Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 533 PM EST Wed Dec 04 2019 Valid 12Z Sun Dec 08 2019 - 12Z Thu Dec 12 2019 ...Heavy precipitation expected across southern mainland Alaska... Models and ensembles showed general consensus at the large scale across Alaska during the extended period. Upper-level ridging should persist/expand from western Canada into eastern mainland Alaska. Meanwhile, an active and relatively progressive flow pattern across the North Pacific will help to reinforce and amplify a mean trough/upper low near/south of the Aleutians. Despite relatively good large scale agreement, the details of specific systems become highly uncertainty past days 4-5 (Sun-Mon), with a relatively high degree of run-to-run variability and inconsistency among deterministic solutions. Sun-Mon, models showed some consensus that a low pressure system should approach the Alaska Peninsula with a surface front crossing the Gulf. Deep southerly flow associated with this system east of the upper trough axis will transport significant amounts of Pacific moisture northward, resulting in heavy precipitation potential for portions of southern mainland Alaska. The GFS suggest the potential for another deep wave of low pressure to develop along the surface front across the Gulf of Alaska Mon night, moving north toward southern mainland Alaska on Tue. Confidence in this scenario is low to to high run-to-run variability, but this scenario would further enhance heavy precipitation potential across southern mainland Alaska if it were to occur. Ensembles show consensus that the next deep surface low should move east near/south of the Aleutians Mon-Tue, approaching the Alaska Peninsula by Wed-Thu. Deterministic solutions diverge significantly on the potential placement, timing, and intensity of any potential triple point lows across the Gulf during this time frame. While development of such a wave of low pressure seems likely given the scenario, the poor consensus makes for very low confidence on the specifics at this time. Given these considerations, the WPC forecast was initially based on a multi-model deterministic blend, including the 12Z GFS/ECMWF/CMC during days 4-5. By days 6-8, low forecast confidence warranted a shift heavily toward ensemble means (ECENS/NAEFS) until some of the forecast specifics become clearer. 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, Sat-Mon, Dec 7-Dec 9. - High significant wave heights for coastal portions of mainland Alaska, Tue, Dec 10. 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