Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 349 PM EDT Fri Sep 13 2019 Valid 12Z Tue Sep 17 2019 - 12Z Sat Sep 21 2019 As has been the case recently at times, guidance shows significant uncertainties with various aspects of the forecast--in today's case flow across and just north of the North Pacific as well as an upper low likely to spend some time over or near the mainland. Already by the start of the period early Tue, models/ensembles diverge considerably for the combined specifics of an upper low forecast to be just south of 40N 170E as of early Sun and northern stream shortwave energy reaching the western Bering Sea by Mon. This leads to major differences for evolution of a possible Aleutians wave. One cluster (initially just the GFS but now joined by the 12Z UKMET/CMC) is sufficiently phased aloft to bring low pressure northeast into the southwestern/central mainland. On the other side of the spectrum the ECMWF (both 00Z and new 12Z run) is so flat and progressive with the northern stream that surface waviness is suppressed and fast. The 00Z UKMET/CMC and now the 12Z GEFS mean (06Z GEFS leaned somewhat more toward the operational GFS) were between the two extremes. Based on guidance available through arrival of the 12Z GFS the preference was for such an intermediate solution with a surface low track from the Aleutians to near Kodiak Island and the Gulf of Alaska. The next system in the series originates from an area of disturbed weather initially south-southeast of Japan (identified as 98W per the Joint Typhoon Warning Center) and possibly another disturbance to the southwest (95W). The extratropical evolution of the system should reach the Aleutians/southern Bering by day 6 Thu. Agreement in principle is better than might sometimes be expected considering the time frame and the fact the originating system(s) is/are merely in the developing stage at this time. The final system of the forecast should come into the picture over the western Bering Sea by day 8 Sat, in response to upper support that emerges from eastern Asia earlier in the period. The ensemble means currently agree remarkably well and most operational runs show low pressure in the vicinity albeit with typical difference in the details. Overall a compromise between the GEFS and ECMWF means provided the best starting point for the upper low expected to meander over or near the mainland during the period. Operational runs display a fair amount of spread/run-to-run variability for specifics in light of the possibility that more than one piece of energy may influence the position of the upper low at any particular time. Recent GFS runs have been somewhat on the northern side of the full guidance spread with the upper low. Based on forecast considerations the forecast started with components of the 00Z ECMWF/UKMET/CMC and 06Z GEFS/00Z ECMWF means early in the forecast, followed by increased ensemble mean emphasis with time (70 percent by day 6 Thu and 100 percent for day 8 Sat). There were some manual adjustments as appropriate. The most coherent model and ensemble signal for enhanced precipitation during the five-day period exists from near the Kenai Peninsula to the Panhandle with one or more low pressure centers (or at the very least cyclonic flow) forecast over the Gulf of Alaska. There is greater uncertainty regarding precise totals across the Aleutians/Peninsula and farther north into the mainland but some embedded relative maxima are likely. 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 the central Aleutians, Tue, Sep 17. - Heavy rain across much of southwestern Alaska, Wed, Sep 18. - Heavy rain across portions of the Alaska Panhandle into southern Alaska, Wed-Thu, Sep 18-Sep 19. 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