Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 751 PM EDT Fri Apr 18 2025 Valid 12Z Tue 22 Apr 2025 - 12Z Sat 26 Apr 2025 ...Overview... The Tuesday-Saturday period will feature two primary features of interest. The first is an eastern Bering Sea and vicinity upper trough/low that likely drops into the northeastern Pacific and away from the mainland, with associated low pressure bringing a period of rain and higher elevation snow primarily to areas along the southern coast. Then a northwestern Pacific system should track along or north of the Aleutians with some enhanced precipitation and brisk winds. Meanwhile, there is reasonable consensus for a high latitude deep-layer ridge that will maintain fairly strong easterly winds across the North Slope. Guidance shows a lot of detail differences for upper flow details over the mainland though. Ensemble means suggest a transition from weak ridging to development of a col region while operational model runs vary with how much ridging or weakness/closed low may exist from day to day. Even with these differences, there is at least better agreement upon most of the mainland staying within the gradient between high pressure over the Arctic and low pressure systems to the south. ...Guidance/Predictability Assessment... For the system forecast to track from the eastern Bering into the northeastern Pacific, latest guidance clusters fairly well as of early Tuesday but then diverges with how quickly the upper low energy drops southeastward and at what longitude. This affects the surface low track along with duration and eastward extent of associated precipitation along the southern coast. Among the dynamical guidance, the 12Z GFS looks particularly quick to pull the system southward and trend drier along the southern coast. Other solutions still vary (12Z UKMET/CMC extending farthest eastward with the front and bringing the most precipitation into the Panhandle--perhaps overdone), while some recent machine learning (ML) models suggest that low pressure could end up farther north in the Gulf versus most dynamical solutions. The 12Z UKMET would be the closest dynamical model to that idea. Initial preference follows a multi-model composite that downplays 12Z GFS influence and otherwise follows an intermediate approach. This leads to only moderate refinement from continuity while monitoring for possible trends in future cycles. There are also some noticeable differences for the handling of the system that may track near the Aleutians during the latter half of the week. In principle the ECMWF/ECens mean have been more consistent with the evolution than GFS/GEFS runs over the past couple days. As the system comes into the picture around Wednesday-Thursday the latest GFS runs lift the low/surface front somewhat north of most other dynamical guidance but some ML models offer potential for a low track that could be a little north of the dynamical majority. This ML model tendency persists into Saturday, supporting exclusion of the GFS/GEFS especially by that time as those solutions stray farther south. The 12Z CMCens and 00Z ECens (with confirmation from the new 12Z ECens run after forecast preparation) are somewhat deeper than the GEFS mean and favor a track along or a little north of the Aleutians, as recommended by the ML models. Meanwhile ECMWF timing differences later in the period favored averaging their two runs for that model's input and the 12Z CMC strays a bit north late but remains close to the northern edge of the ML model spread. There is decent agreement among the means for deep-layer Arctic ridging between 75-80 north latitude, with other models displaying typical spread. 06Z/12Z GFS runs and latest ECMWF runs compare well to the means but the new 18Z GFS displaces the ridge east/south in favor of an upper low lifting up from Siberia. The 12Z CMC becomes elongated/south with the ridge. Meanwhile, confidence is well below average for the fine-scale details aloft over the mainland. Based on other solutions the 18Z GFS has even relatively lower confidence for its upper low that forms over the central/southwestern mainland by the end of the week. Putting together the most common/favored themes of guidance, the first half or so of the forecast consisted of 40 percent 12Z ECMWF and the rest split evenly among the 12Z GFS/CMC/UKMET. After early Thursday the blend rapidly incorporated more 12Z CMCens/00Z ECens input, reaching 60 percent total by Saturday, while also splitting operational ECMWF input between the 12Z and 00Z runs. ...Weather/Hazards Highlights... The system dropping southeast from the eastern Bering Sea and vicinity will help to focus rain and higher elevation snow over the far southwestern mainland/Alaska Peninsula, and then with somewhat higher totals extending eastward along the southern coast and possibly the Panhandle as low pressure reaches the northeastern Pacific. Currently expect potential for some localized enhancement but amounts should remain below hazardous thresholds. Specifics are still very sensitive to the exact track of the low with no clear consensus yet. Some lighter activity could linger along the southern coast into late next week. Upstream Pacific low pressure should bring a period of organized rainfall and brisk winds to the Aleutians from west to east during mid-late next week. Leading flow may bring this precipitation into the Alaska Peninsula and vicinity toward the end of the week, but with decreasing confidence given the guidance spread for low pressure evolution and track. Deep layer ridging over higher latitudes of the Arctic will likely support strong easterly flow across the North Slope through the period. Wind speeds should not reach hazardous criteria but may lead to periods of blowing snow. Expect a fairly stable temperature regime during the period. Northern areas will tend to be below normal due to the proximity of Arctic ridging, while most areas to the south will see above normal readings--especially for morning lows. As for one exception, clouds and precipitation should support below average daytime highs along the Southcentral coast and Panhandle especially during Tuesday-Wednesday. 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