Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Mon Oct 22 2018 Valid 12Z Fri Oct 26 2018 - 12Z Tue Oct 30 2018 ...Continued threat of heavy precipitation for portions of southern/southeastern Alaska... Upper-level flow is expected to become a bit more progressive across Alaska during the extended period as a few of the height anomalies which have been persistent for a while now break down or relocate/reconfigure. North of Alaska in the Arctic, a relatively compact vortex which kept significant cold air from moving south into Alaska will also gradually break down, allowing upper-level flow off the Arctic to play more of a role in weather for Alaska, with the potential for rounds of colder air to eventually move into the state as well. Progression of a few low pressure systems through the Gulf of Alaska during the forecast period will result in continued rounds of potentially heavy precipitation for portions of southern and southeastern Alaska. The extended period of occasionally heavy precipitation may begin to come to an end by early next week as the region comes under more influence of flow off higher latitudes and with less southerly flow of subtropical origin. Relatively low model spread during days 4-5 (Fri-Sat) allowed for deterministic solutions (ECMWF/GFS) to be weighted heavily during that time period as one low pressure system fills/decays over the northern Gulf and a second one enters the Gulf by late Sat. After that time, forecast confidence begins to quickly decrease. The interaction of upper-level flow and shortwave energy from the Arctic with the westerly mid-latitude flow introduces additional chaos and complexity into the forecast. As a result, model solutions quickly begin to differ from Sun onward. An additional low pressure system appears likely to move well south of the Aleutians Sun-Mon with significant timing/intensity differences as the system perhaps nears the Gulf of Alaska by Tue. As a result of the increased spread, the ECENS/GEFS ensemble means were weighted heavily in the forecast for days 6-8 (Sun-Tue), with deterministic solutions eliminated entirely by day 8. Ryan 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