Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 631 PM EDT Wed May 20 2020 Valid 12Z Sun May 24 2020 - 12Z Thu May 28 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, expanding through time, 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 Sun, with an elongated dissipating 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 enters the Gulf of Alaska Wed-Thu, which will spread precipitation and some gusty winds across the eastern Aleutians, the Alaska Peninsula, and portions of southern mainland Alaska by that time. ...Guidance/Predictability Assessment... Overall predictability during the extended forecast period across Alaska was 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 slight 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 Sun and the next one approaching on Wed-Thu. Ensemble spread was below average, and clustering of ensemble members surrounding the midweek low pressure system was quite good considering the time range. Confidence is increasing that a relatively deep low pressure system may enter the Gulf of Alaska by the middle of next week, although there remains some variability on the specific intensity and timing/track. Given these considerations a blend of the 12Z ECMWF/GFS was used as a forecast starting point during days 4-5 (Sun-Mon). After that, a gradual trend toward heavier ensemble mean weighting was shown during days 6-8 (Tue-Thu), 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 southern Alaska, Sat-Sun, May 23-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