Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 756 PM EDT Mon Oct 21 2024 Valid 12Z Fri Oct 25 2024 - 12Z Tue Oct 29 2024 ...Overview... Deepening low pressure over or near the southern Gulf of Alaska late this week into the weekend will support precipitation along the southern coast and especially the Panhandle, along with areas of strong winds primarily over open waters. At the same time mean troughing aloft should prevail over most of the mainland. From late weekend into the first part of next week, mean troughing should persist over the Arctic into northern mainland, but with rapidly increasing spread for the path of an embedded upper low. Flow to the south should become a little more progressive with time, perhaps bringing a weak upper ridge across the southwestern mainland by Sunday. Mean low pressure over the Bering Sea may hold in place while leading fronts stretch eastward in association with shearing then reinforcing dynamics before progression of best defined upper support finally leads to low pressure consolidating over the Gulf by around next Tuesday. ...Guidance Evaluation/Preferences... For the system over or near the southern Gulf of Alaska late this week into the weekend, the most notable trend over the past day is toward better dynamical model agreement that incorporation of small-scale wave development to the southeast of the initial parent low will lead to a deeper and more concentrated low by around early Saturday when the system should at is lowest pressure. 12Z model runs are all in the 960-970 mb range at that time, with typical scatter for position. The 12Z UKMET is somewhat of an eastern extreme, closest to the southern Panhandle. From late Saturday into Sunday there is considerable divergence for latitude of the low as it begins to weaken and move eastward, along with some timing differences. The 00Z/06Z dynamical and machine learning (ML) model spread varied from the northern mainland to south of Haida Gwaii. New 12Z solutions have at least trimmed the northern side of the envelope, suggesting a track over or just south of the Panhandle as most likely. The 12Z ECMWF becomes a little faster than most solutions by Sunday (along with the aforementioned UKMET from a farther east track the day before). An operational model composite through Saturday and with a little ensemble mean input by Sunday provides a reasonable depiction at this time. Future refinements appear likely given the small scale of important details, reducing predictability of some specifics. Guidance remains quite diverse, and in some cases erratic from run to run, for the Arctic upper low that should at least drop down closer to the northern mainland if not actually reach it. Most ML models have been keeping the low north of the mainland and perhaps no farther west than 140W (more similar to the 12Z CMC/UKMET), while latest ECMWF runs bring it into or just northwest of the mainland (as did the 00Z CMC). 6-hourly GFS runs have been all over the place, with the 12Z version particularly far south into the mainland. Ensemble means have been keeping the feature north of the mainland, though the new 12Z ECens has gravitated somewhat closer to the operational runs, now reaching the northern coast by next Tuesday. Preference is to maintain continuity with a low position to the north until better clustering emerges. Clustering is decent for the general scenario of initial western Bering Sea low pressure/fronts getting stretched eastward late week into the weekend as leading dynamics shear out in favor of upstream energy that should arrive into the western Bering by Sunday, holding the main area of low pressure over the western Bering for a while. Then this energy and the surface system should progress eastward (with trends somewhat faster compared to 24 hours ago), weakening upon approaching the mainland. The most common theme in the guidance (especially in the means) is for some phasing of Bering Sea dynamics with mainland troughing by next Tuesday, favoring surface low consolidation over the Gulf. While there are still some detail differences for flow ahead of this system, guidance spread in the Sunday-Monday time frame from the eastern Bering into the mainland is much less pronounced than it was yesterday. As for a couple other forecast considerations of note by next Tuesday, clustering is currently better than average of a potential storm system would affect the western Aleutians so an average of models/means is reasonable here. Meanwhile there has been a signal in some dynamical/ML guidance for central Pacific tropical/hybrid development becoming extratropical and possibly having some influence on the northeastern Pacific by the end of the forecast period. Thus far, ensemble members have varied enough for the means to wash out the system by then while other guidance varies considerably. Will favor a conservative approach until better agreement emerges. Forecast preferences led to starting the early part of the forecast with the 12Z ECMWF/GFS/CMC/UKMET in order of more to less weight, followed by a transition to more 12Z GEFS/CMC and 00Z ECens means with the means comprising 70 percent of the blend by next Tuesday. Some manual edits were made to refine detail of the southern Gulf storm and Arctic upper low. ...Weather/Hazards Highlights... Flow around low pressure expected to deepen over or near the southern Gulf of Alaska Friday into Saturday will likely support precipitation along the southern coast and Panhandle, along with increasing winds and waves offshore. The Panhandle should see relatively higher totals, with latest trends bringing the system a little farther south and reducing amounts along the Southcentral coast. Currently expect precipitation totals with this event to remain below hazardous criteria. The Panhandle should trend drier by Sunday or Monday as low pressure weakens and moves eastward. Some areas of light snow may be possible over the east (ahead of the mean upper trough axis late week into the weekend) and north (around or underneath an Arctic upper low). The Aleutians may see one or more periods of rain from frontal systems during the period, with an increase of rain/wind possible over the western Aleutians by next Tuesday ahead of a stronger system. Bering Sea low pressure will likely weaken sufficiently as it approaches the mainland by early next week to support mostly light precipitation over the far southwestern mainland. Expect most of the state to see below normal highs during the period. The North Slope may remain above normal through the period, though a southward path of the Arctic upper low could yield more moderate anomalies. Min temperatures should be more above average late this week with a trend toward a mix of above/below normal readings to the south of the persistently above normal North Slope. Southwestern areas may see a warmer trend for lows 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