Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 347 PM EDT Fri Oct 18 2019 Valid 12Z Tue Oct 22 2019 - 12Z Sat Oct 26 2019 The dominant focus of the forecast will be on two potentially significant storm systems within moderately progressive and amplified North Pacific flow, including some interaction with northern stream/Bering Sea flow. Most guidance shows the first system likely tracking somewhere between Bristol Bay and the Gulf of Alaska around Wed-Thu followed by another system tracking near the Aleutians Thu-Sat. The first system should bring a period of locally heavy rainfall from the extreme southern mainland into the Panhandle (possibly as far west as parts of the Alaska Peninsula) along with strong winds/high waves. The late week Aleutians system could produce areas of significant precipitation/wind/waves as well. The forecast of the leading system remains more uncertain than desired. Individual models and ensemble members are having difficulty in resolving exactly how or to what degree an emerging western Pacific shortwave/surface low will interact with northern stream/Bering Sea energy. On one end are the latest GFS runs and some earlier CMC runs that are strongest and farther westward toward Bristol Bay or at least the Alaska Peninsula. Other guidance, including the 00-12Z ECMWF/CMC/UKMET, is farther east to varying degrees. GEFS members generally lean toward the western operational run's track while 00Z ECMWF ensemble members are so diverse (including a few that keep the Pacific wave so suppressed that it does not even reach the Gulf) that the ECMWF mean has the weakest low pressure among all models/means. For the purposes of a single deterministic forecast the preference is to depict a track between the two ensemble means (discounting the fast/westward 00Z CMC mean representative of some early CMC runs) with surface low depth reaching down to around 980mb--a middle ground between the very deep GFS and the low probability weak/suppressed scenario. This provides decent track-timing continuity during the first half of the system's evolution. Then the latest cluster of guidance with the upstream Aleutians system would support greater downstream ridging aloft which would lead to quicker weakening of the Alaska Peninsula/Gulf system. As for the Aleutians system, GFS/ECMWF/CMC model runs and the ECMWF mean have been showing a well-defined feature for the past couple days or so, with typical spread for the details given the distant time frame. Latest GEFS/CMC means are starting to hint at a system but in much weaker form at the surface and aloft than any of the aforementioned cluster. Thus the preference is to use a blend of the operational runs and ECMWF mean to represent this system, yielding a deeper low than yesterday's forecast but with fairly similar track and timing. At higher latitudes there is general agreement with a weakness aloft (containing one or more embedded upper lows) along/north of the northern coast of the mainland. Some troughing may curl around into the northwestern corner of the mainland with time. A model/mean blend resolves the medium to smaller scale detail differences that have low predictability at extended time frames. Based on forecast preferences, today's blend started with the 06-12Z GFS and 00Z ECMWF/CMC through early Thu and then gradually increased 00Z ECMWF mean weight to yield an even split between the models and ECMWF mean by the end of day 8 Sat. 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, Wed-Fri, Oct 23-Oct 25. - High winds across portions of mainland Alaska and the Aleutians, Wed-Thu, Oct 23-Oct 24. - High winds across portions of the Aleutians, Fri, Oct 25. - High significant wave heights for coastal portions of mainland Alaska and the Aleutians, Wed-Fri, Oct 23-Oct 25. 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