Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 617 PM EST Sat Dec 28 2019 Valid 12Z Wed Jan 01 2020 - 12Z Sun Jan 05 2020 The best consensus of guidance suggests that elongated short-range energy aloft will consolidate into a closed low that should track from near the northern Alaska Peninsula eastward into the Gulf of Alaska mid-late week, while an east-west ridge extends from northwest Canada into the northern half of the mainland. This northern ridge should then weaken and lingering energy will likely keep upper troughing over the mainland to the end of the forecast period next Sun. The upper level evolution will support a strong surface low that should be near Kodiak Island and the Kenai Peninsula early in the period and then settle into a position a bit to the east for the rest of the period as the initial closed low drifts eastward and the overall upper trough weakens gradually. As was the case yesterday, a blend of the 12Z GFS/ECMWF best reflects the recent southwestward trends for the deep surface low early in the period, at least Wed into early Thu. The UKMET/CMC continue to be farther east as of day 4 Wed. Interestingly the models and means as a whole cluster better by day 5 Thu. After early Thu recent GFS runs stray southward of the means and most other models with the surface low, favoring a reduction of GFS input at that point. Important specifics aloft still exhibit some spread and variability so confidence in an exact solution remains only moderate. Early in the period this storm will produce strong winds/high waves over some areas and promote a precipitation focus over the southeastern coast/Panhandle. These locations will see a drier trend later in the period. Farther west there is good agreement in principle that a very slowly moving trough (anchored by an upper low near Kamchatka) will prevail over the western Pacific with some degree of upper ridging over or near eastern Siberia. Progressive flow around the western Pacific trough should continue across the North Pacific/Aleutians and far southern Bering Sea, though with a tendency for a weak mean ridge aligned over the central Aleutians/Bering. This flow will carry multiple fronts/waves for which individual model runs have been quite diverse and inconsistent. From late week through the weekend the latitude of the axis of lowest pressures has been up for debate as well, corresponding at least part to strength and shape of the eastern Siberia ridge aloft. The one wave that has had a fairly persistent signal for existence reaches the western Aleutians by 12Z Thu. Thus far prefer to lean to the weaker side of guidance, in the realm of the past couple ECMWF runs (weaker trend than prior runs) or recent GEFS means, due to the dynamics passing through a mean ridge. Trailing waves/fronts are likely but with low confidence for details. By late in the period the average of ensemble means would suggest that the latitude of lowest surface pressures over the Aleutians/southern Bering Sea is most likely south of the 12Z CMC/00Z ECMWF but perhaps somewhat north of the 12Z ECMWF that has the strongest eastern Siberia ridge aloft. Exactly how North Pacific wave(s) may interact with mainland/Gulf troughing aloft late in the period is another complexity in the forecast. The aforementioned 12Z GFS/ECMWF blend preferred for the deep storm early in the period transitioned to nearly half ensemble mean weight and lower GFS weight versus the ECMWF by day 6 Fri. Ensemble input continued to increase with time, reaching 100 percent weight by day 8, as the means have remained agreeable for the large scale pattern in the latter half of the extended forecast time frame. 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