Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 617 PM EDT Wed Aug 19 2020 Valid 12Z Sun Aug 23 2020 - 12Z Thu Aug 27 2020 ...Overview and Guidance Evaluation/Preferences... Over the course of the extended period, the primary feature of interest will be a developing and stalling upper level low across the eastern portions of the Bering Sea. At the surface, initial low pressure will skirt across the Aluetions and southern mainland, occluding and stalling a bit before becoming absorbed within the developing large scale low. For the most part, the synoptic pattern showed fairly good agreement between the various deterministic models, at least through day 5/6. Beyond that, typical model biases were seen where the 12Z GFS was faster and ahead of the 12Z ECMWF/CMC. The ensemble means were useful by day 6/7 and were considered a compromise solution given the deterministic model differences later in the period. As such, the WPC blend started off with a near equal weight of the 12Z ECMWF/GFS/CMC then transitioned to higher weights of the ECENS and GEFS means as forecast consistency and confidence lowered. ...Weather Highlights/Threats... Relatively dry conditions and warm temperatures will be in place across much of interior Alaska late in the weekend, before the upper ridge breaks down and transitions to the large upper low across the eastern Bering Sea. A frontal system across the Bering Sea will begin to affect the west coast of Alaska on Sat with scattered showers, before spreading into much of the Interior on Sun, as energy transfers to a new triple point low in the Gulf of Alaska. This system may produce heavy rains across portions of southern mainland Alaska, especially the Alaska Range, Sun-Mon. Into next week, expect scattered to numerous showers to persist across much of mainland Alaska as the ridge weakens and shortwaves traverse the region through midweek. Dropping temperatures will support snows at the higher elevations of the Alaska and Brooks Ranges. High temperatures Sat are forecast to be 5 to 15 deg F above average. From Sun onward, as shortwaves break down the ridge and heights fall, expect temperatures to decrease to near or even several degrees below normal. Ryan/Taylor 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 rain across portions of mainland Alaska, Monday Aug 24. 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