Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 342 PM EDT Tue Apr 30 2019 Valid 12Z Sat May 04 2019 - 12Z Wed May 08 2019 The large scale pattern across Alaska will remain relatively unchanged through the extended forecast period. A mean trough will slowly progress eastward across the Aleutians through the period, with somewhat transient upper ridging centered across the Gulf of Alaska, and a more persistent ridge of high pressure farther north in the Arctic Ocean. This flow pattern results in a constant stream of shortwave energy crossing near/south of the Aleutians and moving into mainland Alaska, with additional shortwave energy rotating around the southern periphery of the arctic ridge affecting the North Slope. The WPC forecast was initially based heavily on the ECMWF during days 4-5 (Sat-Sun), with gradually increasing weight placed on the ECENS/GEFS ensemble means during days 6-8 (Mon-Wed). The ECMWF idea of at least periodically breaking down the northern extent of the Gulf ridge in response to passing shortwave energy remains the preferred solution at this point during the early portion of the forecast period. Teleconnections associated with some of the stronger hemispheric 500 hPa height anomalies do not show significant support for a strong Gulf of Alaska ridge, leading to the conclusion that at least a temporary/periodic weakening of the ridge northern extent seems feasible. An occluded low pressure system is forecast to slowly move east across the Aleutians Sat-Mon, moving into the southeastern Bering Sea by Tue. Models/ensembles show general agreement on this scenario. Differences begin to increase Tue-Wed once again with respect to the Gulf ridge. The ECMWF/CMC and ensemble means support rebuilding the ridge somewhat in response the further upstream trough amplification across the Bering Sea/Aleutians. The GFS differs, weakening the ridge and depicting flatter westerly flow, which would direct more shortwave energy eastward into Southeast Alaska and British Columbia. Also prefer the ECMWF/CMC/ensemble idea here, at least partially restrengthening the ridge and keeping the stream of shortwave energy directed more toward mainland Alaska. The interior of Alaska will remain in a rather chaotic regime in between the north Pacific upper-level easterlies and arctic easterlies. Potential interactions between small-scale features in the two streams introduce additional complications and reduce predictability when it comes to the specific details. The preferred blend should, however, at least provide a broad representation of the forecast for that area. Ryan Bullet points from the WPC Days 3-7 Hazards chart can be found 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