Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 715 PM EDT Wed Sep 15 2021 Valid 12Z Sun Sep 19 2021 - 12Z Thu Sep 23 2021 ...Several Major Storms to Threaten Alaska... ...Guidance and Predictability Assessment... The 12 UTC GFS/ECMWF and GEFS/ECMWF ensembles seem in good agreement Sunday into Monday and a composite blend seems reasonable to smooth the less predictable smaller scale wrinkles in a period with overall above normal predictability. A composite blend also seems reasonable through longer time frames overall, but the 12 UTC ECMWF may be over zealous with stream phasing compared to the more stream separated GFS, GEFS/ECMWF ensemble means and 00 UTC ECMWF run to run continuity. Manual adjustments were applied to WPC progs to ensure deep offshore low depths and wind strengths consistent with a pattern with good system upper support and improving predictability. ...Sensible Weather... A powerhouse storm will track from the Aleutians this weekend to across the Gulf of Alaska early next week. This potent low will present a significant maritime wind and wave threat as well as a heavy precipitation threat from Kodiak Island and south-central coastal Alaska to especially the Southeast Panhandle. Yet another major and large storm will affect the Aleutians and southern Bering Sea early-mid next week before working across the Alaskan Peninsula to the Gulf of Alaska. This moist system is the deep extratropical low associated with current West Pacific Tropical Storm Chanthu, so it presents another heightened threat for high winds/waves and wrapping heavy rains. Farther north in an also active stream flow around/underneath a deep closed low working gradually over the Arctic Ocean, expect quite unsettled conditions with increased rain and snow chances, especially for the Brooks Range, with expected intrusion of Arctic air southward across the North Slope and Interior/Mainland next week. Forecast spread is not stellar, but has improved, bolstering forecast confidence in the pattern. This would also result in below normal temperatures for much of the mainland. 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. - High winds across portions of mainland Alaska and the Aleutians, Sat, Sep 18. - 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