Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 735 PM EDT Sat Aug 07 2021 Valid 12Z Wed Aug 11 2021 - 12Z Sun Aug 15 2021 ...Multi-day Heavy Precipitation Threat for portions of southern Alaska next week... ...Overview and Guidance/Predictability Evaluation... Model and ensemble solutions overall seem reasonably well clustered Wednesday into Friday and a composite blend of the 12 UTC GFS/ECMWF/Canadian was used along with the 19 UTC National Blend of Models to derive the WPC Alaskan product suite in a pattern with above normal predictability. However, steadily increasingly forecast spread and run to run continuity issues become increasingly evident in the models heading into next weekend. Accordingly relied more significantly at these longer time frames on the still compatible 12 UTC GEFS/ECMWF ensemble means as per collaboration with local WFO's, albeit with manual adjustments to maintain lost system organization/detail/comtinuity despite lowered predictability. A cooling upper level trough will be in place over much of the state and the Bering Sea for much of next week. An upper level ridge over British Columbia and the Pacific Northwest will tend to limit the eastward progress of the trough, resulting in a multi-day heavy rainfall threat from south-central into southeast Alaska. This will especially be from the Kenai Peninsula to the northern Southeast Panhandle as fueled by a deep layered and long fetch plume of tropical sourced moisture with extratropical lows associated with Nida, 91W, and Mirinae from the west Pacific. By next weekend, a storm system over the North Pacific may affect the Aleutians. This system would be an extratropical low associated with Lupit. Meanwhile, a closed low over the Arctic Ocean may force a front southward through an unsettled North Slope and Brooks Range mid-later next week. The vigorous closed low may eject next weekend with approach of upstream system energies. ...Weather/Threats Highlights... Multiple rounds of heavy precipitation are expected from south-central Alaska to northern portions of the Southeast Panhandle as strong and deep layered southwesterly flow advects copious moisture inland. This moisture feed will be periodically enhanced with approach of extratropical systems associated with west-Pacific tropical systems Nida, 91W and Mirinae. This could result in some locations potentially getting 3 to 8+ inches of rainfall across topographically favored areas in a long fetch atmospheric river episode. This multi-day event will focus Wednesday-Friday, but uncertainty with moisture/extratropical low track associated with Mirinae opens the door for a continued threat into next weekend. WPC surface progs take a stab at a possible track into the Gulf of Alaska that is not well depicted in guidance, but has decent upstream continuity. This pattern will likely lead to some instances of flooding and enhanced run-off. Meanwhile, upper level troughing over the state will also tend to keep temperatures below normal across most of the state for this forecast period. These anomalies will likely be even greater for an unsettled Brooks Range and points northward of an Arctic front sinking into the Interior mid-later next week, moving through in response to a potent closed low north of the Arctic Coast. The closed Arctic low may eject next weekend in response to approach of upstream upper trough energies that may allow a new low/system work into the Chukchi Sea, Bering Strait, and northern Bering Sea. Schichtel 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