Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 215 PM EST Sat Mar 03 2018 Valid 12Z Wed Mar 07 2018 - 12Z Sun Mar 11 2018 At the start of the period early Wed a shortwave extending from the mainland into the northeast Pacific will be the primary feature of note and the last vestige of the pattern involving a series of progressive systems. Guidance has been having a lot of difficulty in determining amplitude/progression of the energy aloft Tue onward, leading to significant differences for a potential surface wave descending south/southeast from the Gulf of Alaska--including its mere existence. Over multiple days no operational model has been very consistent so confidence is still lower than average for a day 4 forecast. At least today there is better agreement among GFS/ECMWF runs and their means so a GFS/ECMWF compromise provides a good starting point for this system to provide better definition than the means. At least into day 5 Thu the larger scale system heading into the Bering Sea and vicinity shows fairly good consensus and continuity in principle. However an ongoing uncertainty by then involves a potential southern Bering frontal wave which operational runs have been suggesting in one form or another. ECMWF runs had been tending to track this wave toward the Kenai Peninsula but the 00z run adjusted more toward the GFS, which since the 12z/02 run has been tracking the low more northward into the mainland with the possibility of yet another wave forming along the southern coast. Thus far individual ensemble members show more clustering with the parent low well northward and waiting until near the Kenai Peninsula to develop the southern one. For now prefer a model/ensemble mean blend for specifics due to the relatively small scale of important details at the surface and aloft making it difficult to resolve several days out in time. Some clustering/trends in the large scale evolution become notable by the latter half of the period. Compared to the 00z-06z GFS/00z GEFS mean, the 00z ECMWF/CMC and their means form one cluster toward becoming stronger with the ridges aloft both upstream and downstream from the upper low that approaches the western mainland. The strength of the western Canada ridge that could extend into parts of the mainland and sharper flow over the Bering/central Pacific lead to a farther south trend for the upper low versus yesterday's consensus. At the same time this amplified scenario would delay any surface system that would potentially enter/cross the Bering compared to recent GFS runs, likely waiting until around day 8 Sun. In varying ways the 06z GEFS mean and 12z GFS display some of these trends versus their previous runs. Thus it seems reasonable to represent this trend in today's forecast, even if not quite to the degree of some operational runs given typical uncertainty by the days 7-8 time frame. Also some offsetting considerations exist--the overall amplitude of the eastern Asia through western North America pattern supporting the ECMWF cluster but teleconnections relative to the core of positive height anomalies to the south of the Aleutians in D+8 multi-day mean charts offering potential for a somewhat broader mean pattern. Based on considerations over the course of the forecast period, day 4 Wed started with a blend consisting of half 00z ECMWF and the rest 00z/12z GFS. After Wed the forecast incorporated gradually increasing 06z GEFS/00z ECMWF mean weight with time (more ECMWF mean vs GEFS Fri onward) while reducing operational model input so that day 8 started with the ensemble means exclusively. Rausch