Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 344 PM EDT Tue Sep 03 2019 Valid 12Z Sat Sep 07 2019 - 12Z Wed Sep 11 2019 For the large scale pattern the majority of guidance shows a mean trough centered over the Bering Sea into the North Pacific and a mean ridge over mainland Alaska/northwestern Canada. There is significant spread/uncertainty for embedded features--in particular trough energy heading into the mean ridge, and toward the latter half of the period for specifics of low pressure that should have some input from one or more tropical systems. A leading system should be approaching the southwest mainland at the start of the period very early Sat. Even with the relatively shorter lead time there are meaningful differences for how upper support splits and relative strength/position of low centers over the eastern Bering and northern Pacific/Gulf of Alaska. The scale is sufficiently small (for lower than average predictability) that a consensus blend provides the best starting point for the forecast. From Sun onward there is a widening array of possible solutions for the system that starts the period over/near the western Aleutians. Based on the current array of guidance and continuity, the 00Z ECMWF's idea of bringing a fairly strong shortwave progressively across the mainland by Tue appears to be a less probable option. At least the new 12Z run has trended more sheared with the shortwave. Otherwise there is still the significant difference between the ensemble means that have a slow-moving open trough and GFS/CMC that undercut the mainland Alaska/northwest Canada ridge closed low (with important latitude differences determining how much precipitation reaches the Alaska Peninsula/Kenai Peninsula). Especially late in the period prefer leaning toward the ensemble means to represent the favored non-00Z ECMWF scenario for now given the uncertainty in the upstream forecast. During the first half of the period most guidance shows an interaction of mid-latitude flow over eastern Asia and northward-tracking Typhoon Lingling. The best clustering of guidance suggests that low pressure related to this interaction may reach the vicinity of the western Bering Sea by day 8 Wed. Also worth noting to the east is Tropical Storm 14W whose track should approach Japan in a few days per the 12Z JTWC advisory. Most solutions suggest this feature will be absorbed into the larger scale upper trough/surface low. Models/ensembles typically have difficulty resolving mid-latitude/tropical interactions and in this case there are two tropical systems involved, so there is definite the potential for significant changes in the forecast over coming days. Based on the above considerations today's forecast started with a primary emphasis on operational model solutions from early day 4 Sat into early day 6 Mon. Then the forecast rapidly transitioned toward greater ensemble mean weight (more 00Z ECMWF mean versus 06Z GEFS/00Z NAEFS to provide better continuity) and downplaying the 00Z ECMWF over the mainland. While details remain uncertain, the combination of systems originating from the Bering Sea upper trough should promote highest five-day precipitation totals over the southwest corner of the mainland and Kenai Peninsula. It will take additional time to determine wind/rain effects over the Aleutians and Bering Sea by next Tue-Wed. 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 precipitation across portions of mainland Alaska and the Aleutians, Fri-Sat, Sep 6-Sep 7. 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