Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Sat Sep 21 2024 Valid 12Z Wed Sep 25 2024 - 12Z Sun Sep 29 2024 ...Overview... The most agreeable guidance cluster shows an elongated area of low heights (with one or more low centers) over the far western mainland into the Gulf of Alaska/Northeast Pacific as of early Wednesday giving way to a Bering Sea system whose upper low combined with some initial western coast energy should yield a mean low aloft near the western coast by next weekend. This evolution will favor weak upper ridging over eastern and some northern areas. The most prominent forecast uncertainties involve North Pacific low pressure that could affect the southern Panhandle late in the week, and then the combination of details aloft within and around the upper low/trough plus how the northern stream energy could interact with mid-latitude Pacific low pressure. The large scale mean pattern by next weekend suggests an increase of moisture reaching the southern coast and Panhandle but individual solutions diverge significantly for the corresponding surface pattern and timing/magnitude of this potential moisture focus. ...Guidance Evaluation/Preferences... One forecast issue mid-late week will be a mid-latitude Pacific wave that may ultimately track far enough north to affect at least southern parts of the Panhandle. Recent guidance runs have differed for precise details of the initially supporting progressive shortwave energy and what interaction there may be with the upper low lingering over/near the Gulf of Alaska (originally from a strong short-term system). GFS runs through 12Z were somewhat on the weak and southern side of the envelope, closest to most 00Z/06Z machine learning (ML) models that were between Haida Gwaii and Vancouver Island. The 12Z CMC made a pronounced stronger/north adjustment, close to the 00Z ECMWF while the 12Z run nudged a bit south (the two averaging near Haida Gwaii). Latest UKMET runs have been on the northwest side, with the ensemble means generally tracking between the UKMET and 12Z CMC/00Z ECMWF--nearing or reaching the Panhandle. Earlier ML guidance seemed late to trend toward the verifying stronger/northward side of the spread for the system affecting the Panhandle this Friday-Saturday, so it will be interesting to see if similar biases hold for the upcoming system. Preference sides with an intermediate track closest to the 12Z CMC/00Z ECMWF, with the new 12Z ECens mean close to that idea as well. In contrast, from Wednesday into early Friday most guidance is rather well behaved for the system forecast to track into the Bering Sea and approach the western coast of the mainland. An operational model composite represents consensus well for this system during the first half of the forecast, with the only notable continuity adjustment being faster progression of the leading frontal system if not the surface low itself. With some detail issues already starting to arise over the Bering Sea/Aleutians by next Friday, forecast details along the western coast and across the North Pacific become increasingly uncertain by next weekend. There are combined uncertainties regarding possible energy southwest of the Bering Sea system (such as depicted in the 12Z GFS and ECMWF in different ways), the eastern Bering Sea/western coast mean low itself, and what interaction there could be with mid-latitude Pacific low pressure. Surface possibilities include modest low pressure lingering near the western coast into Sunday or translating somewhat southeast to or beyond the Alaska Peninsula, plus other scenarios that involve some incorporation of Pacific low pressure that could deepen into a very strong storm as sporadic GFS/ECMWF/CMC runs have been indicating (but with very poor agreement/continuity). Latest GEFS/CMCens runs suggest some Pacific low incorporation while the ECens means have been minimizing this potential. However all three means end up with low pressure over or just south/southeast of the Alaska Peninsula by Sunday. Although the 12Z CMC is a deep extreme, its track is actually a good compromise among the ensemble means by next Sunday. Preference by this time frame leans more toward the means in principle with a conservative surface low strength, similar to the 12Z GEFS mean. ML models are not very enthusiastic about Pacific surface development late in the period. Considerations for the late week Northeast Pacific storm led to an early period blend consisting of half 00Z/12Z ECMWF runs, 30 percent 12Z CMC, and 20 percent UKMET. Rapidly increasing forecast uncertainty after early Friday led to a quick introduction of the 12Z GEFS/CMCens and 00Z ECens means, reaching 45-60 percent by next weekend. The blend also phased out the 00Z ECMWF by late in the forecast due to its low-confidence deep storm tracking toward the Gulf among other detail questions. ...Weather/Hazards Highlights... The forecast pattern evolution should favor periods of mostly light to locally moderate precipitation over the southwestern/southern mainland and extending into the Panhandle mid-late week, while a Bering Sea system spreads some moisture into the Aleutians and possibly the southwestern mainland. Weak upper ridging should keep precipitation lighter and more scattered over the northwestern half of the state. A leading Pacific storm system may increase precipitation and wind to some extent at least over the southern Panhandle late in the week, but with below average confidence in specifics due to current guidance spread for the system's track. The southern coast and Panhandle may see precipitation increase next weekend, but again with significant uncertainty in the details corresponding to a wide spread in guidance for the expected surface pattern. Latest forecast preference reflects organized low pressure reaching just south of Kodiak Island by early next Sunday, which would promote this anticipated moisture increase along the southern coast. Continue to monitor forecasts in coming days as guidance refines important details. Expect high temperatures to be consistently below normal across southern/southwestern regions and the Panhandle, while weak upper ridging will likely promote some areas of above normal highs over central/northern regions. There should be greater coverage of above normal morning lows, with most of the below normal readings tending to be confined to the southwest and the Panhandle. 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