Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 651 PM EST Sat Jan 20 2024 Valid 12Z Wed Jan 24 2024 - 12Z Sun Jan 28 2024 ...Overview... Over the span of the past few 6-hourly cycles, GFS/GEFS runs have significantly diverged from established continuity and consensus for the large scale pattern expected to develop across Alaska and the Bering Sea into the Pacific. While the GFS scenario raises at least a remote possibility of significant changes to the current forecast at some point, preference currently sides with the overwhelming majority of other guidance that stays close to continuity. Based on the favored solution, progressive Arctic/Siberia flow may develop some degree of upper troughing over the mainland by mid-late week, albeit with persistent uncertainty in details. Meanwhile an upper ridge building over the western half of the Bering Sea should lead to additional amplification/deepening of mainland troughing into next weekend. Meanwhile central Pacific low pressure may track into the northeastern Pacific/Gulf of Alaska by the end of the week. A leading system could brush the southern Panhandle around Wednesday-Thursday. Light to moderate Panhandle/southeastern coast precipitation through midweek should trend heavier Friday onward as low pressure tracks into the Gulf, while interior and southwestern parts of the state see very cold temperatures. A tightening pressure gradient late in the week will likely strengthen winds across southern tier areas. ...Guidance Evaluation/Preferences... Recent GFS runs, and to a fair degree GEFS runs, have strayed remarkably far away from most other guidance. GFS runs are now advertising rapid eastward elongation and then northward building of initial Bering Sea upper ridging, leading to much higher heights over the mainland and vicinity versus the majority depicting a Bering Sea upper ridge (slightly weaker and westward versus the past couple days) and pronounced deepening trough over the mainland by next weekend. Among the non-GFS guidance there is still some east-west spread for the upper trough location (12Z ECMWF a little east of the 00Z run) but that clustering is better than average for a week out in time. ECMWF ensemble mean runs over the past couple days have been trending steadily deeper with the mainland upper trough while 00Z and new 12Z ECMWF-initialized machine learning models provide added support for the dynamical ECMWF/CMC models and their ensemble means, with typical modest differences in trough/upper low details. The result of the ECMWF cluster aloft is similar yesterday's forecast, with mid-latitude Pacific low pressure tracking northward into the Gulf of Alaska by Saturday (somewhat weaker than the 12Z/19 ECMWF) and lingering near the southern coast in weaker form into Sunday. Meanwhile the Bering Sea into North Pacific pattern should remain dominated by high pressure in contrast to the less suppressed GFS/GEFS that would have the Aleutians being affected by flow on the north side of low pressure. Earlier in the period there is still uncertainty over the details of progressive Siberia/Arctic flow reaching the mainland. Even among non-GFS solutions there is some spread and run-to-run variability as to how much troughing initially develops over the mainland mid-late week before the greater amplification occurs. At the very least, what trends exist continue to lean away from the amplified side of the spread. Farther south, model trends have generally been southward for a northeastern Pacific wave that could affect the southern Panhandle by Thursday. ECMWF/CMC runs provide a more moderate adjustment than the farther south GFS/UKMET. Based on guidance comparisons/preferences, today's forecast started with a blend of 12Z-00Z ECMWF/12Z CMC runs for the first half of the period, followed by adding in 30-40 percent of the 12Z CMCens/00Z ECens means to account for gradually increasing detail differences among the favored solution cluster. ...Weather/Hazards Highlights... The forecast continues to show very cold/well below normal temperatures across interior and southwestern areas through this coming week. Overnight lows should drop into the -20s and -30s, with even some -40s and isolated -50s possible in favored valleys of the Yukon Flats. Expect some moderation from daytime Saturday onward. The North Slope and Southcentral/Panhandle will see more moderate temperatures, including some above normal readings. Some colder temperatures may drop into parts of Southcentral for a time. Expect mostly light to locally moderate precipitation along the southeastern coast/Panhandle through Thursday. Then based on the most likely forecast scenario, a system lifting into the northeastern Pacific should bring increasing moisture into the Panhandle by Friday into the weekend. Enhanced precipitation will likely reach at least through the Panhandle. Westward extent between the Panhandle and Prince William Sound will depend on the exact alignment of upper troughing expected to deepen over the mainland and this aspect of the forecast will take more time to resolve. Remaining portions of the state will be relatively dry aside from areas of light/scattered snow possible to the east of the weekend upper trough and over other locations with wavy fronts. During the late week/weekend time frame, the combination of surface high pressure over parts of the mainland and/or low pressure over and south of the Gulf should strengthen the pressure gradient over the southern tier and produce areas of strong northerly winds from the eastern Aleutians through the Alaska Peninsula to around Cook Inlet/Kenai Peninsula. 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