Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 657 PM EST Fri Jan 03 2025 Valid 12Z Tue Jan 07 2025 - 12Z Sat Jan 11 2025 ...Episodes of heavy precipitation and strong winds likely along the southern mainland coast next Monday through Thursday... ...Overview... For the most part, guidance shows various flavors of Bering Sea into Pacific mean troughing and downstream ridging that extends from over or near western Canada into Mainland Alaska through the period. This regime will maintain a wet/snowy pattern along the southern coast of the state, with the most significant multi-day totals likely to be along the Southcentral coast and northern Panhandle. A series of south-to-north waves tracking into or just west of the Gulf of Alaska from early in the week through Thursday may produce periods of strong winds as well, while more low pressure emphasis toward the Aleutians late in the week should lead to a broader flow of moisture into the southern coast after Thursday. Expect above to well above normal temperatures over a majority of the state during the period, aside from some below normal readings through midweek or so over portions of the North Slope and along the western coast toward the end of next week. ...Guidance Evaluation and Preferences... There are some continued differences with specifics of the storm systems expected to lift northward from the Pacific into the southern coast through Thursday, with a fair amount of detail spread persisting for a system reaching the southern mainland by Tuesday and what could be a strong storm reaching the Gulf by Thursday/early Friday. An average among the 12Z GFS/CMC and their ensemble means provided the best intermediate solution and continuity adjustment for the first system, while a 12Z ECMWF/CMC compromise with minority input from the 12Z GEFS/CMCens means provided the best solution with good continuity in principle for the Thursday into early Friday storm. These preferences also take into account prevailing themes from latest machine learning (ML) guidance. After Thursday, guidance displays a lot of spread and variability for system details over the North Pacific/Aleutians along with some differences over and west of the mainland. As a result, by Friday the starting blend reduced 12Z ECMWF weight more quickly than the 12Z CMC while increasing ensemble mean weight (by this time including the 00Z ECens as well) on the way to Saturday reflecting 100 percent ensemble mean input. For the system forecast to reach into or near the southern mainland by early Thursday, overall trends over the past day have been a tad faster. ML models generally show an elongated axis of low pressure from the southeastern Bering Sea through the southern mainland, with variations in where the best defined surface low may be within this axis. A compromise among the 12Z GFS/ECMWF and the 12Z GEFS/CMC means (tilted a bit to the operational models) best fits the ML model theme and the new 12Z ECens mean supports this solution. The 12Z UKMET/CMC are a bit on the slower side. GFS inclusion reflects some guidance hinting at a trailing frontal wave that may lift into the Gulf and enhance winds between it and the Panhandle. Ensemble mean and ML guidance continuity along with tight clustering for the Thursday storm provide added confidence in the forecast. ML runs have steadily averaged in the upper 960s to low 970s mb valid 12Z Thursday and the ensemble means have been trending gradually deeper with the 12Z CMCens mean actually reaching down to the ML model average. Latest dynamical guidance suggests a greatest depth perhaps to around 960 mb before becoming weaker near the southern coast by early Friday. There is little to no support for how far west the 12Z GFS tracks after early Thursday, with the new 18Z GFS run returning somewhat eastward. A 12Z ECMWF/CMC compromise is the closest dynamical input to the majority scenario. Interestingly, the ML model track has been running a tad east of the ensemble means and closest dynamical models. Farther west, guidance generally shows the theme of a North Pacific/Aleutians system Thursday into Friday, most likely weakening or progressing along with the arrival of another (possibly somewhat deeper) system from the west by Saturday. Like yesterday, dynamical and ML models all show various details for how the specifics will look. This favors skewing closer to the ensemble means until individual models start to cluster better. Also during Friday-Saturday, the past couple ECMWF runs have been more aggressive than most other guidance with the eastward push of upper height falls into the mainland from within the overall area of mean troughing expected to persist over the Bering Sea. The old 00Z ECMWF was especially pronounced with this issue, with the 12Z version backing off slightly. Other dynamical and ML models vary with embedded trough details but there is very little hint of an ECMWF-like evolution for now. ...Weather/Hazards Highlights... Continue to expect a series of vigorous south-to-north Pacific systems from early in the week through Thursday, bringing the threat of heavy precipitation (coastal/low elevation rain and snow elsewhere) to areas from the far eastern Alaska Peninsula to the Panhandle, with strong winds also possible over the Gulf of Alaska and extending into areas along the southern coast of the mainland. One system may track from south of Kodiak Island into the southern mainland Monday into Tuesday, followed by a potentially stronger and somewhat larger storm around Thursday. The Days 3-7 Hazards Outlook reflects these threats during the Monday-Thursday period with heavy precipitation indicated from Kodiak Island and areas just west of Cook Inlet east/southeast through the Panhandle, with the best potential for high winds from the southern Kenai Peninsula through the northern Panhandle. There could be a brief lull around Wednesday, depending on system timing. One or more areas of lighter snow may spread northward across the mainland in association with weakening low pressure centers and/or frontal boundaries. The Aleutians will see somewhat unsettled conditions early-mid week but with only scattered light precipitation. One or more surface lows and frontal systems may bring an increase in winds/precipitation to the region by next Thursday-Saturday, but still with lower than average confidence in the details. Flow ahead of these potential North Pacific/Aleutians systems will likely maintain a broad flow of moisture into the southern coast into the weekend, just with somewhat less pronounced focus than provided by the previous systems. Guidance shows significant localized QPF maxima for 5-day totals from the Southcentral coast into northern Panhandle in particular, with some 10+ inch liquid accumulations possible. This time frame is part of a potentially very anomalous longer-term period (10 days from now through next weekend) for precipitation at some locations, particularly over the southeastern part of Southcentral. Much of the state will likely see above to well above normal temperatures during the period. Exceptions include the North Slope which may see below normal readings through midweek or so, and locations along the western coast which may trend a little below normal by the end of the week. Expect the warmest anomalies Wednesday-Thursday, with the most persistent warmth over eastern areas toward the end of the 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