Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 748 PM EDT Tue Mar 16 2021 Valid 12Z Sat Mar 20 2021 - 12Z Wed Mar 24 2021 ...Overview and Guidance/Uncertainty Assessment... Guidance shows a progressive and active pattern across the Bering Sea/North Pacific into the Gulf of Alaska. A leading eastern Bering system should track into the Gulf of Alaska during the weekend and then a North Pacific storm will likely turn northeastward into the Gulf during the first half of next week, followed by another system that tracks into the Bering Sea. These systems are likely to focus the highest precipitation totals over the Panhandle and perhaps to parts of the southeastern coast, with somewhat lesser totals from the Aleutians through the rest of the southern coast. An upper trough initially over Siberia and Kamchatka should play a role in the evolution of the second Gulf system and support reinforcement of below normal temperatures as it settles over the mainland next week. Latest models and ensemble means display better than average agreement for significant features through the period. Thus a 12Z operational model blend provides a good representation of the most likely scenario through the first half of the period, and in spite of some detail or timing differences that develop later in the period the models/means are sufficiently comparable to allow keeping two-thirds operational model weight into day 8 Wednesday with the 12Z GEFS/00Z ECMWF means providing the rest of the input. Even with the decent clustering in today's guidance there are some trends of note. At the start of the period early Saturday the leading Bering Sea into Gulf system is farther north than yesterday's consensus. The combination of stronger low pressure on the trailing edge of this system and somewhat slower progression of the upper trough approaching from Siberia/Kamchatka ultimately leads to a better defined surface wave across the western/northern mainland plus trailing front from the weekend into next week. The slower adjustment of the upper trough also leads to a slightly farther westward track of the storm that initially moves south of the Aleutians and then continues toward the Gulf as the trough phases with the Pacific dynamics. Uncertainty over the specifics of this phasing process will likely provide some lingering detail questions for a while. The primary differences with the late period Bering storm and associated front involve timing, with the GFS characteristically on the faster edge of the envelope. The favored blend provides a good intermediate starting point with operational model variability over the past couple days making it difficult to rule out any individual solution at this point. ...Weather/Hazards Highlights... This weekend's system will generally produce light to moderate precipitation from the Alaska Peninsula/southwestern mainland through the southern coast and Panhandle. Early next week the Aleutians and Peninsula may see a period of precipitation and enhanced winds on the northwest side of the trailing North Pacific system but this will be sensitive to the exact track of the surface low. Then this storm will quickly bring increasing moisture to the southern coast and especially Panhandle with Tuesday-Wednesday being the most likely time frame for potentially heavy precipitation. Models and ensembles currently disagree over the magnitude of precipitable water anomalies (12Z ECMWF being the highest, reaching at least plus 2-3 standard deviations for a time) but this event merits monitoring over the coming days given the lower-latitude origin of the moisture. The system forecast to track across the central latitudes of the Bering Sea late in the period would bring a front across the southern Bering and Aleutians into midweek, accompanied by precipitation and brisk to strong winds. Expect a majority of the state to see below normal temperatures during the period with anomalies tending to be colder for daytime highs relative to morning lows (with some pockets of slightly above normal anomalies possible for the latter). Extreme northern areas may see moderately above normal readings. The upper trough arriving into the mainland during the first half of next week should push temperatures somewhat farther below normal compared to the weekend. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle, Tue, Mar 23. 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