Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 348 PM EDT Thu Oct 10 2019 Valid 12Z Mon Oct 14 2019 - 12Z Fri Oct 18 2019 Most guidance shows a very strong northern Pacific stream to the south of an axis of low heights aloft (containing multiple upper lows) from the Bering Sea into northeastern Pacific/Gulf of Alaska. Meanwhile an initial upper ridge over the mainland should drift to the north or northwest of the mainland by midweek. The overall pattern should favor highest five-day precipitation totals from the eastern Alaska Peninsula and Kodiak Island to the Panhandle with lower but still meaningful amounts across the Aleutians. The most prominent system of interest will be the extratropical evolution of current Super Typhoon Hagibis, likely reaching the western Aleutians by early day 4 Mon and tracking generally eastward thereafter. There are still track and timing uncertainties for this system which will likely produce an episode of strong winds and high waves especially to the south of the low track. Through the 00Z cycle recent trends/ECMWF continuity led to a majority guidance cluster showing a track near the Aleutians and then into the northeastern Pacific--versus a farther north path into the Bering per some earlier ensemble means. However 06Z-12Z guidance is a little more mixed with the GFS and 12Z UKMET having adjusted slower/northward. The 12Z GEFS mean still shows faster timing than the 12Z GFS/UKMET after early Tue though, and the new ECMWF/CMC maintain the faster/south scenario. Based on data available through the arrival of the 12Z GFS, the initial blend started with the 00Z GFS/ECMWF/UKMET that represented the best cluster to provide reasonable detail. Even an intermediate approach yielded a surface low deepening below 960mb near the eastern Aleutians by early day 5 Tue. Stemming in part from differences with extratropical Hagibis earlier in the period, the operational models and ensemble means differ for overall low pressure expected to settle over the northeastern Pacific/Gulf of Alaska by mid-late week. Preferred to trend the forecast toward a model/mean blend (00Z GFS/ECMWF and 06Z GEFS/00Z ECMWF mean) given decreasing confidence in any particular solution for that time frame. Farther upstream some guidance is signaling potential for a system reaching the Bering by day 6 Wed. The ECMWF had been fairly consistent for multiple cycles through the 00Z run while the less consistent GFS agreed in its 00Z run (as did the CMC), offering support for a fair amount of operational model input to enhance the ill-defined means. The full array of 12Z guidance lowers confidence in specifics as the ECMWF/UKMET are weaker and faster. Then another area of general low pressure may develop over the Bering/Pacific by day 8 Fri as an upper trough develops over the western Pacific. Upper flow over the mainland will tend to lack definition (with considerable uncertainty in the fine-scale details) between the ridge that settles into the Arctic and upper lows over the Bering and northeastern Pacific. New 12Z guidance suggests the upper ridge could be a little stronger than the 00-06Z model/model-mean blend that served as the basis for today's forecast. 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 Alaska Hazards... - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Oct 15-Oct 16. - High winds across portions of the Aleutians, Mon-Wed, Oct 14-Oct 16. - High significant wave heights for coastal portions of the Alaska Panhandle, Thu, Oct 17. - High significant wave heights for coastal portions of the Aleutians, Mon-Wed, Oct 14-Oct 16. 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