Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 607 PM EDT Tue Sep 29 2020 Valid 12Z Sat Oct 03 2020 - 12Z Wed Oct 07 2020 ...Heavy rain possible Sunday for portions of coastal Southcentral to the Coast Ranges... ...Overview and Guidance/Predictability Assessment... Upper low is forecast to slowly move eastward from around southwestern Alaska/Bristol Bay this weekend to the Gulf by the middle of next week. Several embedded disturbances will rotate through/around the upper low, deforming it as additional shortwaves drop in from the northwest. The 12Z guidance offered a reasonable starting point for the forecast, transitioning quickly toward the 12Z GFS/ECMWF and their ensemble means for the remainder of the period. After passage of a robust system in the short range, next system will be another well-defined, but probably not as deep, area of low pressure that will likely pass just south of Kodiak toward/into Prince William Sound on Sunday. 12Z Canadian/UKMET drifted away from the tightly-paired 12Z GFS/ECMWF (ensemble means just west) by Mon and their inputs were phased out. Front will lift into NW Canada by early Mon. Next system from the northwestern Pacific will enter the far western Bering around next Tue. By then, the upper pattern will start to flatten, allowing zonal flow to increase. ECMWF ensembles were still much quicker than even the 12Z ECMWF/GFS, though the Canadian was just about as fast (though much different in northern stream flow). Maintained a relatively steady pace on par with the forecast yesterday, near a 12Z ECMWF/GFS middle ground. Uncertainty in how the downstream trough/upper low exit (quicker or slower) will dictate how quickly the pattern can flatten. ...Weather/Threats Highlights... Low pressure will spread modest to locally heavy rainfall over southeastern-facing areas of Kodiak and the Kenai Peninsula toward Prince William Sound eastward to Yakutat Sunday, gradually diminishing as showers on Monday. Several inches of rain are possible along with breezy conditions. Less rainfall is forecast over the mainland after Monday, but will increase over the western and then central Aleutians next Tue/Wed. Temperatures are forecast to be near to well above normal over most of the mainland in southerly flow. Near to below normal temperatures are forecast for the Aleutians under the upper low early in the period and over the Panhandle. Fracasso 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 Hazards: - Heavy precipitation across portions of southern Alaska to the Alaska Panhandle, Sun-Mon, Oct 4-Oct 5. 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