Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 624 PM EST Mon Dec 30 2019 Valid 12Z Fri Jan 03 2020 - 12Z Tue Jan 07 2020 Guidance has continued to provide a fair amount of spread and some changes/trends over portions of the forecast domain but at least as of the 12Z/30 cycle there seems to be a little more clustering in principle. Latest consensus has adjusted somewhat westward for an initial upper low now expected to be near the eastern Alaska Peninsula/Kodiak Island as of early Fri. This feature should progress east-southeast while energy remaining over the southern mainland forms another upper low a little north of the first one. Overall upper trough/low emphasis is likely to shift northward to cover more of the mainland by the latter half of the period in response to westward elongation of flow to the south of an upper ridge that should cover areas from Siberia into the Arctic north of the mainland. There is still some uncertainty over the shape of this ridge though, affecting details over the mainland and Bering Sea. Guidance agrees fairly well that progression of energy within a western Pacific trough aloft will bring height falls into the western Aleutians by early next week, building a ridge into the Aleutians/Bering Sea--providing added support to maintain troughing over the mainland. Forecast evolution should lead to a colder trend over a majority of the mainland late this week into the weekend with temperatures stabilizing early next week. Perhaps with some help from the westward trend of the upper low initially near the Peninsula/Kodiak Island, a number of models have adjusted northwestward for a wave forecast to take shape over the central Pacific Tue and then track northeastward to or near the Alaska Panhandle by around Fri. This trend seemed to begin with the 12Z/29 GFS. Supporting energy is elongated and ensemble spread is considerable so confidence is below average for a system this early in the extended period. The upper dynamics/surface low to the west provide additional potential complications in the forecast. Meanwhile a decent majority of guidance (ECMWF/CMC and their means among other solutions) suggests that initial upper ridging over Siberia will close off to a greater extent than forecast by the operational GFS, leading to some degree of east-west weakness over the Bering in contrast to the nearly north-south ridge in the GFS. The GEFS mean is not much weaker with the ridge than the GFS. The ECMWF/CMC means are not yet confident in the depth of the weakness, so leaning toward the means appears best at this time. Guidance is starting to become a little more similar in principle with a system forecast to track south of the Aleutians during the weekend and then continue into the northeastern Pacific/Gulf of Alaska. An average of operational runs is not far from an average of the GEFS/ECMWF means, though the 00Z ECMWF mean was quite weak so more 12Z GEFS input was preferred among the ensemble solutions. The new 12Z ECMWF mean trended stronger. By early next week there is decent agreement/continuity with the idea of a strong surface gradient and associated brisk winds moving into the Aleutians and southern-western Bering ahead of a front that may reach the Aleutians late in the period. Overall consensus having trended a bit slower with the front versus 24 hours ago would favor phasing out the faster 12Z CMC as part of the blend in this time frame. Forecast preferences led to starting with a nearly even GFS/ECMWF/CMC blend early in the period with operational input still at 80 percent into day 6 Sun. This blend provided good detail for the system forecast to track toward the Panhandle and upper features over/near the Gulf, while yielding a pattern aloft close to the ECMWF mean over the Bering Sea. The forecast transitioned toward slightly more than half ensemble weight by day 8 Tue with more GEFS than ECMWF mean due to the GEFS being stronger with the North Pacific system. Elsewhere the means are either more comparable or an average produces a good intermediate result when included with the operational runs. 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 No significant hazards are expected over Alaska during this forecast period. 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