Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 330 PM EDT Thu Aug 08 2019 Valid 12Z Mon Aug 12 2019 - 12Z Fri Aug 16 2019 Guidance still depicts an amplified/blocky pattern with latest GEFS/ECMWF/CMC ensemble means fairly similar and consistent in principle through days 4-8/Mon-Fri. The ensemble mean cluster shows one closed low just north/northeast of mainland Alaska while an initial Bering Sea closed low is replaced by an upstream western Pacific system that tracks into the western Bering late in the week. Meanwhile upper ridging should rebuild for a time over the northeast Pacific and late in the period an eastern Bering/western mainland ridge should strengthen ahead of the western Pacific/Bering storm. In spite of the relative agreement among the means, the Arctic upper low has exhibited a lot of spread and run-to-run variability in operational model runs. ECMWF runs have been somewhat less erratic than GFS runs over the past couple days. The spread in the guidance keeps confidence lower than desired for resolving exactly how far southward cooler air will push in response to shortwave energy within/around the upper low. The current array of guidance suggests a compromise upper low path leaning a little more toward (but not completely to) the western ECMWF versus the eastern GFS. A moderate eastward adjustment in the new 12Z ECMWF supports this idea. Farther southwest there is lingering uncertainty over exactly what will happen with energy from the initial Bering Sea system. At the very least the 00Z CMC becomes one of the less probable solutions in bringing nearly all of the upper low energy across the mainland/Gulf of Alaska by Wed-Thu. However there is some degree of consensus that flow ahead of this system, as well as the front pushing southward into the northern mainland, may support areas of enhanced precipitation. In contrast to the detail uncertainty from the eastern Bering through the mainland and parts of the Arctic, thus far the models/means have been more agreeable and consistent regarding the system expected to deepen over the western Pacific and eventually track into the western Aleutians/Bering Sea. This relative agreement allows for maintaining some inclusion of operational guidance through the end of the period in order to provide some enhancement of the means. The new 12Z ECMWF adds a question mark though, splitting the system into two separate waves. The gradient ahead of this storm may produce an area of strong winds over parts of the Aleutians/Bering Sea. Downstream the guidance agrees on a strengthening upper ridge over the eastern Bering and western mainland but differs on strength/position. Some recent GFS runs including the 12Z version have been on the strong and/or eastern side of the spread for the ridge. The 06Z GFS is also a bit on the stronger side but with an axis aligned closer to consensus. Based on the guidance evaluation today's forecast started with an operational model blend (ECMWF/GFS/UKMET/CMC) for day 4 Mon and then gradually increased ensemble mean input so that the means provided 60 percent weight by days 7-8 Thu-Fri. The 12Z run was reasonable for the GFS component during the first couple days but the 06Z version compared better to remaining guidance later in the week, requiring a mid-period transition for GFS input. 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 - High winds across portions of the Aleutians, Wed-Thu, Aug 14-Aug 15. - High significant wave heights for coastal portions of the Aleutians, Wed-Thu, Aug 14-Aug 15. 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