Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 654 PM EST Mon Jan 22 2024 Valid 12Z Fri Jan 26 2024 - 12Z Tue Jan 30 2024 ...Multi-day heavy precipitation threat for the Panhandle and portions of the southern coast late this week into early next week... ...Overview... Today's guidance maintains the general theme of a steadily deepening upper trough most likely aligned over the western mainland, with an embedded low likely closing off during the weekend. This upper trough should help to pull a series of surface systems northward into the Northeast Pacific/Gulf of Alaska over the course of the period, bringing multiple days of significant precipitation to the Panhandle and parts of the southern coast. Meanwhile a tight surface gradient across the southern tier into the Aleutians, enhanced at times by individual storm systems, should produce brisk to strong winds at times. Much below normal temperatures over most of the Interior and Southwest late this week will trend warmer during the weekend and early next week, especially over the eastern half of the mainland. ...Guidance Evaluation/Preferences... Dynamical/machine learning models and ensemble means as a whole provide better than average confidence in the amplifying/deepening upper trough scenario along with the potential for a stormy pattern with multiple surface systems lifting northward toward the southern coast. However, thus far guidance spread and run-to-run variability for upper trough alignment and other details have made it challenging to refine the specifics within the likely general pattern. Over the past day, tendencies among the guidance have been for the GFS/CMC and their means to lean somewhat on the western side of the spread for the upper trough alignment (with the CMC/CMCens tending to stray to the southwest side for the embedded upper low) and surface low tracks with the ECMWF/ECens mean to the east. In the 12Z cycle, the CMC/CMCens and GEFS nudged a little eastward (12Z GEFS closer to the average now) and the new 12Z ECens mean has shifted a little southwest with its upper low. Recent runs of ECMWF-initialized machine learning models have been diverse as well but with a relative majority favoring an upper trough/low position over the western mainland--near the average of ensemble means. This intermediate location aligns well with continuity and would lead to a surface low track reaching the southern coast along a corridor centered near Prince William Sound. There is some clustering with the idea of a triple point wave breaking off from a Pacific frontal system and reaching near the southern coast by early Saturday, but otherwise confidence is fairly low for specifics of trailing surface systems. Operational model runs, especially the GFS/CMC, suggest one or more of these latter systems could be quite strong. Early next week the latest GFS/ECMWF/CMC runs suggest potential for additional amplification of flow within the southern side of the upper trough, with the ECMWF/CMC becoming negatively tilted and the 06Z/12Z GFS runs extreme to the point of pulling off a new upper low over the Pacific well south of the Alaska Peninsula and wrapping a very strong mid-latitude surface system into it. The full model/mean array offers little support for the 06Z/12Z GFS scenario. The new 18Z GFS has tempered its solution such that its upper low reaches no farther south than the southwestern Alaska Peninsula--comparable to some CMC/CMCens runs over the past day or so. Lack of confidence in specifics from any of the operational models late in the period favors increasing weight of the ensemble means early next week. Early in the period the 12Z operational model guidance finally came into acceptable enough agreement to allow for a composite of the GFS/ECMWF and slightly less CMC/UKMET input to provide the basis of the forecast. By day 6 Sunday the 12Z GFS started to stray from other guidance, favoring the beginning of a transition from GFS input to the GEFS mean. Then increasing the GEFS/ECens/CMCens ensemble weight to 50-80 percent by days 7-8 Monday-Tuesday helped maintain continuity for the overall pattern with intermediate upper trough/low alignment and surface low track. ...Weather/Hazards Highlights... The most likely pattern evolution still suggests that precipitation will begin to trend heavier over the Panhandle on Friday. Then from the weekend into early next week the Panhandle should continue to see periods of moderate/heavy rain and snow (depending on location/elevation) with this moisture shield also extending westward along the southern coast as multiple surface systems likely track northward into the Gulf of Alaska. Best confidence for highest totals at this time extends from just east of Prince William Sound into the northern Panhandle during the weekend, as denoted by a heavy precipitation area on the Days 3-7 Hazards chart. Over areas that are sufficiently warm, effects of the potentially heavy rain on top of current snow cover will also have to be monitored. The preferred intermediate upper trough position and surface low track would also bring meaningful precipitation as far west as the eastern Kenai 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 periods of strong winds from parts of the far southern mainland into the eastern Aleutians from late this week into the weekend. At this time portions of the central Alaska Peninsula and Kodiak Island have the best potential for hazardous winds during Friday-Saturday. Sufficiently strong surface systems in the Northeast Pacific/Gulf may strengthen winds over some areas during the weekend into early next week as well. Expect very cold/well below normal temperatures over much of the Interior and Southwest into late this week, with coldest anomalies persisting into early Saturday. Overnight lows will likely be in the -20s and -30s F along with some -40s to isolated -50s possible in favored valleys of the Yukon Flats. From daytime Saturday onward, the deepening upper trough over the western mainland should promote a warmer trend over the eastern half of the mainland while keeping below normal (but less extreme) temperatures over the western/southwestern areas. The North Slope and Southcentral/Panhandle will see more moderate temperatures, including some above normal readings that should increase in coverage by the weekend and early next week. 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