Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 731 PM EDT Tue Sep 14 2021 Valid 12Z Sat Sep 18 2021 - 12Z Wed Sep 22 2021 ...Stormy Pattern for Alaska... ...Guidance and Predictability Assessment... The 12 UTC GFS/ECMWF seem in good agreement through this weekend and a composite seems reasonable to smooth the less predictable smaller scale wrinkles in a period with above normal predictability. At longer time frames, The 12 UTC ECMWF ensemble mean now seems a good compromise between the increasingly diverse models and the 12 UTC GEFS mean or 00 UTC ECMWF ensemble mean that may be too progressive with the cold air intrusion into the mainland next week. Any of these ensemble means could be used for the storm track from the North Pacific into the Gulf of AK compared to models that are more varied over time with the main surface low tracks, albeit in a pattern with a seemingly favorable and amplified/energetic upper pattern evolution. Manual adjustments were applied to WPC progs to ensure offshore low depths and wind strengths consistent with the pattern. ...Sensible Weather... A series of major lows are on tap through medium range time scales. A lead system will linger this weekend over the eastern Gulf of Alaska as a maritime threat, albeit with trends for much of associated weather suppressed to the south of the state. The next in a series of deep storms is expected to track from the Aleutians this weekend to across the Gulf of Alaska through early-mid next week. This potent low will again present a maritime threat and also a heavy precipitation threat from south-central Alaska into the Southeast Panhandle. Yet another big low is slated to approach the Aleutians in about a week with an uncertain subsequent track/transition into the southern Bering Sea or into the Gulf of Alaska. This moist system is the extratropical low associated with current West Pacific Tropical Storm Chanthu so there is heightened threat potential at this longer time frame. Farther north in an also active Arctic stream flow, the models have trended warmer and slower than ensembles with an expected intrusion of Arctic air southward across the North Slope and Interior next week. WPC progs offer a compromise solution with an adjustment toward the trend from yesterdays WPC product continuity. Schichtel Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Sep 20-Sep 21. - Heavy rain across portions of the Alaska Panhandle, Mon-Tue, Sep 20-Sep 21. - Much below normal temperatures across portions of mainland Alaska, Mon-Tue, Sep 20-Sep 21. 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