Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 556 PM EDT Sat Sep 30 2023 Valid 12Z Wed Oct 04 2023 - 12Z Sun Oct 08 2023 ......Multi-day heavy precipitation storm threat from Southeast to southern Alaska, with freezing rain potential into the Interior... ...Guidance Evaluation/Preferences... 12 UTC GFS/ECMWF model guidance seems best clustered with GEFS/NAEFS/Canadian and ECMWF ensembles Wednesday into Thursday in a multi-stream pattern with seemingly near normal predictability. The 12 UTC Canadian model seems an outlier in showing more rapid upper ridge building up into the mainland. Forecast spread and run-run continuity issues then grow more significantly, lending transition starting Friday to preference for the still quite compatible ensemble means. Opted to keep some detail from the best ensemble matching 12 UTC ECMWF for Friday, but then focused on the GEFS/ECMWF ensemble means for next weekend. Overall, the forecast plans maintains reasonably good WPC product continuity. ...Pattern Overview and Weather/Hazards Highlights... Expect a strong surge of moisture (possible atmospheric river) by next midweek again into Southeast Alaska as a warm front lifts through the Gulf of Alaska in advance of a slow moving Gulf storm pattern. This is expected to fuel heavy rainfall and mountain snows to mainly impact the Panhandle into Thursday/Friday. The warm front should continue to lift gradually northward and the advent and lingering of deepened northern Gulf of Alaskan low pressure/frontal systems offers a protracted potential for enhanced rainfall, winds and mountain snows extending across the broad southern mainland tier, but mainly Southcentral and the Alaska Range into later next week/weekend. There is a fair amount of uncertainty in exact precipitation amounts in guidance, but the signal is growing. Additionally, much of Mainland Alaska should have below normal temperatures, with daytime highs as much as 20-25 degrees below normal across southern areas. As upper ridging uncertainly builds in, temperatures should moderate and trend warmer over time, but this transition is often too rapid in guidance. This could prolong a pattern favorable to include a threat for impactful freezing rain into the Interior as moisture tries to work around the periphery of the ridge. Schichtel Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php 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