Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Tue Aug 29 2023 Valid 12Z Sat Sep 02 2023 - 12Z Wed Sep 06 2023 ...Guidance Evaluation/Preferences... Guidance has started to diverge quite a bit in the handling of the next series of systems set to effect the state this weekend and into next week in the wake of the stormy weather connected to the remnants of Damrey in the western Pacific. First, the 12Z guidance has trended quite a bit stronger with a wave over the Gulf of Alaska early in the period, with the GFS/ECMWF ahead of the CMC. Attention then turns to a series of systems upstream and their eventual evolution as they impact the state. The 12Z GFS is very aggressive in developing a deep low from an Arctic system diving south over the Interior while the other guidance takes this system on a much more northerly track. This also leads to a much faster lead southern wave over southern portions of the state and the Gulf of Alaska. The ECMWF remains slower with this wave, though with the eventual southward progression of the Arctic system in the GFS, precipitation chances will remain/perhaps increase for the Southeast in either case. The updated forecast initially relies on a combination of the 12Z GFS/ECMWF before a transition to the ECMWF/CMC mid-period given the outlier nature of the GFS forecast, with an eventual contribution from the ensemble means late period as uncertainty increases. ...Weather/Hazards Highlights... The more significant impacts from a late week system tied to the remnants of Damrey from the western Pacific, including heavy rainfall, coastal flooding, high/gusty winds and dangerous maritime conditions, have moved into the short-range. The pattern then looks to transition away from high amplitude as systems are excepted to be embedded within more progressive upstream flows both across the Arctic and underneath out across the Bering Sea, with a trend towards more uncertainty regarding their eventual evolution. However, the signal remains consistent that energies from one or both of these flows will continue the unsettled pattern into the weekend with enhanced precipitation chances and winds. Rainfall will overspread the western coast/Interior as well as into Southcentral, where a split of upper level energies is expected to lead to a southern wave/surface system moving through the Gulf of Alaska. Some of the heaviest rainfall is therefore expected along terrain-favored areas along and near the southern coast, including the Alaska Range. The transition between systems may also lead to some locally gusty winds in the southwest and along Bristol Bay. Otherwise, conditions should remain wet but perhaps not particularly impactful. Recent concerns over high stream flows sourced from the Brooks Range may be somewhat abated by lowering snow levels and reduced runoff. Precipitation chances will also likely increase over the Southeast by early next week as this southern system continues to the southeast over the Gulf of Alaska. There looks to be a brief break for much of the Interior in the Tuesday-Wednesday time frame before another potential system approaches from the Bering Sea late period. Putnam Hazards: - Heavy rain across portions of mainland Alaska, Fri-Mon, Sep 1-Sep 4. - Heavy rain across portions of mainland Alaska, Fri-Sun, Sep 1-Sep 3. - High winds across portions of southwest mainland Alaska and the Alaska Panhandle, Fri-Sat, Sep 1-Sep 2. 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