Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 648 PM EDT Tue May 19 2020 Valid 12Z Sat May 23 2020 - 12Z Wed May 27 2020 ...Overview and Weather Hazards/Threats... The large scale flow pattern across Alaska is expected to remain relatively unchanged through the extended forecast period. An upper-level ridge is forecast to persist from the Arctic Ocean across eastern Alaska and into northwest Canada, resulting in above average temperatures (high temp anomalies of +5 to +15 deg F) across much of the Interior. Farther south, a broad region of cyclonic flow will remain in place across the North Pacific from near/south of the Aleutians east to the Gulf of Alaska, with mid/upper-level shortwave energy amplifying, and associated surface low pressure systems deepening, as they pass through this broad region of cyclonic flow. One such low pressure system is forecast to slowly weaken across the eastern Aleutians and southeastern Bering Sea Sat-Sun, with an elongated occlusion extending east into the Gulf of Alaska. Widespread and potentially heavy precipitation associated with this system is forecast across portions of southern mainland Alaska through the weekend. Precipitation should linger into Monday while gradually diminishing in coverage as the front dissipates, however persistent low-level onshore flow even after the front dissipates will keep precipitation in the picture, especially for mountain ranges in southern mainland Alaska where upslope flow will persist. This pattern of general onshore flow should persist through Tue, before another shortwave and low pressure system approach, forecast to deepen as it passes well south of the Aleutians and approaches the Gulf of Alaska Tue night/Wed, which will start to spread precipitation and some gusty winds across the eastern Aleutians and the Alaska Peninsula by that time. ...Guidance/Predictability Assessment... Overall predictability during the extended forecast period across Alaska was at least slightly above average, at least due in part to a relatively stable and somewhat blocky large scale flow regime. The 12Z ECMWF/GFS showed relatively similar solutions through most of the forecast period, with some differences as to the orientation/intensity of the upper-level ridge axis. Farther south, the GFS and ECMWF were similar in the timing of the low pressure systems, both the initial one Sat-Sun and the next one approaching on Wed. Ensemble spread was perhaps a bit below average, but forecast confidence is not quite as high as the GFS/ECMWF similar solutions would suggest, as both models have shown some degree of run-to-run variability. Given these considerations a blend of the 12Z ECMWF/GFS was used as a forecast starting point during days 4-5 (Sat-Sun). After that, a gradual trend toward heavier ensemble mean weighting was shown during days 6-8 (Mon-Wed), with majority ensemble means on days 7-8. 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 mainland Alaska, Fri-Sun, May 22-May 24. 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