Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 221 PM EDT Wed Oct 17 2018 Valid 12Z Sun Oct 21 2018 - 12Z Thu Oct 25 2018 ...A Prolonged Threat of Heavy Precipitation for Southern/Southeastern Alaska... Highly amplified/blocky larger scale flow over Alaska and vicinity remains stagnant during the medium range period (Sun-Thu). The pattern aloft continues to feature a mean trough centered near/south of the Aleutians/Alaskan Peninsula sandwiched between blocky upper ridges. Expect development and track of a deep Pacific low/storm to the Alaskan Peninsula this weekend. Reinforcing trough energies into early-mid next week will subsequently act to spread/prolong stormy activity across the southeastern Bering Sea/Southwest Alaska and down into the Gulf of Alaska increasingly over time. This pattern will result in a sustained period of southerly flow into Alaska from lower latitudes, with embedded systems enhancing precipitation potential. Models show general agreement on a multi-day heavy precipitation event for portions of southern and southeastern Alaska with multiple inches (liquid equivalent) of precipitation likely, especially at windward locations near the coast. The flow regime will meanwhile inhibit the southward expansion of arctic air across Alaska this period. While models show agreement on the larger scale, there continues to be disagreement on the specific details of individual systems, especially by days 6-8. The WPC Alaskan medium range product suite was accordingly derived from a composite blend of the better clustered GEFS/NAEFS/ECMWF ensemble means with an eye on continuity. Continuity was further maintained by the application of manual modifications to insure deep low pressure consistent with upper support. Schichtel WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html