Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 PM EDT Sat Apr 07 2018 Valid 12Z Wed Apr 11 2018 - 12Z Sun Apr 15 2018 The overall theme of the forecast is consistent, with generally well-defined features across the North Pacific/Bering but greater uncertainty for flow details over Mainland Alaska and to some degree into the Arctic. Regarding North Pacific features, there is a slight nudge southward with the system tracking just south of the Aleutians Wed-Thu. Stronger ensemble mean trends and operational model clustering favor an average of model solutions for this system. On the other hand farther east over the northeastern Pacific/Gulf of Alaska there is an area of mean low pressure but model/individual ensemble solutions are all over the place with respect to placement of individual waves. This favors leaning more toward an ensemble mean depiction until operational model spread decreases. Starting with a blend of models used for the upstream system yields a solution that leans closer to the means. There is a consistent signal that these two areas of low pressure should merge over the northeastern Pacific with the resulting low tracking southeastward by Fri-Sun. The next system whose influence reaches the western Aleutians late in the week is fairly well behaved in terms of agreement/continuity into Fri. After that time a number of operational model runs are suggesting that a frontal wave may strengthen and track into the Aleutians/Bering, possibly becoming the dominant low. As might be expected, the ensemble means show a more consolidated evolution with more emphasis on the parent low. Also of note, over the past day the means have trended noticeably farther northward with the low for forecasts valid day 8 Sun. Prefer tilting the forecast somewhat more toward the ensemble means to provide better continuity until details on frontal waves become more clear. Late in the period there is a split in guidance for timing of northern Pacific flow with the 06z GFS/GEFS mean and 00z CMC mean faster than the 12z GFS and 00z ECMWF/ECMWF mean/CMC. Preferred blends utilize a greater weight of the slower cluster but with only moderate confidence. Meanwhile the ensemble means maintain the steadiest solution for the mainland with a tendency for a col region to develop, between ridges to the northwest and over western Canada, and between cyclonic flow over/north of far northern Canada and Pacific systems. Individual models show a variety of possibilities between a ridge (12z GFS, eastward return of the Arctic ridge in the 00z ECMWF) and a trough (00z CMC). It is possible that mainland flow may become somewhat better defined by day 8 Sun or so, with at least modest mean ridging possibly building ahead of the late period Aleutians/Bering system. The Wed into Thu part of the forecast starts with the 00z ECMWF/12z GFS/00z CMC-UKMET in order from most to least weight. The forecast begins to incorporate ensemble guidance (00z ECMWF mean/06z GEFS/00z NAEFS) late Thu. Ensemble weight increases to 60-70 percent by Sat-Sun with modest 12z GFS/00z ECMWF components maintained through the end of the period. Rausch