Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 537 PM EDT Sun Sep 20 2020 Valid 12Z Thu Sep 24 2020 - 12Z Mon Sep 28 2020 ...Overview and Guidance/Predictability Assessment... An upper low in the Gulf of Alaska to begin the period (Thursday) will weaken in favor of an amplifying shortwave/deepening surface low crossing south of the Aleutians and into the Gulf by next weekend. Meanwhile, another upper low drops through the Bering Strait mid-late week, eventually establishing mean troughing over much of Alaska by early next week. There is decent agreement in the models on the overall large scale pattern, but differences in the details, especially in regards to timing/intensity of the shortwave crossing south of the Aleutians and an eventual surface low into the Gulf. The 12z GFS remains the fastest of the deterministic models with the shortwave, although the 12z ECMWF is a little faster than the previous 00z run. The ensemble means generally follow suit with their deterministic counterparts, but with slightly better agreement. For days 4-5, the WPC forecast relied on a majority deterministic model blend between the 12z GFS/ECMWF, though more weighting on the ECMWF which is closer to the ensemble means. After day 5, leaned increasingly heavier on the ensemble means (00z ECENS/12z GEFS) to mitigate the detail uncertainties. Continued a little weighting with the deterministic models even through day 8, just for some added system definition. ...Weather/Threats Highlights... Heavy rainfall and gusty winds across the Panhandle at the end of the short range should be waning to begin the medium range period, though showers may linger through the period across the southern Mainland/Panhandle region within an otherwise unsettled pattern. The next surface low should keep the heaviest rain south of the Aleutians, but heavy rainfall may again ramp up across the Panhandle/southern Alaska by the weekend/early next week as the system enters the Gulf. Mean troughing over much of the state, especially by late period, should keep temperatures near or below normal. Santorelli 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 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