Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 654 PM EDT Tue Aug 18 2020 Valid 12Z Sat Aug 22 2020 - 12Z Wed Aug 26 2020 ...Overview and Guidance Evaluation/Preferences... The pattern across Alaska will undergo some degree of change during the extended forecast period. An upper-level ridge axis initially across mainland Alaska will be broken down over time as strong westerly flow and embedded shortwaves fairly quickly erode the ridge. Models differ somewhat on the speed with which the ridge breaks down, but differ more so with respect to the timing of individual shortwaves traversing the relatively strong jet. Consensus among the 12Z ECMWF/GFS was sufficient during days 4-5 (Sat-Sun) to use a blend of these two solutions as a basis for the WPC forecast. After that, differences among deterministic guidance quickly increase. The GFS timing with shortwaves crossing the Bering Sea toward the Gulf of Alaska strays from the overall consensus, with the GFS on the fast side. In general, this is a scenario with below average predictability when it comes to the details, so ensemble use was quickly increased through time during days 6-8 (Mon-Wed). The ECMWF solution remained at least somewhat more in line with ensemble means, and thus a minority component of the ECMWF was used through day 8, while the GFS was phased out during that time frame. There is relatively good large scale consensus that by next Mon-Tue, the ridge should be sufficiently weakened to open mainland Alaska up to a barrage of shortwaves associated with the active North Pacific jet, resulting in a transition to a stormier pattern for Alaska by early next week. ...Weather Highlights/Threats... Relatively dry conditions and warm temperatures will be in place across much of interior Alaska on Sat, before the upper-level ridge breaks down. 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 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, Sun-Mon, Aug 23-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