Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 753 PM EDT Fri Aug 16 2024 Valid 12Z Tue Aug 20 2024 - 12Z Sat Aug 24 2024 ...Extratropical Ampil to bring heavy rain and strong winds to portions of western Alaska around the middle of next week... ...Overview... Today's guidance shows only minor refinements for the extratropical reflection of Typhoon Ampil, which will be the dominant system of interest during the extended period. This system should be over the western Bering Sea as the forecast starts early Tuesday and track northeastward with time, most likely passing close to the northwestern coast by early Thursday, and bringing an episode of high winds and significant precipitation to parts of the western mainland around midweek. Expect this to be the last in a series of vigorous southwest-to-northeast systems, as models/ensembles continue to show an evolution toward flat to broadly cyclonic flow aloft from Siberia through Alaska later in the week. Such a pattern will lead to lower predictability for various embedded features. Also of note early in the period, an amplifying upper ridge crossing the mainland ahead of extratropical Ampil will support southward digging of shortwave/closed low energy over the Panhandle Tuesday into Wednesday, likely with fairly modest effects over the region. ...Guidance Evaluation/Preferences... Preferred continuity from yesterday has held up fairly well for most aspects of extratropical Ampil. Over the past day the CMC has trended toward recent GFS/ECMWF runs and machine learning (ML) models as of Tuesday with a fairly strong storm system over the western Bering Sea--at least as deep as the 970s mb with a small number of recent dynamical solutions getting as low as the 960s mb. The weaker ensemble means aligned with the majority cluster for track as well. This leaves the 12Z UKMET as a weak model extreme on Tuesday (before solutions converge better by Wednesday). As the system continues northeastward, the 12Z guidance on average has adjusted farther south close to the northwestern mainland coast as of early Thursday compared to prior consensus. By this time there is somewhat increased uncertainty due to dependence of specifics on interaction with surrounding dynamics aloft. The favored blend adjusts southward from continuity but not quite to some of the 12Z models. After midweek, detail aspects of the forecast rapidly decrease in predictability in the wake of extratropical Ampil as mean flow trends toward a flat to broadly cyclonic configuration. Thus confidence is fairly low with potential Siberia into Bering Sea/Alaska features as well as frontal/wave specifics over the North Pacific and Aleutians, plus late week orientation of flow into the Panhandle and associated precipitation. Amidst this detail uncertainty, the most common theme is for low pressure to settle over the Gulf of Alaska by late next week. Within the northern stream, the most trackable feature is a compact upper low that should start the period over or near Siberia. There is a fair signal in most guidance that this upper low will track eastward eventually with some degree of surface reflection, though the 12Z CMC is currently a pronounced slow/amplified extreme aloft and deep with the surface low. Meanwhile, toward next Friday and especially Saturday the latest GFS runs ultimately develop deeper troughing than remaining guidance from the mainland into the northeastern Pacific (bringing more moisture into the Panhandle). Recent trends of dynamical/ML guidance as a whole seem to suggest that the lingering front over the North Pacific could hang up closer to the Aleutians for a time. However recent ECMWF runs (which have been waffling considerably for timing) are on the deep/northern side of the spread for what waviness may develop in response to shortwave energy on the southern fringe of the westerlies. There may be some dependence on Bering Sea details as well. The preferred blend shows a weak wave tracking near the Aleutians to reflect the general potential of the feature but with minimal confidence in specifics. The first half of the forecast started with a 40/35/25 percent weight of the 12Z ECMWF/GFS/CMC to reflect the best detail for extratropical Ampil. Then with CMC comparisons becoming less favorable and detail predictability rapidly decreasing, the latter half of the forecast phased out the CMC and steadily increased 12Z GEFS/00z ECens mean weight--reaching 70 percent by next Saturday. ...Weather/Hazards Highlights... Most aspects of the forecast of extratropical Ampil have maintained good continuity over the past day, with a track from the western Bering Sea early Tuesday to a position near or north of the northwestern mainland coast by early Thursday. Expect this storm to produce a broad area of strong winds across the Bering Sea and then extending into the western mainland, with the Days 3-7 Hazards Outlook depicting best potential for high winds over the western mainland on Wednesday. Heavy rainfall is also possible, with the Hazards Outlook indicating the best focus over parts of the northwestern mainland also on Wednesday. Some brisk winds and enhanced rainfall may extend into Thursday but with magnitudes below hazardous criteria. 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. The forecast pattern evolution to flat or broadly cyclonic flow aloft should maintain unsettled conditions over most of the state through the end of the week, but with low confidence in details. Expect most of the state to see below normal high temperatures during the Tuesday-Saturday period, with the greatest coverage of coolest anomalies likely to be toward the end of the week. As for exceptions, the southern Panhandle may see well above normal highs around Wednesday-Thursday and the northern coast should be above normal for at least the first 3-4 days of the period. Some pockets of slightly above normal highs are possible over Southcentral on Wednesday as well. Anomalies for morning lows will vary from below normal on Tuesday (and still over the east on Wednesday) under high pressure, rise to above normal for a time ahead of and immediately behind extratropical Ampil's cold front, followed by moderately below normal readings moving in from west to east by Friday-Saturday. 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