Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Mon Nov 13 2023 Valid 12Z Fri Nov 17 2023 - 12Z Tue Nov 21 2023 ...Heavy precipitation threat over portions of the Panhandle late this week... ...High winds likely over the Aleutians on Sunday... ...Overview... The general forecast from late this week through the weekend has been fairly consistent, with a deep upper trough crossing the mainland and bringing a period of below normal temperatures to most areas aside from persistent above normal readings over the North Slope. Leading Northeast Pacific/Gulf of Alaska low pressure/frontal waves will support enhanced precipitation over the Panhandle late this week. However the guidance still differs on the exact character of the upper trough as it extends into the Gulf of Alaska, leading to uncertainty for wave/precipitation details. Meanwhile, a deep storm tracking just east of Kamchatka should push a front and a band of strong winds across the Aleutians and North Pacific/Bering Sea around Sunday. By the first part of next week there is relative agreement/continuity for an upper ridge crossing the Bering Sea to build into the mainland and support a warming trend, but most of the latest models and ensembles are signaling a dramatic change in the Pacific by way of more interaction between the initial northern stream trough and a mid-latitude Pacific system. The result would be much lower surface pressures over the northeastern Pacific than previous forecast along with a rebound of moisture into the Panhandle and perhaps eventually the southern coast. ...Guidance Evaluation and Preferences... Toward the end of this week the most prominent forecast problem involves the exact details of the southern periphery of the mainland upper trough and resulting surface evolution from the northeastern Pacific into the Panhandle. In varying ways over the past three days the GFS has been keeping the axis of the upper trough on the western side of the overall spread, leading to a stronger/northwestern frontal wave development and heavier QPF over the Panhandle. The UKMET shares some similarity to the GFS in today's 12Z cycle, though with a somewhat faster/northward surface development than the GFS. The 12Z ECMWF/CMC show a somewhat more rounded upper trough that keeps the surface low track somewhat farther southeast, though the ECMWF does represent a noticeable northward adjustment from its 00Z run. Recent CMC runs have been on the fast side of the envelope with the surface reflection. Ensemble means show some tilt toward their respective operational runs aloft but with some intermediate ideas, but are less defined at the surface. The persistent spread involving small-scale details that do not have great predictability 4-5 days out in time seems to favor holding onto a compromise solution that is ultimately closest to the 12Z ECMWF in concept and a little northward of yesterday's forecast. Over the past day most guidance has trended significantly deeper with a storm forecast to track just east of Kamchatka on Sunday and push a leading front as well as strong surface gradient/wind field across the Aleutians and adjacent waters, though not quite to the extent of latest GFS runs. This deeper trend may be partially responsible for a pronounced trend in a majority of the 12Z guidance for greater interaction of northern stream flow (a shortwave with surface wave/front dropping through the eastern Bering Sea, and possibly mainland energy) with a mid-latitude Pacific system. In the past day or two there had been some stray model runs from the CMC and GFS that at least broke off a wave that would track northeast to the Panhandle or brought the whole system north, but with the means and other model runs staying suppressed. That has changed by the 12Z cycle, with latest GFS runs and 12Z ECMWF pulling the entire system into the northeastern Pacific, with the 12Z GEFS/CMCens/ECens means bringing lower pressures into the northeastern Pacific in less dramatic fashion. Now the 12Z CMC represents the lone holdout from the prior scenario of keeping higher surface pressures and drier conditions over the northeastern Pacific early next week. Trends of the 12Z GEFS/CMCens at the time of forecast preparation supported the general idea of bringing Pacific low pressure farther northeastward but in less aggressive fashion than the 12Z GFS/ECMWF. The favored blend ultimately ended up closest in principle to the 12Z CMCens mean and the 12Z ECens that arrived after forecast preparation. As for other aspects of the forecast, the ensemble means have trended stronger over the past day or so with the upper ridge crossing the Bering Sea and building into the mainland. This trend supports recent continuity that holds onto mainland/northwestern Canada surface high pressure with less Bering Sea gradient reaching the western mainland than a small minority of solutions. Earlier in the period, preferences lean away from the UKMET/CMC that track an Arctic upper low closer to the northwestern mainland than other models/means. By day 8 next Tuesday the operational runs are suggesting potential for another fairly strong system to reach somewhere between the western Bering Sea and mid-latitude Pacific. Means were in the northern part of this spread though the 12Z ECens has adjusted somewhat south into the Pacific. To reflect the composite of model preferences over the course of the period, today's forecast started with a blend of 40 percent total 12Z GFS/ECMWF and 60 percent evenly split among the CMC/UKMET/GEFS/00Z ECens for the first half of the period and then quickly adjusted to 70 percent total means (including some 12Z CMCens) and the rest divided among the GFS/ECMWF/CMC. ...Weather/Hazards Highlights... The southern periphery of the mainland/Gulf upper trough and associated surface wave(s) will likely promote moderate to locally heavy snow and coastal/southern Panhandle rain late this week. Some moisture may extend somewhat northwest of the Panhandle, ahead of separate Gulf low pressure. Confidence is still lower than desired for precise magnitude of highest totals and northward extent of heaviest precipitation. At this time, relative confidence is highest for the southern Panhandle to see the most precipitation. Expect a much lighter trend by Sunday-Monday but the changes in latest guidance would bring more moisture into the Panhandle and perhaps eventually the southern coast by Tuesday. The Aleutians will see brisk winds and mostly light precipitation into Saturday, followed by a brief period of calmer conditions as a narrow ridge crosses the region from west to east. Then a front approaching/crossing the Aleutians and adjoining waters should spread an area of high winds and precipitation across the region around Sunday. Another potential storm coming into the picture by next Tuesday could help to maintain brisk winds across the Aleutians and vicinity early next week. The deep upper trough crossing the mainland late this week and the weekend will bring below normal temperatures to most of the southern two-thirds of the state, with coldest departures from normal likely to be over southern areas through the weekend and into the Panhandle by early next week. A warming trend should progress from west to east during Monday-Tuesday. The North Slope will be one area that should remain above normal through the period, with anomalies increasing as part of the general warming trend over the mainland early next week. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle, Thu-Fri, Nov 16-Nov 17. - High winds across portions of the Aleutians, Sun, Nov 19. Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook 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