Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Thu Sep 16 2021 Valid 12Z Mon Sep 20 2021 - 12Z Fri Sep 24 2021 ...Several Major Storms to Threaten Alaska... ...Guidance and Predictability Assessment... The 12 UTC models and ensembles seem in good agreement into Monday and a composite blend seems reasonable to smooth the less predictable smaller scale wrinkles in a period with overall above normal predictability. While modest model input offers some detail into days 5-8, opted to switch to mainly the compatible GEFS/ECMWF ensemble means then in a period of increased forecast spread with the details of stream interactions and embedded system specifics. The overall stormy pattern though still offers above normal predictability. 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. This plan maintains reasonable WPC product continuity. ...Sensible Weather... A powerhouse storm will track 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 deep extratropical low is partially associated with current West Pacific Tropical Storm Chanthu and additional onset of upper trough energies, so it presents another heightened threat for high winds/waves and wrapping heavy rains. There is a threat that this system may bring a long fetch moisture plume northeastward into Southeast Alaska by later next week. Farther north in an also active stream flow around/underneath a deep closed low working 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 and Thu, Sep 23. - Heavy rain across portions of the Alaska Panhandle, Mon-Tue, Sep 20-Sep 21 and Thu, Sep 23. - Much below normal temperatures across portions of mainland Alaska, Mon-Thu, Sep 20-Sep 23. 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