Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Sat Aug 17 2024 Valid 12Z Wed Aug 21 2024 - 12Z Sun Aug 25 2024 ...Extratropical Ampil to bring heavy rain and strong winds to portions of western Alaska around midweek... ...Overview... The dominant system of interest early in the period will be the extratropical reflection of Typhoon Ampil, which should track northeastward from the western Bering Sea, reaching close to the northwestern coast of the mainland by late Wednesday/early Thursday and continue onward from there. Expect this storm to bring an episode of high winds and significant precipitation to parts of the western mainland around midweek. A leading ridge will drop southeast from the mainland through the Panhandle Wednesday-Thursday. Then guidance continues to show the pattern settling into broadly flat to cyclonic flow aloft from Siberia through Alaska. This regime will lead to lower predictability for various embedded smaller scale features from the Arctic southward into the North Pacific, though one point of agreement is a tendency for multiple days of cool and unsettled weather across most of the state. By next Sunday a storm system may come into the picture over the western Bering Sea/Aleutians. ...Guidance Evaluation/Preferences... Continuing from yesterday, the GFS/ECMWF/CMC comprise the most compatible operational model cluster for extratropical Ampil, with the 12Z UKMET straying to the weak and southern side of the spread as the system brushes the northern mainland by early Thursday. There has been a subtle faster trend, leaving the CMC/CMCens perhaps a tad slow (but okay as part of a compromise), while the 00Z machine learning (ML) models offer additional support for an average among the non-UKMET models and ensemble means for track. There is still lingering uncertainty for some details due to potential for some influence from smaller scale features to the immediate north and west by Wednesday-Thursday. In the wake of extratropical Ampil, confidence rapidly declines for specifics of various features--including a possible Arctic upper low, a separate compact upper low/trough that could rotate from Siberia across the Bering Sea and into the mainland (with associated surface reflection/s), and Aleutians/North Pacific shortwave energy that would influence frontal waviness over that region. The combination of these issues in turn eventually affect upper heights and orientation of flow/moisture into the Southcentral coast and Panhandle. The various detail uncertainties favor trending the forecast toward a model/ensemble mean blend by the latter half of the period. Even with the model differences and blended solution, there is decent continuity toward the idea of a wavy cold front reaching into the mainland by Friday through the weekend as shortwave energy rotates around, and for modest low pressure to settle over the Gulf of Alaska through late week and next weekend. As for other items of note within this broad area of uncertain details, the CMC continues to look fairly extreme on the slow/amplified side with upper troughing that reaches the Bering Sea by Friday (with a corresponding deeper surface low) while 12Z/18Z GFS runs have finally trended away from some earlier runs that had been on the deep side with Northeast Pacific troughing by late week/next weekend. However the GFS shift to an eastward trough or flatter flow, depending on the run, may be somewhat overdone though. The past couple ECMWF runs have tempered the strength of possible Aleutians waviness but are still inconsistent with the ultimate evolution reaching the Gulf of Alaska by next weekend. Regarding the system potentially reaching the western Aleutians/Bering Sea by next Sunday, the ensemble means have been showing a signal for multiple runs and the 00z ML models offer reasonable support for a similar position. 12Z/18Z GFS runs and the 00Z ECMWF are in that proverbial ballpark as well, leading to a better than average cluster for a day 8 forecast. The 12Z ECMWF is slower while the 12Z CMC is suppressed to the south (after the 00Z run was a fast extreme). The early part of today's forecast started with half 12Z ECMWF and the other half split between the 12Z GFS/CMC to reflect preferences and desired detail for extratropical Ampil. Then the increasing detail uncertainty over much of the domain and considerations for the western Aleutians/Bering Sea system on Sunday led to a mid-late period trend toward splitting ECMWF input between 12Z/00Z runs and raising total ensemble mean input (12Z GEFS/00Z ECens) to 30-50 percent, while holding onto some of the 12Z GFS as well. ...Weather/Hazards Highlights... Extratropical Ampil should track from the northern Bering Sea and then along or near the northwestern mainland coast Wednesday into early Thursday, with continued northeast progression thereafter. Expect this storm to produce a broad area of strong winds across the Bering Sea and then extending into the western mainland, with some areas of heavy rainfall also possible. The most pronounced winds and rain will likely be on Wednesday but some brisk winds and enhanced rainfall may extend into Thursday. The trailing cold front and supporting upper trough should spread rainfall across the rest of the state as well, including the Panhandle after midweek. Some precipitation should fall as snow in the higher elevations of the Brooks Range and on a more localized basis farther south in the Alaska Range (where some localized precipitation enhancement is possible as well). The forecast pattern evolution to flat or broadly cyclonic flow aloft should maintain unsettled conditions over most of the state from late week through next weekend, but with continued low confidence in the details of location/timing/magnitude of relatively higher precipitation totals. Farther west a wavy front near the Aleutians may produce one or more periods of rain. Then a system potentially reaching the western Aleutians or Bering Sea by next Sunday may increase winds and rain in its vicinity. Most of the state will likely see below normal high temperatures Wednesday through next Sunday with the coolest anomalies tending to be Friday-Sunday. Primary exceptions will be on Wednesday-Thursday with the southern Panhandle experiencing well above normal highs as upper ridging passes through, and the northern coast seeing relative warmth due to southerly downslope/offshore flow. Some pockets of slightly above normal highs are possible over Southcentral on Wednesday as well. Anomalies for morning lows will start out somewhat below normal over the east on Wednesday and above normal elsewhere, with above normal lows continuing eastward ahead of and immediately behind extratropical Ampil's cold front, followed by a steady trend toward more below normal readings Friday-Sunday. 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