Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 648 PM EST Sat Jan 16 2021 Valid 12Z Wed Jan 20 2021 - 12Z Sun Jan 24 2021 ...Overview and Guidance/Uncertainty Assessment... Latest models and ensembles continue to advertise a significant pattern change, with greater mean ridging aloft expected to extend into the mainland from the Pacific. This evolution will tend to keep strongest low pressure systems back over the Aleutians/Bering Sea. There will still be some weaker features heading into/through the mean ridge but by next weekend there should be a more pronounced trend toward surface high pressure over the mainland. The pattern will also promote above normal temperatures over most areas with much above normal readings most likely over northern and western areas. In an ongoing theme, individual solutions generally reflect the expected mean pattern evolution but vary with the details of individual systems. At the start of the period early day 4 Wed the majority cluster of new 12Z guidance has a compact wave near the southern coast. The 12Z GFS is on its own (even versus its mean) with a farther south track. Recent days have varied anywhere between the southern coast and Bering Strait. Farther west there has been a fair degree of ambiguity about the specifics of mid-late week low pressure tracking north from the Aleutians through the Bering Sea. The 12Z models have gravitated toward a stronger/consolidated system with a track somewhat east of the ensemble means. This consensus is closest to GFS ideas from the past day or so but not quite as deep. By late in the week the 12Z ECMWF may stray a bit far to the east with its track through the Bering Strait. At the same time there is a bit more of a signal today for waviness that may reach near the eastern Aleutians/Alaska Peninsula by early Fri. This overall area of low pressure may be a combination of a frontal wave and a western low tracking south of the Aleutians. As supporting dynamics aloft continue into the mean ridge, there is better continuity with a consolidating surface system that reaches the Gulf by early Sat. This system will dive southeastward thereafter as upper ridging rebuilds to the west. Continuity is also fairly good into day 7 Sat for a system forecast to track into the western Aleutians/Bering Sea. However specifics become more murky by next Sun, with a general consensus for an axis of lower surface pressures over the western Aleutians/Bering but with a lot of spread for location/strength of any defined lows. Today's blend started with a 12Z operational model composite early, again downplaying the GFS specifics for the wave near the southern coast on Wed. The combination of increasing detail uncertainties as well as various specifics of the 12Z ECMWF becoming more or less favorable relative to the old 00Z run led to trending next weekend's forecast toward a model/mean blend that included both ECMWF runs, the 12Z GFS, and the 12Z GEFS/00Z ECMWF means. The new 12Z ECMWF mean provided good support for the late-period approach. ...Weather/Hazards Highlights... The modest wave forecast to be near the southern coast as of early Wed should be accompanied by light to moderate precipitation. During mid-late week a larger area of precipitation and some brisk to strong winds will spread eastward through the Aleutians/Bering Sea into the western and southern mainland, in association with low pressure tracking northward through the Bering and its leading front. Highest precipitation totals should be along favored terrain from the Alaska Peninsula northward, with meaningful totals also extending along the southern coast as a wave likely reaches/consolidates over the Gulf by early Sat. Some of this moisture may extend into the Panhandle by around Fri night or the weekend. The upper ridge building in behind this system will lead to a drier trend over a majority of the state next weekend. Meanwhile the pattern farther west will promote one or more episodes of precipitation and enhanced winds over the western Aleutians from late Fri through the weekend. Above to much above normal temperatures will prevail over most areas through the period. Expect the greatest anomalies to be focused over northern and western portions of the mainland. Some pockets of near to slightly below normal readings are possible near the Southcentral Alaska coast and below normal temperatures could filter into the southern two-thirds of the Panhandle after midweek. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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