Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 347 PM EDT Sat Mar 23 2019 Valid 12Z Wed Mar 27 2019 - 12Z Sun Mar 31 2019 As was the case yesterday there are some common themes in the forecast of large scale pattern evolution. These include a southward or southeastward drift of an upper high likely centered over the southeast corner of the mainland early Wed followed by another ridge building over and/or northwest from the southwestern mainland by next weekend, and a far western Pacific upper trough (anchored by a low near Kamchatka) that should elongate eastward just south of the Aleutians next weekend. However operational models and individual ensembles go astray with some aspects of the forecast fairly early in the period with differences persisting or increasing with time. The resulting low confidence in any specific forecast solution requires today's forecast to include more ensemble mean guidance than usual early in the period (about half) and exclusively means by next weekend. One major point of contention continues to be with details of shortwave energy initially over the Bering Sea/Aleutians on day 4 Wed and any upstream energy. The combined fast/strong depiction of the upstream shortwave in 00-06Z GFS runs (reaching the Bering on Thu) still seems to be overdone. The GEFS mean has trended somewhat to the GFS idea early but then dampens the feature more by Fri. The 12Z GFS has also trended weaker to add support for toning down the earlier GFS scenario. On the other hand slow upstream progression in the ECMWF/CMC and ECMWF mean lead to a slow moving trough near the western coast of the mainland (in the former two) or moving across the mainland (the latter). The ECMWF mean idea of having a trough crossing the mainland is rather new compared to earlier runs and not very well supported by other guidance or multi-day means which have been showing a very strong core of positive height anomalies centered near the southern coast of the mainland. There are yet other solutions including the 00Z UKMET/12Z GFS which use some of the Bering/Aleutians energy to form a compact upper low near the Alaska Peninsula--another very low confidence scenario. By next weekend the ECMWF and to a lesser degree ECMWF mean/CMC introduce another major difference, pulling the building western ridge far enough northwestward to allow height falls/surface low pressure and moisture to extend into the northeastern Pacific--in contrast to other solutions that show stronger ridging over/near the mainland and keep the northeastern Pacific pattern more suppressed. At the very least the ECMWF mean is not as extreme as latest operational ECMWF runs but it still represents a meaningful change from previously established consensus. The corresponding CMC mean is much closer to the latest GEFS means. Also during next weekend, the ensemble means are somewhat more agreeable among each other with the overall evolution of upper troughing and surface low pressure stretching eastward just south of the Aleutians. Not surprisingly the operational models vary considerably for specifics across the western 2/3 of the North Pacific. Forecast considerations and the desire to reflect as much continuity as possible in light of a very uncertain forecast led to blending the 00-06Z GFS/06Z GEFS and 00Z ECMWF/ECMWF mean early in the period followed by a transition to 2/3 total weight of the 06Z GEFS/00Z NAEFS means versus the 00Z ECMWF mean. Adjustments to the initial blend led to decreasing precipitation probabilities next weekend along the southern coast of the mainland to reflect better the GEFS/NAEFS emphasis. Typically low bias of ensemble mean wind speeds led to some manual increase of wind but balanced against the high uncertainty of speed/direction at any particular location and valid time. Rausch Bullet points from the WPC Days 3-7 Hazards chart can be found at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html