more on its own with a farther eastward solution versus its prior run or other 12Z models. ECMWF runs do converge for longitude (though with a latitude difference) by early day 6 Tue. Over the northeastern Pacific/Gulf, confidence is reasonably high for the initial low expected over the Gulf as the forecast starts early Sun. Then detail confidence declines rapidly for trailing lows, though there is a general signal for one to reach the area by Mon with a potentially stronger development reaching south of the Gulf by early Tue and then lifting northward. The 12Z GFS/CMC and their means cluster fairly well for this Tue-midweek low while the 12Z ECMWF's strongest development is farther east. The new 12Z ECMWF mean is showing somewhat of a trend toward the GFS cluster in principle. Most models/means signal another development to the south of this system by day 8 Thu. Late in the period guidance has been showing potential for another strong system emerging over the western Pacific, with some potential effect on the Aleutians after midweek. Thus far the ECMWF mean has been the most consistent and intermediate guidance source, while various operational model runs have differed considerably for timing and track. The complexity of multiple individual features will likely temper confidence for a while regarding the mainland/Bering/Siberia evolution aloft. CMC weight in the blend is sufficiently low to discount that model's eventual emphasis for lowest heights aloft over the southeastern Bering (not seen in other guidance), though there is agreement on more modest troughing over the southeast Bering/southwestern coast of the mainland by Wed-Thu. ...Weather/Hazards Highlights... The potential for multiple Northeast Pacific-Gulf of Alaska systems will support periods of rain/snow over the Panhandle and southern coast during the period. Specifics of each system will determine precipitation intensity--which could be locally moderate to heavy at times--as well as coverage. One or more of these systems could be sufficiently strong to generate an episode of brisk to strong winds as well. Gradually weakening southern Bering low pressure should track near the Aleutians early in the week, accompanied by some precipitation and locally enhanced winds. The pattern will likely maintain periods of precip over the Aleutians for the rest of the period. The next system emerging into the western Pacific will be worth monitoring after midweek. Current solutions range from a far enough south track to have minimal effects on the Aleutians, to more pronounced winds and precipitation. A majority of the northwestern three-fourths of the mainland should see below normal temperatures--especially for highs--to start next week as upper level energy retrogrades over the area. Readings should moderate toward midweek as heights aloft rise somewhat with north-central locations seeing the best chance for above normal readings. Meanwhile near to above normal temperatures (perhaps locally well above normal for lows) are should prevail over the Panhandle and Southcentral. 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 Hazards: - Heavy precipitation across portions of the Alaska Panhandle, Sat-Sun, Jan 2-Jan 3. 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