Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Fri Mar 29 2024 Valid 12Z Tue Apr 02 2024 - 12Z Sat Apr 06 2024 ...Overview... Guidance continues to show a moderately progressive pattern that should feature two prominent systems tracking into/through the Bering Sea. The leading one will likely spread a broad area of precipitation across the state for a couple days or so around midweek, while separate Gulf of Alaska waves early and late in the period may focus some precipitation along the southern coast and Panhandle but with mostly light to moderate totals. The first Gulf wave may produce strong winds to its immediate northwest. Aloft, midweek upper ridging will replace an initial mainland through Gulf of Alaska trough. Upstream energy should dampen the upper ridge by late week but with some uncertainty over the shape/orientation of upper flow over the mainland. ...Guidance Evaluation and Preferences... In terms of guidance discrepancies through the period, the most prominent focus is on the system forecast to be near the western Aleutians at the start of the period early Tuesday and track northeastward thereafter. Looking at solutions available through the operational 12Z ECMWF, latest guidance had generally sorted itself into two clusters. Operational models show a fairly progressive track that would bring this system into or just west of the northwestern mainland (12Z CMC straying fastest and with more waviness to its south, 12Z UKMET leaning slowest), in contrast to the ensemble means and 00Z ECMWF-initialized machine learning models (MLs) that favored holding the parent low well westward with only weak elongation of some degree of low pressure along an axis represented by the operational model low track. The GEFS mean did nudge its low pressure a little farther east than the 00Z ECens/12Z CMCens means. Especially for the first half of the extended forecast time frame, operational model trends tend to be somewhat more reliable on average so preference sided with the operational model composite, albeit with somewhat lower confidence than usual. This approach yielded stronger definition and slightly faster timing than continuity but with a reasonably comparable track. New guidance available after forecast preparation shows mixed messages. The 12Z ECens mean has trended more to the operational model ideas, while 18Z GFS differences aloft lead to a splitting of low pressure--a slower/southward primary system but a weaker leading frontal wave that ultimately tracks in line with the other operational models. Regarding other aspects of the forecast, guidance shows various ideas for how shortwave energy will ultimately dampen the Wednesday-Thursday upper ridge over the mainland by later in the week. Latest GFS runs are a bit on the deeper/sharper side of the spread for the mainland part of any upper troughing, though there's better agreement on what troughing reaches the northeastern Pacific. Clustering is reasonably good for the Gulf wave on Tuesday while the degree of upper troughing nearby will influence the definition of potential Gulf low pressure around the end of the week. Meanwhile, recent guidance has been showing a decent signal for a fairly strong storm to track into the western-central Aleutians/Bering Sea by next Friday-Saturday. Latest clustering has been gravitating toward the central Bering Sea as of early next Saturday, with a model/ensemble mean average looking like a good starting point. 00Z MLs were mixed on this storm's existence/strength but recently posted 12Z MLs are showing a better signal with most depicting a track more in line with the dynamical guidance. Guidance comparisons/preferences led to starting the forecast with a blend of 70 percent total 12Z GFS/ECMWF and 30 percent UKMET/CMC during the first half of the period, reflecting an intermediate operational model solution for the first Bering Sea storm. The blend quickly added 12Z GEFS/00Z ECens input after early day 6 Thursday, reaching 40-50 percent to account for decreasing confidence in details aloft over the mainland while maintaining somewhat better definition than the means for the second Bering Sea storm. ...Weather/Hazards Highlights... Low pressure evolving near the southern coast early next week will produce an area of high winds in the tight gradient on its northwest side. The Days 3-7 Hazards Chart depicts this wind threat to be most likely over parts of the southwestern mainland/eastern Alaska Peninsula, Kodiak Island, as well as the Kenai Peninsula and areas near Prince William Sound. This system may produce some lingering precipitation along the southern coast and Panhandle into Tuesday. Then the system forecast to track across the western-northern Bering Sea will likely spread a broad area of precipitation and brisk to strong winds across the Aleutians/Bering Sea and then across the mainland. Uncertain details of the storm system lead to moderate to lower confidence in the ultimate magnitude of winds and precipitation. In general, expect relatively higher precipitation totals over the southwestern half of the mainland with best emphasis over windward terrain. Some precipitation focus may also extend to the southeastern coast/Panhandle after midweek, aided by a potential surface wave. The next system reaching the Aleutians/Bering Sea should spread another broad area of precipitation and enhanced winds across that region toward the end of next week. An area of well below normal temperatures will be crossing the state on Tuesday, with some of this cold air lingering into Wednesday over eastern areas. Then expect much warmer air to move in from the west as upper ridging builds overhead around Wednesday-Thursday, with the North Slope likely to see the greatest departures above normal. Flattening of the upper flow toward the end of the week should bring highs back down closer to normal while keeping morning lows above normal. The Panhandle and portions of Southcentral may stay somewhat below normal, especially for highs, through mid-late week. 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