Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 344 PM EDT Fri Sep 21 2018 Valid 12Z Tue Sep 25 2018 - 12Z Sat Sep 29 2018 ...Multi-Day Alaskan Peninsula/Southwest Alaska Heavy Rains mid-later next week... ...Guidance and Predictability Assessment... A favored model and ensemble composite shows a lead northern stream frontal low will focus modest precipitation inland across western Alaskan and in a weakening state to the North Slope and interior early next week. Stormy maritime conditions will meanwhile brew both well south of the Aleutians and off eastern Asia. Subsequently, there is a good larger scale weather pattern transition signal favoring building flow amplitude aloft. This is expected to establish a potent mean trough/closed low centering over the Bering Sea/Aleutians and a deep layered ridge from the eastern Gulf of Alaska up through the Alaskan interior by mid-later next week. Models continue to struggle with the handling of deepened embedded systems moving through the increasingly blocky flow day by the 5-8 timeframe and reinforced lows will likely further develop considering potential energy then lift on the western peripjery of the ridge. Given the inconsistent models, decided in collaboration with Alaskan weather offices to use a composite of the reasonably well clustered 00 UTC NAEFS/ECMWF ensemble means that maintain good WPC continuity. ...Pattern Overview and Weather/Hazards Highlights... The upper pattern will support blocky high pressure centered just southeast of the Gulf of Alaska that will inhibit eastward surface frontal progressions over the northeast Pacific/Gulf of Alaska. Fronts will hang up parallel to the flow on the western periphery of the high as coincident with a persistent moisture conduit to fuel a prolonged period of heavy rain to the AKPen and southwest/southern Alaska midweek through later next week as increasingly stormy lows over the Aleutians/Bering Sea and the northeast Pacific solidify and lift across the region. Schichtel WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html