Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 639 PM EDT Wed Aug 31 2022 Valid 12Z Sun Sep 04 2022 - 12Z Thu Sep 08 2022 ...Atmospheric river to cause heavy rain across southeastern Alaska into the weekend... ...Overview... Latest guidance is generally consistent with an upper low/surface system near Kodiak Island and the Kenai Peninsula through the weekend, with the overall feature likely opening up thereafter. This system will focus an atmospheric river of deep Pacific moisture into areas along and near the southeastern coast of the Alaska mainland and into the Panhandle, producing heavy rain and possible flooding in those areas. Also during the weekend a separate system should track along the Aleutians. By the first half of next week the primary emphasis of the forecast will be on an upper trough and likely embedded low moving from Siberia into the Bering Sea along with how it may interact with North Pacific flow and a possible surface wave. ...Guidance/Predictability Assessment... The WPC Alaskan Day 4-8 product suite was primarily derived from a blend of the 12 UTC GFS/ECMWF and 12 UTC GEFS/ECMWF ensemble means. It was decided via collaboration with Alaskan weather offices to apply blend weights tipped in favor of the GFS/ECMWF into days 4/5 (Sunday/Monday) before shifting strongly toward the ensemble means by days 6-8 amid steadily growing forecast spread. WPC product continuity was reasonably well maintained in this manner in a pattern with near normal overall predictability. ...Sensible Weather and Potential Hazards... The atmospheric river event that should be in progress from the southern coast into the Panhandle at the start of the period will likely persist into the weekend, but gradually become less pronounced in terms of rainfall totals relative to the short-range period (through Friday night) and primarily shift southward over the Panhandle by Sunday when a frontal wave could produce some enhancement. Potential for flooding and enhanced runoff will continue into the weekend given the significant totals already expected by the start of the extended period. Meanwhile, the system tracking along the Aleutians during the weekend will bring a brief period of rainfall to the region. Some locations from the Aleutians into the Alaska Peninsula/far western mainland may subsequently see rain develop toward Tuesday-Wednesday depending on the Bering Sea-North Pacific evolution which currently has fairly low confidence in specifics. Latest progs still bring an organized low/frontal system across the region that could then reorganize over the northern Gulf of Alaska next Wednesday/Thursday. This could offer an additional maritime and southern Alaskan threat of enhanced rains/winds/waves, albeit with details to emerge for this longer time frame. Elsewhere, the overall pattern with lowered heights and passing upper systems should also support some scattered areas of mostly light precipitation over interior portions of the mainland over the period. Schichtel Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Sun, Sep 3-Sep 4. 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