Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 648 PM EDT Sat Aug 29 2020 Valid 12Z Wed Sep 02 2020 - 12Z Sun Sep 06 2020 ...Guidance/Predictability Assessment... Model and ensemble guidance agrees upon maintaining a mean trough aloft from west of the mainland into the central Pacific, with a general amplifying trend from midweek into next weekend. Incorporation of tropical cyclone Maysak into an upper trough over Asia will aid the evolution toward greater amplitude. Northeast Pacific upper ridging should also build by next weekend, becoming anchored by a 594-600 dm high forecast to be south or southwest of Haida Gwaii by day 8 next Sun. There are two primary features of interest and both involve details that are small enough in scale to yield low predictability in resolving differences among the guidance. Early in the period low pressure near the Kenai Peninsula may anchor a front that attaches to a Northeast Pacific wave which is most likely to track south of the Panhandle. The overall pattern at the surface and aloft has fairly good continuity but there are still noticeable differences with track/timing of the southern wave. Then most guidance expects a wave (with leading warm front) to track somewhere between Bristol Bay and the Gulf of Alaska next weekend. The 12Z CMC along with the 12Z GEFS-CMC/00Z ECMWF means and some earlier ECMWF/GFS runs favor the Bristol Bay route while latest GFS runs and the 12Z ECMWF are farther east. Also of note, consolidating North Pacific low pressure may reach far enough north to have some effect over/east of the eastern Aleutians at some point next Sun. A 12Z operational blend represented the consensus/continuity evolution early in the period. Trending to a nearly even blend of models and means (including a little 00Z ECMWF in addition to the 12Z run for that model's contribution) by days 7-8 Sat-Sun provided a good depiction of the agreeable amplified pattern while staying conservative with finer details. The resulting PMSL/thickness pattern next Sun provided more support for the eastern cluster with the frontal wave at that time, though a separate southeastern Bering wave could anchor a forming stationary front that extends across the southern mainland. ...Weather Highlights/Threats... Expect areas from the Kenai Peninsula to the Panhandle to see the highest precipitation totals for the Wed-Sun period. Low pressure near the Kenai Peninsula during mid-late week will provide the first focus for activity. Then another surge of moisture will likely accompany an approaching warm front and possible anchoring wave next weekend. Other parts of the mainland will see some episodes of precip with varying intensity though with lighter total amounts. Most of the mainland should see below normal highs through the period with the greatest negative anomalies over the south and Panhandle. There may be very localized pockets of above normal readings on one or more days over the central/northern mainland or northern coast. Meanwhile moderately below normal low temperatures pushing into the southwest quadrant of the mainland and then across the central mainland will trim some of the coverage of above normal lows with time. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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