Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 555 PM EDT Thu Sep 24 2020 Valid 12Z Mon Sep 28 2020 - 12Z Fri Oct 02 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 strong ridge over western Canada and a mean trough (with one or more 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 (Monday-Tuesday), eventually rotating north and westward towards western AK/eastern Bering Sea. The latest 12z operational GFS and ECMWF show enough agreement both regarding strength and timing of this low that a majority weighting of these in the blend works well. After day 5, operational models show increased chaos with a couple of shortwaves rounding the base of the mean trough, and especially with extra tropical remnants of cyclone Dolphin (currently located east of Japan). This low enters the picture by day 5 and should stay well south of the Aleutians as it translates east, and then begin to shift northward into the Gulf by days 7/8 aided by the blocky ridge over Western Canada. The 12z GFS is faster and stronger with the surface low, however the 12z ECMWF is a hair faster/stronger than its previous 00z run. However, given the time range and continued uncertainty, the WPC blend leaned more towards the ensemble means by days 7 and 8, with some inclusion of the operational models just to maintain some definition to the system. ...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 winds likely to accompany. Trailing cold front could contain one or more waves for localized enhancement 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 southern mainland/Panhandle with some pockets of warmer than average highs could exist over the south. Other locations should see above normal highs during the period. Expect widespread coverage of above normal lows with greatest anomalies tending to be over some central/northern areas. Any modestly below normal lows should be localized and limited to the extreme western/southwestern 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 the Alaska Panhandle and mainland Alaska, Sun, Sep 27. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Sep 29-Sep 30. - High winds across portions of the Alaska Panhandle and mainland Alaska, Sun, Sep 27. 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