Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Thu Dec 31 2020 Valid 12Z Mon Jan 04 2021 - 12Z Fri Jan 08 2021 ...Overview and Guidance/Uncertainty Assessment... Systems carried along or influenced by progressive North Pacific flow, and feeding into a general area of northeastern Pacific mean troughing aloft, continue to provide the main uncertainties of the forecast in terms of sensible weather effects. Farther north there are ongoing detail differences but reasonable agreement that heights over the mainland should gradually rise as initial upper low/shortwave energy retrogrades and merges into the circulation around an Arctic upper low that drops into and retrogrades over Siberia. Beyond midweek some residual upper troughing will likely persist over the eastern Bering and southwestern coast of the mainland. The early/mid period emphasis on operational guidance used somewhat lower weighting of recent ECMWF runs than average. This is due in part to the average of other guidance generally holding onto yesterday's ideas for a deepening system expected to track south of the eastern Aleutians and Alaska Peninsula Mon into early Tue followed by a track that approaches the eastern side of Kodiak Island--an evolution somewhat slower/westward of the 12Z ECMWF. The 00Z run was closer in principle. Confidence is still not very high given potential dependence on interaction of separate features over the North Pacific and Aleutians as of early Mon. Eastward timing of the low over the southern Bering/Aleutians on Mon (a much-weakened form of today's low analyzed at 921mb) continues to be troublesome with a range of possibilities depending on the specifics of the northern fringe of North Pacific flow. The ECMWF remains on the eastern side of the envelope initially with the GFS eventually catching up, while the 12Z UKMET/CMC are slower. Current forecast depicts it becoming absorbed into the mean flow by early Wed but there is potential it could still be better defined at that time. Increasing spread for some details within a fairly agreeable large scale pattern favored a rapid increase of 12Z GEFS/00Z ECMWF mean input by days 7-8 Thu-Fri. Of particular note there is not yet much support for the low near Kodiak Island by midweek continuing northwest into Bristol Bay per the 06Z/12Z GFS. There is still a loose signal for another system farther to the south after midweek but with poor agreement from operational runs. The last couple ECMWF mean runs are the most coherent with this system. Farther west the trends from the past day have extended some early indications 24 hours ago that low pressure reaching the central Pacific by Thu will track far enough south to have little influence on the Aleutians. However models and ensembles currently show better than average clustering with the next western Pacific storm that could extend its influence into the Aleutians after early next Fri. ...Weather/Hazards Highlights... The primary focus for meaningful precipitation through much of next week will be along the southern coast and Panhandle. Coverage and intensity of rain/snow will continue to be dependent on lower confidence track and depth specifics of individual surface lows reaching the northeastern Pacific/Gulf of Alaska. Currently the best consensus of guidance suggests some of the locally enhanced precip should reach at least as far west as the Kenai Peninsula. The system expected to track somewhat south/east of the Alaska Peninsula around Tue-Wed may have the best potential to produce areas of strong winds. Gradually weakening southern Bering/Aleutians low pressure early-mid week will likely maintain periods of precipitation over the Aleutians through the first half of the week. After a western Pacific system likely passes well south of the Aleutians after midweek, the next in the series may bring increasing winds/precipitation into the western Aleutians next Fri. At the start of the period on Mon expect most of the mainland to see below to well below normal temperatures (especially for highs), with the southeast corner of the mainland and Panhandle likely to be the exception with above normal readings. Pattern evolution aloft will support a steady warming trend from east to west, leaving only the far western part of the mainland below normal by late week. 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