Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 556 PM EST Wed Feb 19 2020 Valid 12Z Sun Feb 23 2020 - 12Z Thu Feb 27 2020 The large scale flow pattern across Alaska and the North Pacific will undergo some degree of modulation during the extended forecast period, transitioning from a relatively progressive pattern to one with at least some degree of blocking. Increased atmospheric blocking around the northern hemisphere next week is expected to result in a relatively strong positive height anomaly center across northeast Asia, and a negative height anomaly center across northern Europe. Teleconnections associated with both of these strong hemispheric anomalies support development and persistence of a mean upper-level trough across the Aleutians/Bering Sea, and some degree of ridging across western Canada. This setup is in fact what appears in the ensemble means as we look forward to next week, despite quite a bit of variability on the details of individual low pressure systems. A blend of the 12Z ECMWF/06Z GFS was heavily used as a basis for the forecast during days 4-5 (Sun-Mon), as these solutions showed relatively good consensus with each other and with ensemble means. A compact low pressure system is forecast to cross the southeastern Gulf of Alaska and move inland across British Columbia on Sun, potentially bringing a period of enhanced precipitation to portions of Southeast Alaska. As upper-level ridging begins to build by Sun-Mon, heights should briefly rise across much of mainland Alaska before the ridge axis shifts east into its favored position across western Canada and the aforementioned mean trough/upper low sets up across the Aleutians. A weakening low pressure/frontal system is forecast to approach the Alaska Peninsula and southern mainland Alaska on Tue, bringing a round of potentially heavy precipitation and high winds, with another system perhaps approaching by Thu. Deterministic solutions differed by the middle of next week as to the strength of the ridge across western Canada, with the GFS allowing a strong North Pacific jet to break down the ridge, and the ECMWF keeping a stronger ridge in place. Ensemble means support some moderate degree of ridge persisting, which as mentioned is also supported by hemispheric teleconnections. Additionally, rapidly increasing spread among deterministic solutions by the early to middle portion of next week with respect to the track/intensity of individual low pressure systems warranted increasingly heavy use of ECENS/GEFS ensemble means during the day 6-8 (Tue-Thu) time period. Ryan 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 the Alaska Panhandle and mainland Alaska, Tue-Wed, Feb 25-Feb 26. - High winds across portions of mainland Alaska, Tue-Wed, Feb 25-Feb 26. - Much below normal temperatures across portions of mainland Alaska, Sat, Feb 22. 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