Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Mon Aug 29 2022 Valid 12Z Fri Sep 02 2022 - 12Z Tue Sep 06 2022 ...Atmospheric river to cause heavy rain across southeastern Alaska late this week into early next week... ...Overview... An upper low shifting southeastward across the Alaska Peninsula Friday and spinning over the Gulf of Alaska through early next week, along with low pressure at the surface, will direct an atmospheric river of deep Pacific moisture into the southeastern Alaska mainland and into the Panhandle, causing heavy rain and possible flooding in those areas. The overall pattern may relax somewhat by early next week as the Gulf low weakens and troughing is forecast to track from eastern Siberia and the Bering Sea toward western Alaska by the end of the period next Tuesday. ...Guidance/Predictability Assessment... Recent model guidance continues to show an agreeable overall depiction of the closed upper low and its movement from the southeastern Bering Sea early Friday into the Gulf of Alaska over the weekend and early next week before weakening. Though there was some variability with exact placement and strength of the associated surface low, a blend of the latest deterministic models was able to be used as there were no clear outliers, though perhaps the UKMET was too deep with the surface low. Then upstream/farther west, a potent shortwave or possible compact closed low is forecast to dive southeast across the Aleutians, but model timing varies. The 12Z ECMWF seemed to be a good middle ground in between the slower GFS (especially the 12Z run) and the faster UKMET and CMC around Sunday. That feature shifts eastward into the northern Pacific/Gulf with some continued model variability, though GFS runs almost have the shortwave trough catching up to the faster models by Mon/Tue. This is certainly one of the more uncertain parts of the forecast, as shown by considerable variability in the ensemble members from each model suite creating almost flat ensemble means. By early next week, there is general agreement for troughing to deepen west of Alaska, but the 12Z CMC seemed too aggressive compared to other guidance with an upper low farther south centered along the Aleutians by Tuesday. For the latter part of the period, incorporated the GEFS and EC ensemble means into the forecast blend, but was able to keep about 60 percent deterministic models led by the 12Z ECMWF in the blend to maintain some strength of systems. ...Sensible Weather and Potential Hazards... Multiple rounds of heavy rainfall will be ongoing as the period begins Friday from the Kenai Peninsula and greater Anchorage area toward the southeast Panhandle region given ample moisture from the atmospheric river and its orientation orthogonal to the terrain leading to upslope flow. Rainfall maxima could be 1-2 inches per day, creating possible multi-day accumulations of 3-6 inches and potential for flooding and enhanced runoff. The atmospheric river will gradually shift eastward through early next week and bring moderate to locally heavy rain to the Panhandle, especially northern portions, but the southern Panhandle may also see heavy rain. Scattered light showers are possible for the interior mainland. Temperature-wise, the southern part of the state can expect a bit cooler than normal temperatures especially for highs, with closer to normal and more mixed above/below average temperature areas farther north into the interior. Tate 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 Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Thu-Sun, Sep 1-Sep 4. 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