Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Tue Oct 16 2018 Valid 12Z Sat Oct 20 2018 - 12Z Wed Oct 24 2018 ...Another Threat of Multi-day Heavy Precipitation for Southern/Southeastern Alaska... Larger scale flow over Alaska and the north Pacific remains relatively in place during the medium range period (Sat-Wed). The pattern aloft will feature a mean trough centered near/south of the Aleutians/Alaskan Peninsula sandwiched between blocky upper ridges. In the wake of a short range deep surface low, expect development and track of yet another deep Pacific low to the Alaskan Peninsula this weekend. Reinforcing trough energies into early-mid next week will also 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 the subtropics, with each embedded system enhancing precipitation potential. Models show general agreement on a multi-day heavy precipitation event for portions of southern and southeastern Alaska with several inches (liquid equivalent) of precipitation looking likely, especially at windward locations near the coast. The flow regime will 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 7/8. The WPC Alaskan medium range product suite was accordingly derived from a composite blend of the well clustered 12 UTC GFS/GEFS and 00 UTC ECMWF/ECMWF ensemble mean days 4-6 before leaning more strongly on the ensemble means days 7/8 with an eye on continuity. Continuity was further maintained by the application of manual modifications to insure deep low pressure consistent with upper support that was otherwise compromised by the composite blend process. 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