Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Sun Aug 18 2024 Valid 12Z Thu Aug 22 2024 - 12Z Mon Aug 26 2024 ...Overview... As the extratropical reflection of Ampil lifts northeastward from near the northwestern coast of the mainland early Thursday onward and a narrow upper ridge drops south of the southern Panhandle, guidance maintains the idea of the upper pattern settling into flat to broadly cyclonic flow late week into the weekend. Upper troughing may become somewhat more pronounced from the mainland into the Pacific Saturday onward, with developing uncertainty over eastward progression corresponding to how quickly a storm system tracks into/across the Bering Sea by the start of next week. This pattern should produce multiple days of cool and unsettled weather for most of the state, and lead to increasing precipitation over the Panhandle and Southcentral coast from the weekend into early next week. ...Guidance Evaluation/Preferences... At the start of the period early Thursday, consensus has held up well for the position of extratropical Ampil which should be near the northwestern mainland coast at that time. Latest UKMET runs have joined other recent solutions though the 12Z CMC may be a bit on the fast side. In the wake of extratropical Ampil, predictability declines for features within the progressive flow. There is decent model clustering for a compact shortwave to cross the Bering Sea and reach the mainland by Friday, albeit with typical timing uncertainties. Then guidance generally signals potential for one or more pieces of upstream energy to amplify into a better defined upper trough from the mainland into the Pacific. Details of initial flow and this potential amplification/sharpening aloft will determine the surface evolution across the North Pacific into the Gulf of Alaska along with the magnitude/timing of moisture focus into the southern coast and Panhandle by Saturday-Monday. Meanwhile there is decent continuity with a wavy cold front pushing into the mainland, though of course with typical variability in details. The 12Z CMC appears most at odds with other dynamical and machine learning (ML) guidance, with a stronger Friday Aleutians wave which tracks just south of the Alaska Peninsula and into the Gulf--in contrast to consensus that is more suppressed. By Saturday-Monday the operational models have been waffling with respect to trough evolution and associated surface reflection over the Pacific. The 12Z GFS/ECMWF both adjusted toward greater trough amplitude into the Pacific with a closed low, helping to draw developing Pacific low pressure northward into the Gulf. The new 18Z GFS has pushed the upper trough and surface low farther eastward. In contrast, other dynamical model runs, ensemble means, and 00Z/06Z ML models simply maintain persistent low pressure over the Gulf without input from Pacific development. The latter scenario would lead to a less pronounced moisture surge along the southern coast/Panhandle, but still with a favorable pattern for precipitation in general. The favored blend leans more toward the majority cluster but with some incorporation of a modest Pacific frontal wave into the more dominant low. The Sunday-Monday system forecast to track into the Bering Sea has held up well in concept over the past day, but with fairly typical timing/latitude spread for a days 7-8 forecast. By Monday the best clustering among dynamical/ML models is around 170W-180 longitude and the central/southern Bering Sea. The 12Z ECMWF is somewhat on the slow side of the spread, but there has been a slightly slower trend from yesterday and the new 12Z ECens mean adjusted somewhat slower than its prior run as well (but is still faster than the operational run). A model/ensemble mean compromise remains a reasonable starting point while awaiting better agreement to allow for a deeper trend toward an operational model average depth. With the CMC not comparing well to other guidance for some aspects of the forecast, the first half of the period incorporated mostly the 12Z GFS/ECMWF with some 12Z UKMET input. Given the differences that develop after early Saturday, the forecast then shifted to a model/ensemble mean mix the rest of the way through Monday. 12Z GEFS/00Z ECens weight reached 45 percent by Monday while GFS input was split between 12Z/06Z runs and the ECMWF between the 12Z/00Z runs. ...Weather/Hazards Highlights... The forecast pattern will favor decent coverage of precipitation over Mainland Alaska during the period, with some areas of localized focus especially along favored terrain. 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. Other ranges could see isolated high elevation snow as upper troughing becomes more prominent around the weekend. Farther west, a wavy front near the Aleutians may produce some light to moderate rain but still with some uncertainty over northward extent/totals. Then it becomes a significant question mark whether this waviness and Pacific moisture ultimately reach into the Northeast Pacific/Gulf of Alaska to enhance rainfall along the Southcentral coast/Panhandle by Saturday-Monday. At the very least, there is reasonable guidance consensus for a wetter period over that region as mean low pressure persists over the Gulf. The potential input of Pacific surface development/moisture is a less confident aspect of the forecast. The system forecast to track into the Bering Sea by Sunday-Monday may increase winds and rain over the Bering Sea and Aleutians. Expect much of the state to see below normal high temperatures Thursday-Monday, with the coolest anomalies from late week into the weekend. Primary exceptions will be on Thursday with the southern Panhandle experiencing above normal highs as upper ridging passes over/south of the area, and the northern coast seeing relative warmth due to southerly downslope/offshore flow. Anomalies for morning lows will start out more above than below normal on Thursday but trend mostly below normal thereafter, with some lingering above normal readings along the northern coast and southern 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