Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Mon Apr 01 2024 Valid 12Z Fri Apr 05 2024 - 12Z Tue Apr 09 2024 ...Overview... While some guidance diverges by early next week, the most agreeable and consistent solutions continue to show the upper pattern decelerating toward a western-central mainland through Gulf of Alaska upper trough from an initial moderately progressive pattern late this week. Leading shortwave energy will support a Gulf of Alaska surface wave and Southeastern coast/Panhandle precipitation focus around the end of the week. A fairly vigorous storm system remains likely to track over the western Aleutians and southern Bering Sea around late Friday into Saturday, and emerge into or near the Gulf of Alaska by early next week. This system should produce strong winds especially near the Aleutians, with highest precipitation totals tending to be along parts of the southern coast and Panhandle later in the period. Guidance continues to differ for precise upper flow details over the Arctic and into the mainland during the weekend into early next week, with the most likely scenario being an Arctic upper low helping to anchor a deepening trough that extends south over the northwestern mainland. ...Guidance Evaluation and Preferences... Guidance clustering for timing has improved noticeably for the strong storm whose track on average has been fairly well forecast to be across the western Aleutians into the southern Bering Sea and then across the Alaska Peninsula and into/near the Gulf. Latest GFS runs have nudged a little slower than yesterday's runs, and while the ECMWF has been nudging a bit slower after its faster jump 24 hours ago, the overall consensus has trended slightly faster by the time the system crosses the Alaska Peninsula and reaches the Gulf by Sunday-Monday. Dynamical guidance and ECMWF-initialized machine learning models (MLs) agree the storm should be strongest as it crosses the western Aleutians/southern Bering Sea with a central pressure in the 970s mb with gradual weakening thereafter. A composite of latest guidance provides a reasonable starting point at this time. Ahead of this feature, guidance shows good agreement and continuity in principle for a Friday-Saturday Gulf wave supported by progressive shortwave energy. There are lingering detail differences, with most dynamical models/ensembles and MLs favoring a weaker surface system than GFS runs thus far but perhaps slightly faster shortwave progression than seen in ECMWF runs. Some guidance continues to be very diverse and erratic with respect to Arctic through western mainland flow from the weekend into next week. Among the operational models, recent ECMWF runs have been the most consistent in depicting a deep upper low drifting fairly close to the northwestern mainland. Consecutive GFS runs have been all over the place, between keeping the upper low well north like or at least closer to the means (ECens means farthest south but not quite as far south as the operational ECMWF runs) as in the 00Z/06Z and new 18Z run or dropping it south through the entire western mainland such as seen in the 12Z version. The 12Z CMC also brings the upper low well south into the mainland after the past couple runs just brought weaker sheared energy south into the mainland while keeping the main upper low farther north. Thus far the MLs have favored keeping the upper low well north but with varying ideas for how much troughing reaches the mainland and when. The ensemble means have at least been fairly stable with their northwestern mainland upper trough and have trended a little deeper over the past day, so preference sides closest to the means toward the end of the period with ECMWF runs meriting minority inclusion as the operational scenario closest to the means in principle. On the western side of the mean trough by next Tuesday, some operational models and most 00Z MLs have been hinting at the potential for some shortwave energy to be embedded within the flow. Most solutions depicting this energy suggest that any surface reflection should be fairly modest. With this part of the forecast having low predictability and larger-scale preferences siding more with the means, the current forecast does not depict any well-defined feature at this time. The first half of the period started with a blend of slightly more 12Z GFS/ECMWF relative to the 12Z CMC/UKMET, to yield an intermediate solution for the late week Gulf wave and represent consensus for the Aleutians/southern Bering Sea storm. Days 7-8 Monday-Tuesday quickly increased total 12Z GEFS/00Z ECens weight to 40-70 percent while splitting ECMWF input between the 00Z/12Z runs due to developing detail differences and eliminating the GFS/CMC after Monday due to their questionable mainland upper low. ...Weather/Hazards Highlights... Expect a late week Gulf of Alaska wave to support precipitation focus along the southeastern coast/Panhandle especially on Friday, with most locations seeing moderate totals. The system forecast to track across the southern Bering Sea/Aleutians should spread a broad area of precipitation and strong winds (especially to the south of the low track) across that region late this week into Saturday. Winds could reach storm force at some locations within the axis of strongest winds. Gradual weakening of the surface low as it tracks across the Alaska Peninsula and settles into or near the Gulf should reduce wind speeds with time, though the Aleutians/Alaska Peninsula may see a period of brisk northwesterly winds behind the system. The storm's winds combined with higher than average tides due to the lunar phase around this weekend also raise a concern for coastal flooding at some locations. Beyond the Aleutians, moisture from this system should extend across the far southwestern mainland/Alaska Peninsula and then the southern coast plus the Panhandle, with the latter seeing the highest totals around Sunday-Monday. Also over the course of the period, an upper trough currently forecast to deepen over the northwestern mainland should push a front southeastward across the state. The upper trough/front could generate some areas of light snow. From late this week into the weekend, expect highs to be above normal over the North Slope and below normal over the Panhandle and parts of Southcentral, with a mix of anomalies in-between. Most areas should see above normal morning lows. Temperature trends from late weekend into the first half of next week will depend uncertain flow details aloft. Currently the most likely scenario is for a deepening upper trough over the northwestern mainland to promote a colder trend from west to east, bringing temperatures down to below normal levels especially over western-central areas by next Monday-Tuesday. 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