Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 740 PM EDT Sat Aug 14 2021 Valid 12Z Wed Aug 18 2021 - 12Z Sun Aug 22 2021 ...Overview... Guidance as a whole advertises low-amplitude mean flow aloft between an upper low over the Arctic Ocean and a ridge over the eastern Pacific. Embedded shortwaves should move along in multiple streams, with considerable uncertainty over the amplitude and progression of most of these features as well as for what interaction may occur between streams at any particular time. The average of guidance shows upper heights declining some over the Gulf of Alaska and vicinity by next weekend. This pattern should promote a cool and unsettled period over much of the state but with less confidence in the specifics. ...Guidance/Predictability Evaluation... The medium to smaller scale of individual shortwaves along with the possibility for some stream interaction lead to fairly low predictability for details during the period. The most agreeable feature in principle (at least early in the period) is a shortwave aloft and associated surface wave likely to emerge over the Bering Sea by around midweek and then continue toward/into the mainland. GFS runs stray somewhat to the faster side of the guidance spread as is the model's occasional tendency, though not to such an extreme extent as to preclude at least minority inclusion in the forecast. Meanwhile by day 6 Friday the 12Z CMC/UKMET both become more amplified with the shortwave versus other models and the ensemble means. Through the latter half of the period solutions increasingly diverge among each other and over consecutive runs for features within the North Pacific, Asia/Bering, and Arctic streams. The 12Z CMC is generally on its own with its upper low that reaches the northwestern mainland by next Saturday. Differences in the North Pacific stream lead to a full range of possibilities between meaningful precipitation reaching areas from the eastern Aleutians into the southern coast/Panhandle and fairly dry conditions. Starting with a 12Z operational model blend early in the period reflected the most agreeable detail for the initial Bering Sea system tracking toward the mainland. Then transitioning toward more 12Z GEFS/00Z ECMWF mean weight (reaching 70 percent by day 8 Sunday) with lingering input from the past two ECMWF runs and 12Z GFS provided a more conservative approach given the low predictability of individual features. ...Weather/Threats Highlights... The Bering Sea system tracking toward/into the mainland as well as a leading front already draped over the mainland will likely help to focus organized precipitation over portions of the central and southern mainland during mid-late week. Some pockets of enhanced activity are possible. The magnitude of totals from the eastern side of the Kenai Peninsula into the Panhandle will be sensitive to the upper shortwave's exact amplitude which is yet to be determined with much confidence. A more amplified shortwave that would lead to higher totals currently appears to be less probable but such a scenario is still worth monitoring. One or more upstream features in the Arctic-Bering streams and/or North Pacific stream may produce other areas of organized precipitation during the late week into weekend time frame but again with minimal confidence in the details. The unsettled pattern should keep much of the state below normal for high temperatures during the period. Lows will generally be more moderate with best potential for above normal readings over the southeastern half of the state. Rausch 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