Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 807 PM EDT Tue Aug 30 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... Guidance is agreeable and 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 models and ensembles show good clustering and continuity for the initial Gulf of Alaska system, and while timing spread persists for the weekend Aleutians system, the general low track is more agreeable. For the Aleutians system the 12Z ECMWF trended slower than its previous run (and the new 12Z ECMWF mean) while the 12Z GFS/UKMET were on the faster side of the spread. The new 18Z GFS has come in with slower timing, closer to the 00Z ECMWF and 12Z ECMWF mean. From the weekend into early Monday, an operational model composite (splitting the ECMWF component among the past two runs) represented consensus/continuity for the Gulf system and yielded an intermediate timing for the Aleutians system. This blend also helped to resolve differences for a compact frontal wave that could affect parts of the Panhandle around Sunday. Multiple uncertainties become increasingly evident through the rest of the forecast. Individual model runs have been variable for the ultimate path of the upper low emerging from Siberia, ranging anywhere between the Bering Strait (some GFS runs including those from 06Z and 18Z) and the Aleutians (00Z ECMWF). The uncertainty over the upper low's track and specifics of surrounding flow in turn leads to question marks over what kind of interaction may occur with North Pacific flow and a possible surface low. Issues with this upper low also correspond to significant differences for flow details over the Arctic and the northern mainland, with current preferences siding closer to the ensemble means that bring the overall trough axis close to the western coast of the mainland by next Wednesday. Meanwhile there is a wide north-south spread for the weekend Aleutians system as it reaches farther east into the Pacific. The most common theme is for the surface low to track into the northeastern Pacific but remain far enough away to have minimal influence on the Panhandle. The forecast blend trended to 40-50 percent total weight of the 12Z GEFS/00Z ECMWF mean to go along with lingering input from the 12Z GFS/CMC and 12Z/00Z ECMWF runs, providing somewhat more detail than some of the means but still leaning closer to the means in principle where significant model differences exist. Especially aloft, the new 12Z ECMWF mean compares favorably to this approach by early next Wednesday. ...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 more to 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. Some areas of mostly light to moderate precipitation will be possible over interior portions of the mainland. 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 see rain develop toward Tuesday-Wednesday depending on the Bering Sea-North Pacific evolution which currently has fairly low confidence in specifics. Expect southern portions of the mainland and the northern Panhandle to see below average highs for most of the period while eastern parts of the interior as well as the northern coast will tend to see above average highs. Other areas should see somewhat below normal to near normal highs. Morning lows should be more broadly above normal, with highest anomalies mostly over the northeast. Rausch Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Fri-Sun, Sep 2-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