Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 230 PM EST Sun Dec 09 2018 Valid 12Z Thu Dec 13 2018 - 12Z Mon Dec 17 2018 At the start of the period early Thu consensus shows a Gulf system nearing the southeastern coast/Panhandle. There are still finer-scale detail uncertainties with guidance generally split between the 00Z ECMWF/CMC means and to some degree 00Z CMC that emphasize low pressure just east of the Kenai Peninsula and the 06Z GEFS mean/other operational models that show more southeastward elongation/farther southeastward low track. These differences are well within typical error for day 4 forecasts so an intermediate solution would be preferable. Meanwhile the upper low over/near the western mainland has adjusted a bit east from yesterday's consensus as of Thu. After that time there is a fair degree of spread regarding the path of it or a significant portion of its energy. The 00Z ECMWF is somewhat extreme with its relatively fast northward progression but GFS runs suggest an eventual northward path as well. Ensemble means as well as the 00Z CMC show a more gradual movement or even just settling over the Bering Strait. Specifics of this type of evolution tend to have below average predictability so prefer an intermediate approach with the first priority being to lean away from the 00Z ECMWF. By the end of the period next Mon any lingering energy may end up being incorporated into a larger scale elongated northwest-southeast upper trough. During the first half of the period the forecast is still generally on track for the very strong northern Pacific system expected to pass just south of the Aleutians late this week. Most operational guidance continues to suggest the central pressure may reach down at least into the 950's mb at the storm's most intense stage. Latest GFS/GEFS mean runs provide the one question mark. 06Z/12Z GFS runs split the upper dynamics late in the short range time frame, leading to two surface lows with the leading one ultimately becoming the one that strongly deepens--and tracking south of the more consolidated system represented by consensus. Although the GEFS means are more consolidated, some members may be exhibiting an evolution like the GFS resulting in latest GEFS mean runs trending weaker than other means or yesterday's GEFS means for the surface low. As a result prefer a starting point consisting of the 00Z GFS/ECMWF/ECMWF mean for this system from Thu into early Sat. Then over the course of the weekend the 00Z ECMWF as well as GFS runs since 00Z have adjusted toward a stronger ridge extending from Canada across much of the mainland, hindering the eventual northward progression of the surface low. Ensemble means also show evidence of a slower northward progression into the Gulf of Alaska but so far do not yet show a pronounced stronger trend with what ridging aloft extends into the mainland. The CMC shows more southerly flow aloft over the mainland. A further complexity involves how approaching Bering Sea/Aleutians shortwave energy interacts with the system by next Sun-Mon with specifics remaining far from being resolved. Based on these considerations the forecast preference by Sun-Mon adjusts toward 70 percent total 00Z ECMWF mean/06Z GEFS mean input relative to a lingering 00Z GFS/ECMWF component. By this time frame the GEFS mean's weakness is not as extreme so it could be including in the blend, albeit at a lower weight than the ECMWF mean. The leading Gulf system will produce some enhanced precipitation and strong winds early in the period. The trailing deep northern Pacific system will generate a broad area of strong winds and high waves. Precipitation amounts over the Aleutians will be sensitive to exact storm track. Aforementioned uncertainties currently lower confidence in the timing and magnitude of associated moisture that could reach areas from the Alaska Peninsula through the southern coast and Panhandle from about late Fri onward. Rausch WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html