Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 753 PM EDT Tue Aug 31 2021 Valid 12Z Sat Sep 04 2021 - 12Z Wed Sep 08 2021 ...Pattern Overview and Model Guidance... Today's guidance offers some similarity in the large scale evolution but a number of meaningful embedded uncertainties. A leading northeastern Pacific system may track near Haida Gwaii early in the weekend. Farther west initial southern Bering Sea/Aleutians energy will likely evolve as an upper low/trough and eventually bring a system into the northeastern Pacific while some height falls should extend into the mainland to the southeast of a mean upper low that becomes established to the north of eastern Siberia. Additional energy emerging from Asia may generate an Aleutians/Bering Sea system by days 7-8 Tuesday-Wednesday. A composite of 12Z operational models early in the period followed by a trend toward more 12Z GEFS/00Z ECMWF mean input provided the best balance of detail while reflecting only the somewhat more confident aspects of the large scale evolution. Of particular note, the blend shows less aggressive trough amplification into the mainland during the weekend into early next week compared to the GFS--thus toning down somewhat the precipitation amounts over and east of the eastern Kenai Peninsula. There is another vigorous system that the 12Z GFS rotates through the northwestern coast of the mainland around Tuesday and confidence for that is fairly low given the full range of other guidance. By late in the period the preferred blend provides a compromise for the possible northeastern Pacific system, between the slower ECMWF/CMC and fast GFS (ensemble means generally between the two). There is also considerable uncertainty over the character of upper dynamics that will support low pressure expected to reach into the Aleutians/Bering Sea late in the period. Thus confidence is low for the strength and track of the surface low. ...Sensible Weather... The forecast pattern will promote a fairly unsettled period with multiple areas of precipitation from the weekend into the middle of next week. A system tracking near Haida Gwaii early in the weekend may bring some moisture to the southern mainland. Lingering low pressure/surface troughing could help to focus areas of precipitation along the southern coast during the weekend into the start of next week, with some uncertainty for amounts but more likely with totals lower than forecast by latest GFS runs. Also expect precipitation to accompany a front that pushes into the northwestern mainland during the weekend and then stalls before lifting back to the north. Toward Tuesday and/or Wednesday low pressure reaching the northeastern Pacific could increase precipitation over the Panhandle while an Aleutians/Bering Sea system may extend its accompanying moisture into the western mainland. Expect a colder trend toward below normal high temperatures to progress from west to east across the mainland while areas over and near the Panhandle likely see below normal highs through the period. Min temperatures should still be above normal through the weekend followed by some areas of below normal mins reaching portions of western and interior areas. Rausch Hazards: - Heavy rain across portions of mainland Alaska, Fri, Sep 3. 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