Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Mon Dec 18 2023 Valid 12Z Fri Dec 22 2023 - 12Z Tue Dec 26 2023 ...Stormy pattern expected to develop over the North Pacific/Aleutians and settle into the Northeast Pacific/southern coast of the mainland... ...Overview... Today's guidance continues to show the establishment of a more amplified upper pattern. An overall mean trough will likely consist of slowly digging initial Bering Strait/northern Bering Sea troughing and farther south leading North Pacific/Aleutians dynamics plus trailing flow. Downstream ridging over western Canada may extend into the mainland at times, most likely during the weekend. This evolution should produce a strong storm tracking near the eastern Aleutians late this week, with this and/or additional systems expected to gravitate toward the northeastern Pacific/Gulf of Alaska by the weekend and early next week. Expect these systems to produce areas of strong winds across the North Pacific and vicinity along with a threat of heavy precipitation along the southern coast of the mainland into the Panhandle. ...Guidance Evaluation/Preferences... For the North Pacific/Aleutians into Northeast Pacific, over the past day the GFS/GEFS have been trending away from their previous minority solution (southern leading system and deep/northern trailing storm) and closer to established consensus that has been represented by most recent runs of the ECMWF/UKMET/CMC and the ECens/CMCens means for the leading storm forecast to be near the eastern Aleutians as of Friday. The 12Z GFS/GEFS have not fully reached that cluster (due to remaining somewhat weaker/more open aloft) but the GEFS mean was actually a little closer versus the operational run as of Friday, and the new 18Z GFS has improved a little more. Consensus remains close to continuity by Saturday, tracking the eastern Aleutians storm just south of the Alaska Peninsula. Model scatter is fairly typical for the time frame with the means staying near the middle of the spread. Then the 12Z runs are starting to refine some details, suggesting that trailing mid-latitude low pressure should intensify into an even deeper low than the first one and reaching a position just south or southeast of Kodiak Island by early Sunday. Even with the different details, continuity looks better than average for the consensus day-to-day low positions. However individual solutions rapidly diverge after early Sunday, both for the ultimate path of the Sunday storm and the existence/path of another potential northward-moving Pacific system early next week (as suggested by some runs of operational models and ECens/CMCens means). Current preference is to keep weakening low pressure near the southern coast into Monday/Christmas Day and have a conservative representation of the possible trailing system. Meanwhile guidance has varied among each other and from run to run within the overall upper trough forecast to be aligned over or somewhat west of the western coast of the mainland. The 12Z CMC brought its upper low farther east into the mainland than other guidance after Friday, while on the opposite side the new 18Z GFS pushes it to the western extreme via more mainland ridging. The ensemble means have been fairly agreeable and consistent with the overall trough axis. Farther west, there continues to be a signal for low pressure to track over the western Aleutians but with spread for specifics to keep confidence low. The Friday portion of the forecast started with a 12Z model composite aside from splitting the GFS component with the GEFS. Remaining days eliminated the CMC due to its questionable western mainland upper low. After early Sunday the blend rapidly increased 12Z GEFS/CMCens and 00Z ECens mean input in light of detail spread, with means reaching 75 percent total weight (with small lingering components of the 12Z GFS/ECMWF) by day 8 next Tuesday. ...Weather/Hazards Highlights... Confidence is gradually increasing that a strong storm should track near the eastern Aleutians around Friday and reach just south of the Alaska Peninsula by Saturday. This system will likely produce organized precipitation from the Aleutians eastward along with a broad area of brisk to strong winds. Whether by way of this system's continued progression or replacement by another potentially deeper one, the overall area of low pressure should continue south/southeast of Kodiak Island and perhaps into the Gulf of Alaska by Sunday-Monday. This storm track will focus heavy precipitation along the southern coast from the Kenai Peninsula eastward during the weekend, including the southern Panhandle by Sunday. The intensity of precipitation may ease up some after the weekend and confidence rapidly decreases for details of any surface systems, but the pattern should remain favorable for some activity to persist through early next week. Most snow that falls over the central/northern mainland should be light and scattered though with some continued uncertainty over whether much of the moisture reaching the south will reach farther northward. Beyond the initial system affecting the eastern Aleutians, the Aleutians and vicinity will see unsettled conditions for most of the period frequent light precipitation. Evolution of the upper pattern should favor above normal temperatures especially over the eastern half of the state. Anomalies for lows should generally be greater relative to those for highs, and there should be a warmer trend after Saturday. Some areas of slightly below normal highs will be possible over portions of the interior. The deepening mean trough aloft near the western mainland will likely promote a colder/below normal trend over that part of the state as well as the Aleutians. Rausch 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