Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 341 PM EDT Fri Apr 26 2019 Valid 12Z Tue Apr 30 2019 - 12Z Sat May 04 2019 Recent runs of model and ensemble guidance are increasing confidence in the idea that an upper high over the northern mainland at the start of the period early Tue will rapidly lift northward into the Arctic. As this occurs a weakness aloft may develop over the mainland and/or Gulf of Alaska. The weakness would consist of some combination of energy from within/around a Canadian upper low and/or Bering Sea-Aleutians energy approaching from the west. This type of evolution has low predictability so confidence regarding this developing weakness is much lower than for the upper high. Some signals toward North Pacific flow possibly becoming more progressive late in the period suggest low predictability as well. At the surface, the evolution aloft should transfer high pressure emphasis from the mainland into the Arctic and northern Canada while promoting lower pressures over the southern mainland and Gulf of Alaska. Regarding the upper high, today's clustering early in the period is much better than seen yesterday. After Wed there is still spread/run-to-run variability, such as 00Z ECMWF mean runs pulling the high farther north than 12Z runs and the new 12Z GFS positioning the high eastward of most other models/means. At the same time there is a wide variety of possibilities for what will become of the Canadian upper low and surrounding flow. Historically the most aggressive solutions for energy retrograding south of a mid-high latitude upper high tend to be overdone--favoring less weight of the 00Z ECMWF/CMC with time. On the other side of the spectrum the 12Z GFS is among the least retrogressive solutions. An intermediate forecast looks most reasonable at this time as the majority position of the upper high should promote at least some weakness to develop to the south--and push a cold front into the Panhandle/eastern mainland. Evolution across the Bering Sea and North Atlantic is also quite uncertain by mid-late period within/ahead of general mean troughing aloft. Thus far there has been better consensus toward the core of energy aloft remaining over the Bering versus the 12Z GFS that forms an upper low that slowly tracks south of the Aleutians. Another alternative is a North Pacific wave lifting into the Bering. Solutions are so diverse that a conservative approach closer to the ensemble means would appear best until better clustering emerges. In order to reflect consensus early in the period and then increasingly downplay uncertain specifics farther out in time, the forecast blend started with the 00Z ECMWF and 06-12Z GFS runs on Tue, followed by gradually increasing 06Z GEFS/00Z ECMWF mean input (and replacing the less favored 12Z GFS with the 00Z run after Wed) so that the means reached 70-80 percent total weight by days 7-8 Fri-Sat. Rausch Bullet points from the WPC Days 3-7 Hazards chart can be found at: https://www.wpc.ncep.noaa.gov/threats/threats.php - High winds across portions of the Aleutians, Mon, Apr 29. 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