Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 655 PM EST Mon Jan 18 2021 Valid 12Z Fri Jan 22 2021 - 12Z Tue Jan 26 2021 ...Overview and Guidance/Uncertainty Assessment... The majority of today's guidance shows a mean ridge aloft prevailing over the northeastern Pacific and Mainland Alaska with a couple shortwaves most likely splitting as they head into the mean ridge. Models and means continue to refine/adjust their forecasts for individual features but the continuity for the overall pattern is generally somewhat better than was the case during the latter half of the period 24 hours ago. The first system of interest will track northeastward from the eastern Aleutians/southeastern Bering Sea early Fri onward. An operational model consensus continues to lead the ensemble means for this system. Among the means the GEFS was earliest to improve its definition while it has taken until the new 12Z run of the ECMWF mean to start reflecting this feature. Even among the operational models there are detail uncertainties that will likely take into the short range to be resolved, given the importance of small scale shortwave details that have low predictability until a shorter time frame. Latest solutions hint at the system splitting into two waves, one that lifts quickly northward and a second slower/southward one that tracks toward the southwestern mainland. The 12Z GFS is a deep extreme with this southern wave. Recent trends for a stronger downstream ridge aloft lead to a slower adjustment for the supporting dynamics aloft and somewhat later development and/or departure of a southern coast/Panhandle wave during the weekend. Farther upstream the majority of guidance has trended noticeably stronger and somewhat slower (the latter trend continuing from yesterday) for the system forecast to track into the western Aleutians and then the Bering Sea during the weekend and early next week. Recent ECMWF/CMC runs have displayed a more coherent evolution and better comparison to the means versus the 06Z/12Z GFS runs. Not surprisingly, the stronger trend for this storm supports a somewhat stronger/sharper downstream ridge but the ridge is still not as strong as most guidance was showing two days ago. There is decent continuity with the leading front reaching the eastern Bering/Aleutians by early Mon and then dissipating (though with weak waviness still possible across the Alaska Peninsula later Mon) as the upper support weakens/splits to the north and southeast. Finally, there has been a signal in the guidance for another storm to emerge from the western Pacific during the first half of next week but with significant differences in track/timing. The 00Z ECMWF, 12Z CMC, and 06Z/12Z GFS runs cluster well for a day 8 forecast with arrival into the Aleutians/Bering Sea by next Tue. Some ensemble means have been hinting at this system as well but have been eager to merge it with the preceding system. Latest GEFS means are the closest in principle to the aforementioned operational cluster. After being ill-defined in the 00Z run, the new 12Z ECMWF mean has trended toward this idea as well. The 12Z ECMWF is slower than other solutions. During the first half of the period a 12Z operational model blend reflects the most common ideas of guidance while downplaying less confident details of any particular solution. Then the forecast adjusts to a model/mean blend that maintains some CMC input, split ECMWF input between the last two runs and eventually more toward the old 00Z version, and uses a little more 12Z GEFS versus 00Z ECMWF mean that was the latest run available at preparation time. ...Weather/Hazards Highlights... The wave(s) tracking northeastward from the eastern Aleutians/southeastern Bering Sea late this week will bring meaningful precipitation to locations from the Alaska Peninsula through the Kenai Peninsula and northward over the western mainland. Expect some locally moderate to heavy amounts over favored terrain. Then a wave forecast to develop along the southern coast and track into the Panhandle during the weekend should extend precipitation into the southeastern coast/Panhandle. There is a fair amount of spread for precip intensity with this wave but at least some moderate totals appear likely. Meanwhile precipitation should gradually become lighter and more scattered with time over the mainland. The Aleutians should see an episode of precipitation/stronger winds during the weekend as low pressure tracks into the western Aleutians/Bering Sea and the leading front extends out to the east. Some moisture from this system may reach into the southwest corner of the mainland/Alaska Peninsula by early next week but likely not extend any farther eastward. Another system forecast to track into the Aleutians/Bering Sea by next Tue may bring another period of enhanced precipitation/winds. The majority of the state will see above to much above normal temperatures late this week/early weekend, with the most extreme anomalies likely to be over northern and western areas. Then expect a cooling trend from late weekend into next week. The Panhandle may see some below normal readings as early as late this week/weekend while parts of Southcentral could drop below normal by Sun onward. Other parts of the eastern mainland could decline to below normal readings by next Tue. 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