Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EDT Wed Jul 31 2019 Valid 12Z Sun Aug 04 2019 - 12Z Thu Aug 08 2019 Guidance has continued to vary with the forecast of the strong system tracking across/near the extreme northern mainland during the weekend (late short range/early extended time frame). An episode of heavy precipitation and strong winds may accompany this system. After a significant faster trend through the 12Z/30 cycle, the majority cluster adjusted back somewhat to varying degrees of compromise. On a favorable note the new 12Z runs seem to exhibit less dramatic adjustments versus yesterday. There is still some spread with the 12Z ECMWF a bit on the slower side of the current envelope. The 06Z GFS was a northwestern extreme. The 00Z ECMWF and 12Z GFS/00Z CMC provided the best ensemble mean-supported starting point for this system, though questionable CMC particulars with approaching Arctic energy required phasing out its solution fairly quickly after the start of the forecast. For the overall Sun-Thu period the ensemble means agree fairly well that upper ridging should persist to the south of the mainland while a ridge also builds over the eastern Bering Sea/extreme western mainland, in response to troughing/possible embedded upper low reaching the western Bering. Aside from varying run-to-run details the ECMWF/CMC runs have generally been the most consistent in signaling this western Bering trough/upper low. The 12Z GFS is closest to the means and ECMWF/CMC in principle versus the 00Z GFS that was quite flat with Bering/mainland flow and 06Z GFS that was quite weak over the western Bering but very strong with the downstream ridge. The 12Z GFS is also strong with the ridge but reasonable comparison to the mean pattern merits minority inclusion in the forecast blend. Ahead of the western Bering trough upper low there is still a reasonable signal that a weak wave will track northward over the Bering. The front extending south from the wave should serve as a focus for rainfall over parts of the eastern Aleutians from the weekend into the first half of next week. Based on the above considerations the day 4 Sun forecast started with a blend of half 00Z ECMWF and the remainder 12Z GFS/00Z CMC. After phasing out the CMC the remainder of the period was based on lingering input from the 00Z ECMWF/12Z GFS along with the 00Z ECMWF/NAEFS means and 06Z GEFS mean. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php - Heavy rain across portions of mainland Alaska, Sat, Aug 3. - High winds across portions of mainland Alaska, Sat, Aug 3. - High winds across portions of mainland Alaska, Sun, Aug 4. 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