Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Sun Sep 11 2022 Valid 12Z Thu Sep 15 2022 - 12Z Mon Sep 19 2022 ...Typhoon 15W is forecast to become extratropical and move across the Aleutians late this week... ...Lingering low pressure system centered near eastern Bering Sea/western mainland will likely bring windy and unsettled weather across much of western and southern half of Alaska into early next week... ...Guidance/Predictability Assessment... Uncertainty remains abound in the vicinity of the Aleutians late this week as models continue to show great difficulties in resolving the interaction between tropical cyclone 15W, currently located well to the northeast of Guam, and various baroclinic waves that are forecast to move offshore from eastern Siberia. The GFS/GEFS agrees pretty well with the CMC/CMC mean by taking the cyclone on a faster track across the Aleutians on Thursday, whereas the ECMWF has been distinctively slow due to more robust interactions of the cyclone with the baroclinic waves. However, the ECMWF has been progressively faster with the cyclone in each successive run. An intermediate solution is adopted with a preference toward the faster solutions. For the Bering Sea low on Thu-Fri, the ECMWF has been far to the southeast than other guidance. However, the latest (12Z) EC shifted the low farther northwest while the GFS shifted it farther east over the western Bering Sea. A blend of the two models should yield a good starting point. Despite the rather poor model agreement on Days 4-6, ensemble means resolve themselves and attain rather good agreement by Day 8 when a broad cyclone centered over the eastern Bering Sea is forecast to drift east into western mainland Alaska. The WPC medium-range forecasts were composed based on 40% from the 12Z ECMWF/00Z or 12Z EC mean, 40% from the 12Z GFS/GEFS and 20% from the 12Z CMC and CMC mean, transitioning to mostly a blend of the ensemble means by Day 8. ...Pattern Overview and Weather/Hazards Highlights... The forecast period will begin on Thursday with a Bering Sea cyclone bringing windy and rainy conditions eastward across the Aleutians toward western mainland. In the meantime, a robust extratropical cyclone originated from typhoon 15W is forecast to track across the Aleutians around late Thursday to early Friday with gale force winds and heavy rain possible. The two moisture plumes associated with the cyclones could merge over the western half of Alaska on Friday with widespread heavy rain and heavy snow possible in the higher elevations. This broad cyclone will likely continue to bring windy and unsettled conditions for at least the western half of mainland into early next week, with occasional intrusion of impulses sliding eastward across the Aleutians toward the Gulf of Alaska, bringing rounds of unsettled weather at times into the southern coastal section and the Alaska Panhandle. The driest portion of Alaska appears to be the North Slope where rain chances are minimal despite the presence of a stationary front along the Brooks Range. Kong Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php 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