Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 228 PM EST Sat Feb 16 2019 Valid 12Z Wed Feb 20 2019 - 12Z Sun Feb 24 2019 Most guidance continues to advertise a transition in the pattern from progressive flow to a more amplified trough-ridge regime. Thus the series of Pacific/Bering Sea systems reaching the mainland/Gulf of Alaska (with accompanying enhanced precipitation/wind) extending into the early part of the days 4-8 forecast should give way to somewhat drier weather over the mainland and vicinity next weekend. At the same time low pressure should tend to remain over the western Bering Sea while leading wavy front(s) crossing the Bering and Aleutians serve to focus moisture/precipitation and potentially strong winds. Early in the period the 12Z GFS and 00Z ECMWF/UKMET/CMC provide decent agreement for the system reaching the eastern Bering Sea by Wed, with a northeastward track into Thu while a triple point wave tracks across the northern Gulf. Using a blend of these solutions essentially yields a stronger version of yesterday's continuity. From Thu onward the 06-12Z GFS stray from the majority of guidance, as those GFS runs bring another shortwave aloft/surface system completely across the Bering. Only a small number of individual ensemble members suggest the 06-12Z GFS scenario while remaining ensembles and other operational models (including the 00Z ECMWF/UKMET/CMC/NAVGEM) show more ridging aloft over the Bering thus holding low pressure westward--consistent with established continuity. Thus heading into late week the GFS input switches from the 12Z run to 00Z run while the forecast begins to incorporate the ensemble means. Along the trailing front there is a decent model/ensemble signal toward a strong wave tracking from the western Pacific into western Bering Thu-Sat, likely becoming the dominant low by the start of the weekend. Additional frontal waviness appears likely as the front slowly pushes eastward Sat-Sun but with much lower confidence on timing/strength so manual forecasts are conservative with this aspect of the forecast. Farther eastward, during the latter half of the period there is an ongoing trend toward greater strength for the ridge that amplifies into the mainland and Arctic. The ECMWF mean has generally been leading this trend compared to GEFS mean runs. On the other hand GEFS means are somewhat deeper than the ECMWF mean for western Bering low pressure by next weekend. This favors an even balance among the ECMWF/GEFS means. Also the 00Z GEFS run is closer to the ECMWF mean strength for the upper ridge versus the 06Z run, favoring the 00Z run for GEFS input. Minority input of the 00Z GFS/ECMWF (30-40 percent total) late in the period ultimately helps to account for the aforementioned stronger trend. Around the end of the forecast period and beyond it will be worth watching to see if the ridge becomes sharp/amplified enough to allow for undercutting mid-latitude flow. Across the Arctic the flow details remain somewhat ambiguous before the late period ridge builds into the higher latitudes. Mid-late week Arctic flow is moderately progressive from a mean perspective but over the past day ECMWF/GFS runs have flipped to some degree regarding amplitude of high latitude shortwave energy and track of anchoring upper low. A blended approach provides the best starting point given the uncertainty in specifics. Rausch WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html