Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 655 PM EST Sun Jan 21 2024 Valid 12Z Thu Jan 25 2024 - 12Z Mon Jan 29 2024 ...Overview... With latest GFS/GEFS runs reverting toward the majority guidance cluster in principle, confidence has further increased in the expected evolution toward significant deepening of an upper trough over or near the western mainland from late this week onward, with an embedded closed low also likely. This upper pattern should favor a series of surface lows tracking into the northeastern Pacific/Gulf of Alaska along with a multi-day period of moderate to potentially heavy precipitation across the Panhandle and at least portions of the southern coast from around Friday into early next week. Much below normal temperatures over much of the Interior and Southwest late this week will moderate by the weekend/early next week, especially over the eastern half of the mainland. ...Guidance Evaluation/Preferences... GFS/GEFS runs through the 12Z cycle appear to be in the process of recovering from a 24-48 hour period of going astray from consensus for the forecast pattern evolution (showing more of an upper ridge versus the deep trough in other guidance). As of late this week there is still some guidance spread for how amplified leading mainland flow will be, with the GFS/GEFS/UKMET flatter and slower with initial troughing. There is general consensus upon some trailing energy feeding into the upper trough by around Sunday and the GFS/GEFS essentially catch up to the ECMWF/CMC-based guidance as the latter energy arrives. As is typical for forecasts several days out in time there is meaningful spread for where the upper trough axis and embedded closed low end up. The 12Z ECMWF has strayed to the eastern side of the spread for the trough (eastern mainland) and north for the upper low late in the period while the 12Z CMC brings its upper low as far southwest as the southwestern Alaska Peninsula by Sunday, with the CMCens mean only slightly farther east. Interestingly, ECens mean runs through the 00Z cycle have been trending northward with the embedded upper low, but with a steady deeper trend for the overall trough to add confidence to a deep solution. Meanwhile latest runs of ECMWF-initialized machine learning models offer minimal support for an upper low as far southwest as the CMC/CMCens (none from the 00Z cycle and one from the new 12Z cycle), instead favoring the western mainland on average (but with typical spread). The differences in longitude for the upper trough axis lead to corresponding spread for the track of what may be a series of surface systems whose individual details have fairly low predictability at this time. Current preference resides with an intermediate track aimed toward the vicinity of the Kenai Peninsula/Prince William Sound. The western CMC/CMCens/GFS lean more to Kodiak Island while the 12Z ECMWF strays eastward for its lingering frontal boundary late in the period. The aforementioned trailing energy feeding into the trough may support a wave/front in some fashion. Such a feature has been showing up in the guidance over recent days but with poor continuity for position and timing. Finally, there is a persistent indication of weak low pressure to the south of the Panhandle as of early Thursday. This feature should rapidly weaken after that time. For the first half of the period a blend of the 00Z/12Z ECMWF runs, some 12Z CMC input, and 30 percent total 00Z ECens/12Z CMCens means provided the preferred forecast scenario, with the idea that the GFS/GEFS are still catching up to consensus. A deeper trend in the new 18Z GFS seems to provide some early confirmation. Later in the period the ECens/CMCens weight increased a bit to 40 percent while operational ECMWF input phased out the eastern trough of the 12Z run. This provided a reasonably consistent and intermediate position for the upper trough/low, in light of persistence in the new 12Z ECens mean and a westward shift in the 18Z GFS. ...Weather/Hazards Highlights... The forecast pattern evolution continues to suggest that starting Friday precipitation should trend heavier along the Panhandle and to some extent farther west along the southern coast, with the potential for a series of surface systems to extend this wet/snowy weather into early next week. Currently the relatively higher confidence in significant multi-day totals is over the Panhandle and the far eastern coast of Southcentral. The preferred intermediate upper trough position and surface low track would bring meaningful precipitation as far west as the eastern Kenai Peninsula, while the western side of the guidance spread would bring the moisture shield as far back as Kodiak Island and eastern Alaska Peninsula. Lesser amounts of moisture streaming northward may produce some snow over at least the eastern mainland. Meanwhile, a tightening surface gradient across the southern tier may yield a period of strong winds from parts of the far southern mainland into the eastern Aleutians from late this week into the weekend. Sufficiently strong surface systems in the Northeast Pacific/Gulf may enhance winds as well. Expect very cold/well below normal temperatures over much of the Interior/Southwest into late this week with overnight lows in the -20s and -30s along with some -40s to isolated -50s possible in favored valleys of the Yukon Flats. The deepening upper trough over the western mainland should promote a warmer trend over the eastern half of the mainland from the weekend into next week while keeping below normal temperatures over the western/southwestern areas. The North Slope and Southcentral/Panhandle will see more moderate temperatures, including some above normal readings. 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