Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 PM EDT Wed Mar 21 2018 Valid 12Z Sun Mar 25 2018 - 12Z Thu Mar 29 2018 Guidance still displays significant spread for details of flow aloft over the Arctic into mainland Alaska. Compared to yesterday when guidance was more evenly split or slightly in favor of an ECMWF cluster, now there is less support for the 00z ECMWF/ECMWF mean scenario of a persistent closed high over the Arctic with a significant weakness over the northern mainland. While support for that exact evolution has waned somewhat, there is still decent consensus that what ridging exists should be somewhat stronger than seen in a number of recent GFS/GEFS mean runs. Farther south there is improved clustering for the system forecast to track across the southeastern Bering Sea toward the southwestern mainland. An operational model compromise among the 06z GFS and 00z ECMWF/CMC/UKMET compares well to the typically weaker ensemble means--though for a time the ECMWF mean is not far from a blend of those model runs. This blend--including enough ECMWF input to reflect its consistency thus far in the Arctic and per coordination with WFO AFG--provides some adjustment from continuity for details over the Arctic into the mainland but maintains a non-GFS consensus. By the latter half of the period the most confident aspect of the forecast appears to be the development of a deep central Pacific trough/embedded closed low with a downstream ridge covering the eastern Pacific. Recent operational runs suggest the central Pacific low surface/aloft may end up farther south than the ensemble means, favoring at least modest inclusion of operational guidance through the end of the period. Elsewhere model/ensemble spread is sufficiently great to require more emphasis on the ensemble means. Confidence rapidly decreases in determining shortwave details over the mainland and Bering Sea. Also there is significant uncertainty regarding the potential for a mid-latitude Pacific wave to develop/track into the northeast Pacific/Gulf of Alaska during the Tue-Thu time frame with possibly significant wind/precip effects on the southern coast and/or Panhandle. ECMWF runs have been showing decent development but with a 15-20 degrees longitude eastward trend over the past day. GFS runs had been more muted/suppressed but the 12z run came in with a strong system west of the 00z and 12z/20 ECMWF runs. Support aloft comes from progressive mid-latitude Pacific flow that tends to have lower predictability at the extended time frame. Overall there seems to be at least moderate potential for such a system but so far the spread/variability in models and ensembles has been too great to depict this system in a deterministic forecast. A blend of 70 percent total 06z GEFS/00z ECMWF mean with the rest lingering components of the 06z GFS-00z ECMWF/CMC (plus a little manual adjustment) yielded a compact northern Gulf of Alaska system as of 12z Wed, though with at least as much support aloft from remaining northern stream energy as what may travel northeastward from the mid-latitude Pacific. This provides a reasonable starting point for depicting system potential while waiting for better agreement/continuity in the guidance. Rausch