Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 727 PM EDT Tue Oct 08 2024 Valid 12Z Sat Oct 12 2024 - 12Z Wed Oct 16 2024 ...Deep western Gulf low to bring strong winds, heavy rain, and maritime threats late this week through the weekend... ...Overview... The dominant focus of the forecast will be a deep low likely tracking south of the Alaska Peninsula as of early Saturday and then gradually weakening as it settles close to Kodiak Island. This large storm will produce a broad area of high winds into the weekend, along with heavy precipitation along the southern coast and Panhandle. Meanwhile the northern side of the storm's upper low should open up during the weekend, yielding a mean trough over parts of the mainland through the first half of next week. Farther west the guidance shows a less impactful Aleutians/North Pacific system early next week, with questions regarding potential interaction with mainland trough energy by next Wednesday. ...Guidance Evaluation/Preferences... Over/near the Gulf of Alaska, the majority of guidance clusters fairly well for the initially deep storm south of the Alaska Peninsula with typical detail spread for exact track. Overall the latest consensus has adjusted the system's position a little southeast of yesterday's continuity, though some machine learning (ML) models still offer potential for a path slightly east of the current majority. The one stray solution of note is the 12Z GFS which shows much stronger frontal wave development to the south and west of the Panhandle next Monday, with corresponding south/southeast track of the Kodiak Island low. There is little if any support from other dynamical/ML guidance for the 12Z GFS evolution, with what signal exists being for a weak frontal wave that could track over or south of the Panhandle. The new 18Z GFS compares better to other guidance, as did the 06Z run. After shortwave energy and surface low pressure depart from the northeastern mainland on Sunday, most guidance maintains weak troughing aloft over the Arctic with a ridge to the west. This pattern supports surface high pressure to the north/northwest of the mainland by Tuesday-Wednesday. The 12Z ECMWF strays from the majority, showing a faster progression of high pressure and a trailing wave. Dynamical and ML models/ensembles show average or better clustering for the system tracking across the North Pacific close to the Aleutians early next week, with today's guidance average showing a slightly northward trend from yesterday. The primary discrepancy arises by next Wednesday when the ensemble means show quicker merging of this system with the lingering western Gulf low versus the operational models. This evolution will depend in part on low-predictability details of energy within and dropping down the western side of the mainland upper trough. 00Z/06Z ML model spread included the dynamical model/ensemble mean envelope but the new 12Z ML runs show a little better signal toward the operational model scenario of keeping the two system separate at least into early Wednesday. A blend of the 12Z GFS/ECMWF and lesser weight of the 12Z UKMET/CMC represented consensus well into Sunday. Then the forecast used the 06Z GFS to replace the 12Z run given the latter's questionable Gulf solution by Monday. Tuesday-Wednesday incorporated a modest weight of the 12Z GEFS/00Z ECens means while splitting ECMWF input among the 12Z and 00Z runs due to the questionable Arctic progression in the 12Z run and developing detail differences elsewhere. ...Weather/Hazards Highlights... The storm reaching the western Gulf of Alaska by the end of this week will produce a broad area of high winds from the eastern Bering Sea/Aleutians to the Northeast Pacific/Panhandle. Highest wind speeds should be on Saturday, in the northerly flow over the eastern Bering/Aleutians and in easterly/southeasterly flow near the Southcentral coast and Panhandle. Significant waves will also present a threat for maritime interests. Meanwhile the eastern side of the storm's circulation will bring a period of heavy precipitation to the southern coast and Panhandle, with the best atmospheric river signal evident on Saturday when precipitable water values should be most anomalous. Both winds and precipitation should trend less extreme with time, though mean troughing aloft over parts of the mainland and lingering low pressure near Kodiak Island may support some southern coast/Panhandle precipitation through the first half of next week. The Days 3-7 Hazards Outlook reflects the most confident areas for high wind and heavy precipitation threats during the Friday-Sunday period. Shortwave energy and associated surface low breaking off from the northern side of the western Gulf storm may produce an area of organized precipitation, mostly snow, across parts of the northern mainland during the weekend. The system expected to track just south of the Aleutians early next week may bring light to moderate rainfall to the region, most likely over the central/western islands. Expect daytime high temperatures to be below normal from the Aleutians through the southern coast (including some locations somewhat inland) and the Panhandle. Most areas from the Interior through North Slope will likely see above normal highs through the period. A majority of the state will see above normal min temperatures, with the greatest anomalies tending to be over the North Slope. 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