Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Mon Aug 19 2024 Valid 12Z Fri Aug 23 2024 - 12Z Tue Aug 27 2024 ...Overview... Latest models and ensembles show the upper flow pattern transitioning from flat or broadly cyclonic flow on Friday toward somewhat more pronounced troughing over and south of the mainland during the weekend. Within this evolution, recent solutions have varied considerably for the specifics of the overall trough and thus Northeast Pacific/Gulf of Alaska surface evolution and associated increasing precipitation along the Southcentral coast and Panhandle. Albeit with typical uncertainty for the distant time frame, the forecast remains on track for an upstream system to move across the southern half of the Bering Sea early next week with enhanced winds/rainfall, possibly approaching the southwestern mainland by next Tuesday. Expect this system to help push the downstream trough eastward by that time. ...Guidance Evaluation/Preferences... Predictability and confidence remain below average for shortwave specifics within the overall trough evolution over/near and south of the mainland late this week onward. GFS/ECMWF/CMC runs have all been varying among each other and from run to run for trough amplitude and timing. Recent runs of machine learning (ML) models have varied as well, but have at least tended to lean away from the more amplified side of the spread (such as earlier 12Z/18 GFS-ECMWF runs and today's 06Z GFS). Ensemble members are diverse as expected, with today's means generally having trended a little flatter with the overall trough versus 24 hours ago. Uncertainty over specifics of the upper trough continues to translate to spread for Northeast Pacific/Gulf of Alaska surface details. While the 12Z CMC has finally dampened development of an initial North Pacific/Aleutians wave that the model had been tracking into the Gulf, there is still some signal in the ECMWF/UKMET for a weak Pacific wave to reach at least far enough north to have an influence on the Panhandle by Sunday and the combined slow/sharp upper trough in the 12Z ECMWF supports development of another wave late Sunday into Monday. These waves are still less pronounced than seen in the 12Z GFS/ECMWF runs from 24 hours ago. Given minimal agreement among ensemble members for any mobile small-scale waves, the means have consistently been depicting stable low pressure over the Gulf of Alaska and recent runs of ML models have agreed with that scenario. Accounting for the full range of guidance, preference favors a blended approach for the upper trough and surface reflection as guidance diverges. This yields moderate upper troughing and a more dominant Gulf low with just a hint of Pacific waviness getting incorporated into the overall frontal system during the weekend. Likewise, this would correspond to a more modest moisture increase along the Southcentral coast/Panhandle than if a more pronounced wave were to track northward into the Gulf. So far the system expected to track across the southern half of the Bering Sea early next week has been fairly well behaved in the guidance relative to typical 6-8 day spread/variability. There are still a few moderately extreme solutions, such as the farther north 12Z CMC and slower 06Z/12Z/18Z GFS (with the 12Z run still acceptable as part of a compromise). The average of GEFS/ECens/CMCens means has been relatively stable over the past day and 3/5 of the ML models are similar, reaching the southeastern Bering Sea by early next Tuesday. Among the 00Z ML minority, one was fast and the other north. A quick look at the new 12Z ML runs shows those two extremes trending toward consensus. Today's forecast started with a blend of more 12Z GFS/ECMWF relative to 12Z CMC/UKMET early in the period. Lack of confidence in the UKMET's wave nearing the Gulf by early day 6 Sunday and in the CMC's Bering Sea system led to removing those models after Saturday, while diverging ECMWF details favored a split of 12Z/00Z runs for ECMWF input. Otherwise, the mid-late period forecast added 25-35 percent total of the 12Z GEFS/CMCens and 00Z ECens means while maintaining some 12Z GFS input. With minor editing, this provided Bering Sea system depth closer to the operational runs than the weaker ensemble means (with the ECens/CMCens stronger than the GEFS). ...Weather/Hazards Highlights... With the upper pattern leaning toward troughing late week through the weekend, expect fairly broad precipitation coverage across the mainland in that time frame, with some areas of localized focus especially along favored terrain. Precipitation should fall as snow in the higher elevations of the Brooks Range and farther south over portions of the Alaska Range as well, with other ranges possibly seeing isolated high elevation snow depending on depth of the upper trough. The evolving upper trough and Gulf of Alaska surface low pressure will likely support increasing precipitation along the Southcentral coast and Panhandle by the weekend. However there is still a fair amount of uncertainty over exactly how much moisture may reach the area. The current majority cluster would maintain Gulf of Alaska low pressure with any Pacific waves tending to stay more suppressed, keeping rainfall moderate aside from perhaps some greater enhancement over the southern Panhandle. A lower-probability potential (and reduced somewhat from 24 hours ago) would be for one or more Pacific waves to track into the Gulf and bring higher rainfall totals. This activity should trend lighter early next week, depending on upper trough timing. The system forecast to track across the southern Bering Sea Sunday onward will likely increase winds and rain over the Bering Sea and Aleutians, and then into the southwestern mainland and vicinity by early next work week. Most of the state should see below normal high temperatures late this week into the first part of next week, with the coolest anomalies likely during Friday-Sunday. Morning lows will tend to be below normal Saturday-Monday with more moderate anomalies than for highs. Friday and next Tuesday may feature a more even mix of moderately above/below normal mins. 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