Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 PM EDT Tue Aug 07 2018 Valid 12Z Sat Aug 11 2018 - 12Z Wed Aug 15 2018 The dominant focus of the forecast--both in terms of weather effects and detail uncertainty--involves the extratropical evolution of Typhoon Shanshan. This system may bring a period of heavy precipitation and strong winds to locations from the Aleutians/Bering Sea into Mainland Alaska. In the short range time frame the Joint Typhoon Warming Center track has Shanshan brushing Japan and then recurving northeastward into the northwestern Pacific. Once it makes its extratropical transition there is still significant uncertainty regarding exactly how northern stream flow will interact with the system, affecting strength and track. Looking at guidance through the arrival of the 12Z GFS, it appears that the 00Z ECMWF may be too weak/suppressed relative to continuity and recent GEFS/ECMWF ensemble means. Tendency toward stronger rather than weaker development when there is stream interaction also favors leaning away from the 00Z ECMWF scenario. The individual GEFS/ECMWF individual members are still very diverse but the mean track, through the Bering Sea and into the mainland, seems to provide a good consistent basis for that aspect of the forecast. This leaves the 12Z GFS a bit south/southeast for a time. Overall prefer the 00Z-06Z GFS and 12Z/06 ECMWF as the best operational input for the forecast with only modest ensemble mean inclusion early in the period. This produces a low pressure system with depth between the GFS runs and much weaker ensemble means. Most guidance has adjusted noticeably faster from 24 hours ago so this blend captures the trend. Other notes... The CMC has had difficulty in properly resolving/tracking Shanshan near Japan so its forecasts would be considered less reliable. Meanwhile the CMC and parallel (FV3) GFS runs show much stronger downstream ridging aloft versus other guidance. This is certainly plausible assuming a strong Bering Sea system but their surface solutions are sufficiently different from other guidance and continuity to preclude their use in the forecast. Behind this system there is good consensus regarding high pressure building into the Aleutians/Bering Sea. Then the 12Z GFS becomes somewhat faster than consensus with low pressure that may emerge into the northwestern Pacific toward the end of the period. The favored forecast blend was adjusted to reflect a better-detailed compromise of an upper low expected to be near the Bering Strait as of early day 4 Sat. This area has high sensitivity in terms of steering flow as the 06Z GFS and to a greater extent 12Z GFS kick the low eastward across the northern mainland while the ECMWF/CMC hold it westward and incorporate it into the evolving northeast-southwest mean trough aloft. So far there are no clear trends recommending one side or other of this broad envelope. Once extratropical Shanshan moves inland and weakens there is decent agreement in principle for gradual eastward progression of the northeast-southwest mean trough aloft. The 06Z GEFS/00Z ECMWF means represent this well and agree for the upstream pattern so the forecast increased total ensemble weight to 50-70 percent later in the period. 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