Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 656 PM EST Sun Dec 17 2023 Valid 12Z Thu Dec 21 2023 - 12Z Mon Dec 25 2023 ...Stormy pattern expected to develop from the North Pacific/Aleutians into Northeast Pacific/southern coast of the mainland... ...Overview... Late this week will see the last vestiges of the short-term pattern favoring a succession of waves tracking eastward into the Gulf of Alaska. Then guidance is continuing to suggest that the upper pattern will become more amplified from late this week through next weekend, albeit with continued question marks for the details. This would be via the persistent trough/upper low just west of the northern mainland extending farther south and then southeast as incoming energy settles into position while western Canada ridging may begin to extend into the mainland during the weekend. The anticipated evolution aloft would support one or more strong surface systems tracking from the North Pacific/Aleutians into the northeastern Pacific, accompanied by a broad area of strong winds and potential for significant precipitation along the southern coast of the mainland and possibly extending into the Panhandle. ...Guidance Evaluation/Preferences... At the start of the period early Thursday, a 12Z model composite looks reasonable for resolving continued detail differences for the overall area of surface low pressure expected to be over the Gulf of Alaska. The 12Z ECMWF is in the minority with a trailing western wave near the southwestern coast of the mainland. The resulting forecast offers fairly good continuity relative to previous days. Aside from some longitude differences with the northern part of the upper trough offshore the western coast of the mainland (GFS runs tending to be a little east of consensus), the rest of the forecast focuses on continued differences for Aleutians/North Pacific storm development and its evolution to the east/northeast. As has been the case in recent days, the Friday-Saturday time frame continues to be contentious for the details. However with the arrival of 12Z guidance (and support from 00Z ECMWF-initialized machine learning models), there is an increasing signal for greater emphasis of the leading wave of two potential features (but slower than seen in some earlier ECMWF/CMC runs). This would be a fairly strong storm tracking just south of the eastern Aleutians as of early Friday and the Alaska Peninsula as of early Saturday. The GFS/GEFS runs offer the one competing cluster, tracking the leading system well south of the other cluster while strongly deepening the trailing wave--which generally gets sheared out in the majority scenario. At least as of early Friday the 12Z GEFS hinted at the leading wave (and tracking closer to the ECens/CMCens than the operational GFS) whereas it had no reflection of it 24 hours ago. In spite of the differences, like yesterday all three ensemble means still converge just south of Kodiak Island by early Sunday. From later Sunday into next Monday/Christmas Day, additional uncertainties come into play. To varying degrees the 12Z ECMWF/CMC track the leading low pressure farther north into the mainland while the ensemble means continue to linger near Kodiak Island. The farther north track potential will have to be monitored as the shape of upper flow could allow for that. Meanwhile the new 12Z ECens mean is starting to show a more explicit potential for a mid-latitude system to track into the mean surface low position as the past couple operational ECMWF have depicted. Toward the latter part of the period GFS runs also gravitate closer to the centroid of potential surface low positions and would likewise bring enhanced moisture to the southern coast, just with different details than some of the other models/means. One or more compact lows may exist/track over parts of the western half of the Aleutians/Bering Sea during the period. These have low predictability given their small scale and have had poor agreement/continuity among the various model runs. Enough of a model signal exists for one or more modestly defined features during the first half of the period but the mere surface troughs seen in the means after Saturday are a reasonable starting point for now given preferences farther east. Composite preferences led to starting today's forecast with a 12Z operational model blend for day 4 Thursday. Then majority clustering for North Pacific development favored adjusting to input from the 12Z ECMWF/UKMET/CMC and 30 percent total of the 12z CMCens/00z ECens means for days 5-6 Friday-Saturday. With late-period ECMWF/CMC runs diverging from the means, day 7 Sunday increased ensemble mean input (including the GEFS mean upon convergence with the other means) to 60 percent and day 8 to 100 percent. ...Weather/Hazards Highlights... Lingering low pressure over the Gulf of Alaska late this week should produce areas of moderate precipitation along the southern coast/Panhandle. By late Thursday-Friday onward, attention will turn to the Aleutians-North Pacific region for potential strong surface development, with this system currently expected to track just south of the Alaska Peninsula/Kodiak Island into the weekend. There is still meaningful uncertainty regarding the ultimate track of the storm, as well as a lower probability of a trailing wave developing. Amidst the various possibilities for specifics from late this week through next weekend, there is a general theme toward areas of strong winds over the Aleutians/surrounding waters (turning northwesterly after system passage) as well as extending into the northeastern Pacific, along with areas of significant precipitation focus along and near the southern coast of the mainland. Some of the moisture may extend eastward into the Panhandle. The late-period track of low pressure will determine how far northward into the mainland meaningful snow may reach. Before then, most snow that falls over the northern two-thirds or so of the mainland should be light and scattered. Evolution of the upper pattern will tend to favor maintenance of above normal temperatures especially over the eastern half of the state, with anomalies for lows likely to be greater relative to those for highs. There could be pockets of slightly below normal highs over portions of the interior. The deepening mean trough aloft just west/southwest of the mainland by the weekend should promote a colder/below normal trend over the western mainland and into 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