Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 756 PM EDT Sat Oct 07 2023 Valid 12Z Wed Oct 11 2023 - 12Z Sun Oct 15 2023 ...Deep storm likely to bring strong winds and high seas to the Aleutians and surrounding waters by Thursday... ...Overview... Guidance is still having some difficulty in resolving the ultimate path of an upper low initially forecast to be over the western mainland as of early Wednesday (most likely to drift northward), as well as how much upper ridging could build westward into the mainland from Canada later in the week and the weekend if the upper low departs. Details of these features will likely influence the path of an elongated area of low pressure (with one or more centers) expected to track northward into or near the Gulf late this week and possibly into the weekend. This evolution would most likely bring another episode of enhanced precipitation and winds to the Panhandle, with exact track determining how much moisture reaches the southern coast west of the Panhandle. Meanwhile the models continue to show very deep low pressure reaching the Aleutians/southern Bering Sea by late Wednesday/early Thursday, bringing strong winds/high seas to the region. Most forecasts suggest this system should gradually weaken as it wobbles over the region another couple days. Upstream energy may interact or combine with this system to yield a mean low just south of the Alaska Peninsula by the end of the period next Sunday. ...Guidance Evaluation and Preferences... After about Thursday, the models are still varying considerably for if/how quickly the upper low initially over the western mainland departs and how much ridging builds in behind it. Among latest runs the 00Z/12Z/18Z GFS are closest to the ensemble means that have consistently been showing a gradual northward drift for the low, reaching a position north of the western mainland by day 7 Saturday. A modest degree of ridging builds in from the east for the weekend. ECMWF runs have been on the slow side for the upper low, though the 12Z run did nudge north a bit from the prior run. The 06Z GFS was quite persistent as well. On the other extreme, the 12Z CMC ejects the low quickly and builds a strong east-west ridge over the mainland by Friday-Saturday. Meanwhile low-predictability shortwave details will be important in resolving some aspects of the general area of low pressure expected to move north into or near the Gulf of Alaska late in the week and possibly into the weekend, with track determined in part by relative emphasis of the mainland upper low versus westward extent of the Canada ridge. The ensemble means have been the most stable recently for the main leading wave and the 12Z GFS was closest to the means in principle for a track in the middle of the spread. However it remains to be seen whether QPF will be as high as the GFS forecast over the southern coast to the west of the northern Panhandle. There may be another wave trailing behind, with the favored blend showing a modest second wave to account for this possibility. The strong ridge to the north in the CMC pulls that model's storm track westward, while the UKMET and to a lesser degree ECMWF are east of the means. Multi-day trends have been to nudge the low track/precipitation axis a little to the west. Most guidance has held onto fairly good clustering for the track/timing of the deep Aleutians-Bering Sea storm into Thursday. The 12Z CMC/CMCens and 18Z GFS are somewhat west or northwest of consensus at that time. Operational model runs suggest a most likely central pressure in the 950s mb as of Thursday with an occasional run reaching into the upper 940s mb. After Thursday the guidance diverges with respect to the rate of weakening (GFS and UKMET holding onto a stronger low than other models for a while), and how upstream energy with possible surface reflection could interact or combine with this system. There is loose agreement that the one or more surface lows should ultimately yield a mean low a little to the south of the Alaska Peninsula by day 8 next Sunday. The early part of the forecast emphasized an operational model composite with more 12Z GFS/ECMWF weight relative to the UKMET/CMC. After day 5 Thursday, the blend removed the 12Z CMC in favor of the 12Z GEFS/00Z ECens means and then steadily increased ensemble weight so that the means had 60 percent influence by day 8 Sunday. Lingering GFS/ECMWF weight tilted somewhat more to the GFS. ...Weather and Hazards Highlights... Confidence remains above average for the forecast of deep low pressure expected to reach the Aleutians and southern Bering Sea by late Wednesday-Thursday. This system will likely bring strong winds and high seas to the Aleutians and surrounding waters, with continued potential to reach hazardous criteria at some locations. A broad shield of focused rainfall will sweep across the Aleutians/Bering Sea as well. After Thursday this storm should weaken somewhat as it wobbles over the region for a couple days but the rate of weakening is still uncertain. This system and its lingering lighter rainfall along with another possible feature supported by upstream energy should ultimately track south of the eastern Aleutians/Alaska Peninsula by next Sunday. After a brief midweek lull in precipitation along the southern coast and Panhandle around midweek, one or more surface waves tracking northward offshore the Panhandle (into or near the Gulf) late in the week and possibly into the weekend may bring another episode of enhanced precipitation and winds. This late week waviness should focus highest totals over portions of the Panhandle. Recent trends shifting slightly to the west have gradually increased potential amounts along the southern coast west of the Panhandle, but current guidance spread for exact track tempers confidence for details at this time. Light North Slope snow may continue into Wednesday, while any additional precipitation over the Interior/northern mainland will depend on uncertain specifics of flow aloft. High temperatures should be below normal over the southern half or two-thirds of the state at least into the latter half of the week, with areas near the northern coast seeing the best potential for some above normal anomalies. Highs may moderate by the weekend as the western mainland upper low potentially lifts northward. Low temperatures will tend to have warmer anomalies, leading to a mix of above and below normal readings that should also trend a little warmer with time. Below normal lows are most likely over southwestern areas and into parts of the Interior. 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