Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Fri Aug 09 2019 Valid 12Z Tue Aug 13 2019 - 12Z Sat Aug 17 2019 Over the past 24 hours the most prominent trend in the guidance is that the ensemble means show a stronger ridge building into the eastern Bering Sea and vicinity, leading to increasing amplitude within an already blocky pattern. This adjustment leads to a greater northerly component of flow aloft over the mainland and Gulf of Alaska by the latter half of the period while holding back strong western Pacific low pressure that had been expected to track into the western Bering Sea by late week. Individual operational model runs over the past couple days have hinted at the potential for a fairly strong ridge but with differences/variability in longitude. Overall recent ECMWF/ECMWF ensemble mean runs have been closer to the current clustering than some GFS/GEFS mean runs that were somewhat farther east. The new 12Z ECMWF has shifted the ridge eastward though. Prefer to update the forecast with a model/mean blend to reflect the latest guidance trends, while remaining less aggressive than the ECMWF/CMC runs that have the strongest ridge. Newest GFS runs are weaker than the 12Z/08 run so confidence is not quite high enough to go with the strongest side of the spread yet. Meanwhile predictability is low for small scale details within/around the upper low forecast to be northeast of the mainland. At the moment there is a common signal that energy associated with a compact upper low initially over the western Canadian Archipelago will loop around the larger scale circulation during the short range and track over/near the northern mainland by next Tue-Wed. A blend emphasizing operational guidance reflects this detail better than the means but specifics will likely change to some degree over future runs. Forecast considerations led to starting out with a blend tilted 80 percent toward the 12Z GFS and 00Z ECMWF/UKMET for day 4 Tue into day 5 Wed, with the rest consisting of the 06Z GEFS/00Z ECMWF means. Then the forecast quickly trended to 50-60 percent ensemble weight while holding onto some GFS/ECMWF input. During the first half of the period portions of the southern half of the mainland may see an episode of enhanced precipitation with a band of moisture associated with shortwave energy aloft and a wavy cold front crossing the extreme southern mainland/Gulf of Alaska. The pattern over the northern mainland may also promote some locally significant precipitation but specifics are more unclear. The western Pacific storm tracking toward the western Aleutians is slower than in yesterday's guidance but there is still reasonable consensus on the potential for the leading gradient to generate a band of strong winds over parts of the Aleutians and Bering Sea, with some focused rainfall possible as well. 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 interior Alaska, Tue-Wed, Aug 13-Aug 14. - 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