Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 634 PM EDT Sun Aug 16 2020 Valid 12Z Thu Aug 20 2020 - 12Z Mon Aug 24 2020 ...Guidance Evaluation and Preferences... The amplified pattern which has shown a strong signal in the extended guidance in recent days, appears likely to continue through the end of next week. An upper-level low is expected to persist across the Gulf of Alaska, with an upper ridge initially over the eastern Bering Sea early in the extended gradually shifting east across mainland Alaska through late in the week. By next weekend, guidance shows some degree of weakening/flatting of the ridge as strong westerlies and embedded shortwave energy erode the northern/western side of the ridge. While the pattern will briefly resemble a Rex Block, model/ensemble guidance suggest this will not be a feature with much in the way of persistence given the suggested relatively quick flattening of the ridge. Models/ensembles continued to relatively good consensus from recent days through the first part of the extended period (days 4-5), but showed more degradation compared to recent days later in the forecast period. A blend of the 12Z GFS/ECMWF was used during days 4-5 (Thu-Fri), with more weight on the GFS relative to the ECMWF. The solutions were not fundamentally different, but the GFS was perhaps a bit more well-centered within the overall consensus for features through that time period. The primary point of contention relates to timing of smaller-scale shortwave energy in the westerlies across the Bering Sea, and how quickly this energy erodes the ridge axis across mainland Alaska. While deterministic solutions showed greater differences, a blend of ensemble means likely represents the best consensus approach here by later in the forecast period. Thus, starting on day 6 (Sat) and continuing through day 8 (Mon), a gradual shift toward heavier use of ECENS/GEFS ensemble means was shown. ...Weather Highlights/Threats... A low pressure system is forecast to cross the southern Gulf of Alaska through late this week. This system will keep widespread precipitation in the picture across the Panhandle. As upper ridging builds across the Bering Sea and western mainland Alaska, drier conditions should prevail across much of mainland through late in the week, with only the Panhandle/Southeast Alaska remaining wet (on the eastern side of the persistent Gulf low). Additionally, another frontal system is expected to cross the Bering Sea late in the week, moving onshore in western mainland Alaska Sun night into Mon, spreading showers inland with it. In general, this pattern should favor temperatures several degrees above average across much of the state through late in the week, although temperatures across much of mainland/interior Alaska will begin to decrease closer to seasonal norms by the weekend into early next week as the incoming shortwave energy flattens the ridge, along with increased clouds/precipitation. Relatively widespread clouds/precipitation across the Panhandle will favor below average temperatures across that area through the extended forecast period. Ryan 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