Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Tue Apr 03 2018 Valid 12Z Sat Apr 07 2018 - 12Z Wed Apr 11 2018 The primary area of interest for significant systems will extend from the northern Pacific/Aleutians/Bering Sea into the northeastern Pacific in association with a mean axis of relatively low heights aloft. Broadly speaking there is a semblance of consensus on the existence of the largest scale systems but a lot of spread for specific details of each feature, plus uncertainty regarding whether one or more waves could stray as far north as the Gulf of Alaska. Farther north, lingering energy aloft over the mainland (from the eastern periphery of a short range upper low) may close off into a compact low for a time before departing after Mon. With typical spread in the guidance there is a general signal toward retrogression of the Arctic ridge allowing some degree of cyclonic flow aloft to extend into the northern/northeastern mainland toward next Tue-Wed. Based on the array of latest guidance, today's forecast starts with a mostly operational model emphasis (06z-12z GFS, 00z ECMWF, 00z CMC) versus 20 percent ensemble mean input (06z GEFS/00z ECMWF mean) for day 4 Sat into day 5 Sun followed by a transition toward the ensemble means so that days 7-8 Tue-Wed incorporate 80 percent means. Relative to yesterday this consensus shows a slightly more elongated depiction of energy supporting the initial Aleutians/northern Pacific system as of Sat and averages out some significant spread for what ultimately happens with any residual low pressure over the Bering. For the next system near the Aleutians around day 6 Mon the favored model/mean blend helps to keep the forecast close to continuity. The ECMWF/GFS and other models/ensemble members vary considerably on the exact structure of the system, with these details tending to have lower predictability that far out in time. However there is majority support (including from the 00z ECMWF mean) for a low track to the north of the 00z ECMWF to some degree. This system should then feed into the area of mean low pressure over the eastern Pacific while yet another system comes into the picture over the western Aleutians by day 8 Wed. Recent GFS runs/00z ECMWF compare fairly well with the ensemble means for this feature. Over the mainland, decent clustering extends into day 5 Sun for the energy aloft that may close off a compact low. After Sun solutions diverge for how this feature departs from the mainland and it appears to be very sensitive to specifics of the Arctic ridge and surrounding flow. Today's forecast blend provides a non-committal depiction of the one common idea being the departure/shearing of this energy in some fashion. Toward the end of the period the majority of guidance suggests that the Arctic high will retrograde enough to allow for cyclonic flow to extend into the northern/northeastern mainland. By Wed a number of operational model runs develop lower heights within this trough versus the ensemble means, but recent trends in the means have been for a slower retrogression of the Arctic high. Thus at the moment the means seem to provide a good conservative depiction of the evolving cyclonic flow. Rausch