Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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343 FXAK02 KWNH 222355 PMDAK Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Sun Sep 22 2024 Valid 12Z Thu Sep 26 2024 - 12Z Mon Sep 30 2024 ...Overview... Latest guidance continues to show a mean upper trough/low setting up near the western coast of the mainland with the arrival of a Bering Sea system and incorporation of lingering west coast energy. Some degree of troughing should also extend south into the Pacific from late week onward. Meanwhile, upper ridging in various forms should prevail over eastern and northern parts of the mainland through the period while an upper low/trough will eject out of the Northeast Pacific/Gulf of Alaska late this week. A storm system ahead of this ejecting trough may affect parts of the southern Panhandle late this week. Then flow ahead of the Bering Sea system may increase precipitation along the southern coast by the weekend, followed by still-uncertain details of potentially very strong Northeast Pacific storm development that could bring heavier precipitation and strong winds to the southern coast/Panhandle. ...Guidance Evaluation/Preferences... Recent model runs have continued to display significant north-south spread for the northeastern Pacific system that may affect the southern Panhandle, with some adjustments among individual solutions. System evolution and track remain sensitive to exactly how initial North Pacific shortwave energy interacts with upper low/trough lingering over and just south of the Gulf of Alaska. Ensemble means have tended to be on the northern side of the spread with a track into the southern Panhandle, with the 12Z CMC similar to that cluster. GFS runs had been rather far south but the 12Z/18Z runs have nudged up toward Haida Gwaii. The 12Z ECMWF (north of its 00Z run) is between Haida Gwaii and Vancouver Island--a track favored by most of the machine learning (ML) models. Prefer an intermediate track near Haida Gwaii, near or a fraction north of the GFS. This represents only a slight southward shift from continuity, with trends running a little faster versus yesterday as well. The system tracking into the Bering Sea continues to display fairly typical spread and variability for the type of system and time frame. Today`s runs show a little more southwestward elongation of upper dynamics, pulling the surface system a bit southwest of continuity (and the average of ML models). The latest GFS is a little stronger than most previous runs, bringing it more in line with established consensus. The dominant forecast issue from the weekend into the start of next week continues to be the details of potentially strong development of low pressure (possibly reaching at least into the 960s mb) initially tracking across the mid-latitude Pacific and then lifting into the Northeast Pacific/Gulf of Alaska. In a very broad sense the operational guidance has been showing such a signal for development over the past couple days but with a lot of spread for track and timing. Recent CMC/ECMWF runs have generally been more consistent than the GFS (though the 12Z/21 ECMWF was a weak exception). Today`s guidance generally delays the evolution by about a day. This yields an extra day of the initial Bering Sea system lingering over the Bristol Bay/Alaska Peninsula region before deepening Pacific low pressure becomes the dominant feature south or east of Kodiak Island by Monday. Corresponding to what is still considerable ensemble member spread, the means are still somewhat ill-defined with how this evolution takes place. However the remarkable clustering of the means (GEFS/CMCens/ECens plus the ECMWF AIFS mean) at the surface by day 8 Monday provides some degree of confidence in surface low existence and a favorable pattern for increasing precipitation along the southern coast and Panhandle. ML models have varied a lot as well, though new 12Z runs seem to be showing a little more of a deepening signal overall. Uncertainty/predictability at that extended time frame favors a conservative deterministic forecast as a starting point, about three-fourths toward the ensemble means relative to the operational dynamical models (though as noted below, requiring about a half model/half mean blend due to position differences). Improved clustering and continuity going forward would provide support for a deeper solution in the official forecast. Forecast considerations favoring an intermediate solution for significant features led to the first part of the forecast period starting with an evenly weighted composite of the 12Z GFS/ECMWF/UKMET/CMC. The blend started to incorporate a little ensemble mean guidance (12Z GEFS/CMCens and 00Z ECens) by day 6 Saturday, quickly settling at nearly half means thereafter with lingering input from the 12Z GFS/CMC and more so 12Z ECMWF (since its surface low was a little closer to the means). ...Weather/Hazards Highlights... A rapidly moving northeastern Pacific system most likely tracking over or near Haida Gwaii may briefly increase precipitation/winds over parts of the southern Panhandle late this week. There is still a fair amount of latitude uncertainty for the system`s track so confidence remains low to medium for the magnitude of effects. Farther west, the system initially tracking across the Bering Sea will spread light to locally moderate precipitation (mostly rain) into the Aleutians and Alaska Peninsula late this week. Leading flow may start to enhance rain and high elevation snow from the eastern Alaska Peninsula to Kenai Peninsula and nearby areas during the weekend. The southern coast and Panhandle may see a more significant increase of precipitation by Sunday-Monday in association with low pressure reaching toward/into the Gulf of Alaska, while potential also exists for this low to be quite deep and produce strong winds. Continue to monitor forecast closely as details become better refined in coming days. The central and northern mainland may see some light and scattered precipitation late this week, followed by a drier trend. The forecast continues to show consistently below normal high temperatures across southern/southwestern regions and the Panhandle, while upper ridging will likely promote more areas of above normal highs (with just some pockets of slightly below normal) over central and northern regions. Expect greater coverage of above normal morning lows, with most of the below normal readings 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 $$