Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 601 PM EDT Fri Sep 25 2020 Valid 12Z Tue Sep 29 2020 - 12Z Sat Oct 03 2020 ...Overview and Guidance/Predictability Assessment... Model/ensemble guidance is agreeable and consistent in showing a transition to a very amplified upper pattern during the medium range, featuring a strengthening ridge over western Canada and a mean trough (with a couple of embedded lows) over and extending well south from the western mainland. This configuration should lead to a very wet period from portions of the Peninsula region to the Southern Coast and Panhandle. Guidance behavior thus far suggests well above average confidence in the mean pattern but models continue to have considerable difficulty with the specifics for individual systems. Two main forecast concerns for the period across Alaska. The first is a deep surface low lifting northward towards the Peninsula region days 4-5 (Tuesday-Wednesday). The latest 12z operational GFS is the slowest of the solutions, although the 12z ECMWF did trend slower from its 00z run. Despite the minor timing differences, a compromise of the two provided a good starting point. After day 5, operational models continue to struggle with a couple of shortwaves rounding the base of the mean trough, and especially with extra tropical remnants of cyclone Dolphin. This low enters the domain by day 5 and should stay well south of the Aleutians as it translates east, and then begins to shift towards the Alaska Peninsula by days 6/7 steered by the blocky ridge over Western Canada. The 12z ECMWF is faster with the surface low, while the positioning of the 12z GFS is much closer to that of the ensemble means (which do show very good agreement). Given the time range and continued uncertainty, the WPC blend leaned more towards the ensemble means by days 7 and 8, with some small contributions of the 12z GFS just for some added pattern definition. ...Weather/Threats Highlights... The general pattern evolution will favor increasing potential for heavy precipitation from the Peninsula to the northern Panhandle. The details of timing, magnitude, and coverage are far from resolved though. The first surface low tracking northward towards Kodiak Island, should pull significant moisture into the Gulf of Alaska and eventually the southern Coast/northern Panhandle early next week, with gusty, gale force, winds likely to accompany. Trailing cold front may provide localized enhancement of precipitation beyond midweek as well. The next system (remnants of Dolphin) will probably bring another round of moderate to heavy rainfall late next week, but the timing and intensity details remain in question. Elsewhere, expect periods of mostly light precipitation over the Aleutians with some scattered activity over the mainland away from the main focus in the south. Below normal high temperatures will prevail over the North Slope (though gradually moderating with time) and pockets of the mainland. Expect widespread coverage of above normal lows prevailing across much of the Alaska mainland. Santorelli 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 mainland Alaska, Tue-Fri, Sep 29-Oct 2. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Fri, Sep 28-Oct 2. - High winds across portions of mainland Alaska, Tue-Wed, Sep 29-Sep 30. 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