Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 745 PM EDT Sat Aug 13 2022 Valid 12Z Wed Aug 17 2022 - 12Z Sun Aug 21 2022 ...Multiple days of heavy precipitation potential along/near the southern coast and Panhandle... ...Overview... Latest GEFS/ECMWF/CMC ensemble means all show a similar large scale evolution, with an upper low near the western coast of the mainland wobbling south/southeast to a position near Kodiak Island by early next Sunday. The overall circulation should incorporate incoming energy approaching from the western Bering Sea/Pacific. A leading system will likely reach the Gulf of Alaska by the start of the period early Wednesday. Additional surface lows will likely track within an area encompassing the North Pacific/Aleutians/southern Bering Sea into the Gulf of Alaska. This evolution will yield an extended period of significant precipitation potential over some areas near the southern coast and Panhandle, while less extreme amounts of rain/high elevation snow may extend over the rest of the state. ...Guidance/Predictability Assessment... By the latter half of the period, ensemble means have adjusted somewhat southeastward with the upper low over the past day or so. Otherwise the means have provided reasonable continuity and the aforementioned current agreement provides decent confidence in the large scale evolution. However individual model solutions show enough spread and run-to-run differences to temper confidence in some important details at the surface and aloft, and in turn specifics of the precipitation forecast. Complexities that have lower predictability include one or more shortwaves within the initial cyclonic flow around the upper low as well as the details of shortwave energy/low pressure that track in from the west and become a part of the overall circulation. In light of the relative stability/agreement of the means, today's forecast approach was to incorporate operational model runs that aligned most closely with the means early-mid period and then trend toward a half models/half means solution by day 8 Sunday. Guidance comparisons led to favoring more of the 00Z ECMWF relative to the 12Z run, plus the 06Z GFS and 12Z CMC/UKMET for the early half of the period. The 12Z ECMWF/GFS strayed farther north of consensus for the main upper low, with the new 18Z GFS partially adjusting back somewhat closer to the means. Trending the blend to a half models/means solution by day 8 Sunday provided a good balance between typically better operational model detail and the agreeable evolution of the means. The resulting forecast also yielded a reasonable intermediate forecast within the existing spread for surface lows between the North Pacific into Bering Sea and east into the Gulf of Alaska. ...Weather/Hazard Highlights... Low pressure expected to reach near the southern coast by the start of the period early Wednesday will bring one episode of focused precipitation to the southern coast and Panhandle around midweek. Specifics of individual lows approaching from the west are less certain but overall the guidance is signaling that the rest of the period could feature significant additional precipitation across the southern coast/Panhandle and nearby areas, whether from a persistent flow of moisture into the region or multiple episodes that vary in magnitude. Favored terrain may see a total of at least several inches during the course of the period. The remainder of the state should see less extreme amounts of rain and high elevation snow, with details to be determined by uncertain specifics of flow aloft. A majority of the state should see below average highs, with the greatest negative anomalies likely to be over the North Slope and over the far south/Panhandle. Some slightly above normal highs could extend across central latitudes of the mainland by late week and next weekend. Lows will tend to be below normal over western areas and somewhat above normal over the east and the Panhandle. 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