Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 542 PM EST Sat Dec 05 2020 Valid 12Z Wed Dec 09 2020 - 12Z Sun Dec 13 2020 ...Guidance and Uncertainty Assessment... Models and ensembles are suggesting the large scale pattern aloft during Wed-Sun will be fairly complex and blocky. The best consensus of guidance shows closed lows for at least a portion of the period over the North Pacific and northern/western Alaska mainland while ridging evolves, with one or more possible embedded closed highs, over the Bering Sea/Siberia as well as over the Arctic. Uncertainties with each individual feature and their possible interactions already lower confidence in some forecast specifics, while shortwave/interaction issues in the Northeast Pacific also complicate the forecast. The first significant discrepancy of note arises by day 5 Thu and involves the surface pattern over the Gulf of Alaska/northeastern Pacific. The full spectrum of guidance ranges from maintaining emphasis on southern coast/northern Gulf low pressure from Wed, to weakening this feature in deference to a stronger system that develops to the southeast (tracking anywhere from offshore the Panhandle to Vancouver Island)--depending on low-predictability specifics of leading shortwave energy. Preferred operational model blend early in the period (12Z GFS/UKMET/CMC and 12Z/00Z split of ECMWF input) provides an intermediate solution to enhance flexibility pending future guidance changes. Model runs diverge by late week on potential evolution of the early-period upper low over the mainland, and then next weekend for specifics of Bering/Siberia upper ridging and how/whether an initially deep North Pacific upper low/storm system may weaken and ultimately become part of the upper trough surrounding the mainland upper low. Especially by next weekend the GEFS/ECMWF/CMC ensemble means provide the most agreeable and consistent evolution of the upper low center curling into the western mainland while the broader circulation incorporates the weakening North Pacific feature. Thus the forecast trends more to the means in principle, with a transition of operational model components toward the 00Z ECMWF and 06Z GFS that compare best to the means. The 12Z ECMWF ultimately brings the upper low farther south than most solutions while the 12Z GFS is weaker/northward with initial mainland upper low energy as it builds more ridging in from the south. Meanwhile the past couple CMC runs have wanted to pull the upper low into the Gulf. The preferred evolution should lead to surface low pressure evolving near/just southeast of Kodiak Island and the Kenai Peninsula by next weekend, but again with modest confidence given model spread and pattern complexity. The one aspect of the forecast having better agreement/confidence is the high pressure expected over the Arctic through the period. ...Weather/Hazards Highlights... Surface details over the Gulf/northeastern Pacific become more uncertain already by Thu, but the general pattern favoring one or more surface lows over this region should promote some periods of focused precipitation over the southern coast and Panhandle between midweek and next weekend. Precip should be somewhat lighter and more scattered farther north across the mainland. Meanwhile during mid-late week a strong North Pacific storm will bring precipitation to the Aleutians and vicinity. The gradient between this storm and strengthening high pressure over higher latitudes will also support a period of strong winds over the Aleutians and Bering Sea. The most likely pattern evolution during the period should promote below normal temperatures over southwestern/central portions of the mainland, while above normal readings prevail over the North Slope and southeastern mainland. The Panhandle may see a mix of above/below normal anomalies depending on location with min temps more likely to be above normal. Rausch 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 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