Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Sun Mar 31 2024 Valid 12Z Thu Apr 04 2024 - 12Z Mon Apr 08 2024 ...Overview... Models and ensembles continue to show a moderately progressive pattern to start with late this week but generally indicate a deceleration with time, ultimately leading to a mean trough axis aloft settling over the western mainland by around the start of next week. Within the initial progressive pattern, one or more surface systems near the eastern coast of the mainland and far eastern Siberia should spread a broad area of moisture across the mainland while supporting upper troughing generates a Gulf of Alaska wave and Southeastern coast/Panhandle precipitation focus around the end of the week. A fairly vigorous and windy storm system still looks likely to track over the southern Bering Sea or Aleutians around late Friday into the weekend, reaching the Kodiak Island/Gulf of Alaska region with its surrounding moisture shield by early next Monday. Meanwhile guidance continues to differ for the precise upper flow details over the Arctic and into parts of the mainland from the weekend into early next week. ...Guidance Evaluation and Preferences... Guidance continues to show some spread and timing variability for the system reaching near the eastern tip of Siberia as of the start of the period early Thursday. Today the GFS/GEFS runs lie between the slower ECMWF/ECens and faster UKMET. Holding onto an intermediate solution provides the best stability compared to continuity. At the same time the signal persists for a fast-moving wave to the south, tracking from the North Pacific through Bering Sea and reaching near the western coast of the mainland by early Thursday. The surface low itself is not extremely well defined but guidance thickness fields are much more prominent with the frontal system that this wave anchors. Consensus yields a solution similar to continuity for this feature. By Friday the latest GFS runs become deeper than remaining guidance (other dynamical models and 00Z ECMWF-initialized machine learning models/MLs) with upper dynamics crossing the mainland into northeastern Pacific, thus leading to a stronger Gulf of Alaska surface wave. This discrepancy becomes less noticeable as the wave lingers into Saturday. On the other hand, the 12Z CMC is on the weak side of the envelope. The preferred blend maintained good continuity for this feature. Guidance continues to suggest high confidence in the existence of the late Friday onward system tracking across the southern Bering Sea/Aleutians and eventually emerging into or near the Gulf of Alaska by next Monday. However the dynamical models and ensembles along with the MLs are still waffling for timing. Latest GFS runs have been leaning on the faster side of the spread. The prior couple ECMWF runs and the 00Z ECens mean were on the slow side but the new 12Z ECMWF run shifted significantly faster but not to the extent of the GFS by Sunday-Monday. The 12Z CMC/CMCens are in the middle. 00Z MLs showed a fair amount of spread as well, lowering confidence in a particular side of the envelope. Preference sides with an intermediate timing closest to the CMC and generally between the 00Z/12Z ECMWF runs. The 12Z ECens mean arriving after forecast preparation (with a somewhat faster trend from the 00Z run) generally supports this idea. The favored forecast blend yielded slightly faster progression than yesterday, with the surface low reaching close to Bristol Bay as of early Sunday and near Kodiak Island early Monday. Consensus indicates the storm should be deepest around the time it reaches the western Aleutians/southern Bering Sea with a central pressure in the 970s mb as of early Saturday, with gradual weakening thereafter. Model/ensemble spread and run-to-run variability continue for upper flow details over the Arctic into the mainland. The primary general signal/trend of note is for mean troughing to extend from the high latitudes through the vicinity of the Bering Strait. Within this pattern, some GFS/ECMWF runs have been bringing a deep upper low close to or even into the northwestern mainland, versus keeping it well northward in other models and the means. The 06Z/12Z GFS actually dropped an upper low all the way south through the Alaska Peninsula but the new 18Z run has at least returned back north near the 00Z/12Z ECMWF runs. Prefer a conservative approach closer to the means until the models achieve better clustering and stability. Upper trough definition has improved enough to yield a better signal for a leading surface front over the mainland from the weekend into early next week. Preferences for the various facets of today's forecast led to an evolving blend with time. Day 4 Thursday incorporated the 12Z GFS/ECMWF/CMC/UKMET in order from more to less weight, followed by switching GFS/ECMWF emphasis somewhat by day 5 Friday. As timing differences arose for the southern Bering Sea/Aleutians storm, day 6 Saturday split ECMWF input among the 00Z/12Z runs and began weighting the CMC more than the GFS. Days 7-8 Sunday-Monday eliminated the GFS and gradually lowered ECMWF influence while increasing total 12Z GEFS/CMCens plus 00Z ECens weight to 30-50 percent. ...Weather/Hazards Highlights... The combination of northern Bering Sea/eastern Siberia low pressure and a separate potential wave/frontal system reaching the western coast of the mainland by early Thursday will likely spread a broad area of precipitation and brisk winds into the mainland on Thursday with the supporting upper trough helping to maintain some of the precipitation into Friday. Expect relatively higher precipitation totals over the southwestern half of the mainland with best emphasis over windward terrain. Precipitation focus should also extend along the southeastern coast/Panhandle for a couple days late in the week, aided by a Gulf of Alaska surface wave which guidance has been consistent in depicting, albeit with some uncertainty for strength. The system forecast to track across the southern Bering Sea or Aleutians should spread another broad area of precipitation and strong winds (especially to the south of the low track) across that region late in the week into next weekend. Farther eastward, moisture from this system should be confined more to the far southwestern mainland and southern coast by next weekend. Snow and coastal/low elevation rain will likely reach the Panhandle by around next Sunday-Monday. The pattern evolution from late this week into early next week will support mostly above normal morning lows across the state, with the greatest anomalies likely over northern and western areas late this week. The North Slope should see above normal highs through the period as well but the rest of the mainland will likely see more of a mix between above/below normal highs. Expect the Panhandle and areas near the Southcentral coast to see highs remain somewhat below normal through the period. 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