Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Tue Mar 20 2018 Valid 12Z Sat Mar 24 2018 - 12Z Wed Mar 28 2018 Already during the first half of the period guidance develops some fundamental differences in the pattern over the Arctic and mainland Alaska, with two general clusters evident. The GFS/GEFS/NAVGEM form one group which displays more of an omega block pattern with a narrow ridge. The GFS/GEFS in particular move the ridge rapidly northward leading to quicker arrival of westerly mean flow. On the other hand the ECMWF/ECMWF mean/CMC/UKMET show a stronger/southward closed high with retrograding upper low energy crossing the mainland leading to a rex block configuration. Recent ECMWF mean runs have trended noticeably stronger/southward with the upper ridge. This trend along with tendency for upper ridges to be slower/more persistent than at least the weaker side of the spread, plus yesterday's coordinated forecast favoring more persistent Arctic ridging, provide support for holding onto more Arctic ridging than GFS runs. There is still a lot of uncertainty for how much energy does retrograde across the mainland given guidance spread and occasional tendency for models to be overly aggressive with strength/speed of energy retrograding beneath upper highs. Therefore prefer a blended approach that maintains the integrity of the Arctic ridge but tones down the detail of any energy that retrogrades across the mainland. There is also meaningful uncertainty with northern Pacific/Bering flow. As a first consideration, recent GFS runs have been faster than most other solutions with a shortwave forecast to be emerging from eastern Asia at the start of the period. This difference likely plays a role in the forecast downstream with the majority of solutions tending to be somewhat more amplified than the GFS with the upper trough crossing the Bering and showing somewhat more phasing with short range northwestern Pacific energy. The majority evolution aloft leads to potential for a farther north track of the Aleutians system versus continuity but it is still a question to what extent. The 00z ECMWF/ECMWF mean made a fairly significant northward adjustment from previous runs while CMC runs have been showing a compromise between the ECMWF and more suppressed GFS. For a time the 12z GFS does show a farther north track than its previous runs. 00z ensembles include the 00z ECMWF solution but somewhat more members favor a track at least a little farther southward. Overall an intermediate solution looks best here. Toward the end of the period there is better than average model/ensemble agreement on the idea of an amplified central Pacific upper trough with best defined surface low pressure well south of the Aleutians by the end of day 8 Wed. Confidence is much lower in any leading surface lows such as what the past couple ECMWF runs bring into the vicinity of the southwestern Alaska Peninsula. Thus would like to dampen out this feature before it reaches 50N latitude. Guidance agrees on the existence of leading system over the Gulf of Alaska early in the period but with differences in strength/track. Ensemble means and other operational models generally suggest some combination of a better defined and/or farther north wave than some recent GFS runs. Combining considerations for each area/system of interest, the forecast blend started with input from the 00z and 12z/19 ECMWF, 12z GFS, and 00z CMC/UKMET early in the period. From mid-period onward the forecast included a steadily increasing weight of the 06z GEFS/00z ECMWF mean (higher percentage for the latter) so that day 8 Wed used the means exclusively. Rausch