Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Tue Sep 03 2024 Valid 12Z Sat Sep 07 2024 - 12Z Wed Sep 11 2024 ...Overview... The Gulf of Alaska system producing heavy precipitation along the southern coast and Panhandle into late this week will be in the process of weakening by the start of the period Saturday, though upper troughing should linger over the Northeast Pacific through at least the weekend. Upstream energy over eastern Siberia and vicinity should support a surface low/frontal system but with a fair amount of spread for if/how low pressure reaches the mainland. By the early to middle part of next week, the primary forecast problem will be a potential Bering Sea or Aleutians system which so far has exhibited a lot of spread and run-to-run variability in the guidance. Also with a lot of detail uncertainty, guidance suggests there will be a general weakness aloft near the northern coast of the mainland. The forecast pattern will favor a broad area of light to locally moderate precipitation during the weekend and an eventual transition toward more focus over at least parts of the southwestern half of the state. Temperatures will tend to be above normal for morning lows and below normal to slightly above for daytime highs. ...Guidance Evaluation/Preferences... Regarding the dynamics and surface system initially over and/or near Siberia, there is a lot of spread regarding low-predictability details that will influence how upper energy will evolve/track with corresponding effects at the surface. 00Z/06Z ECMWF-initialized machine learning (ML) models generally suggest some shearing of the dynamics aloft, leading to less-defined low pressure and possibly less eastward progression than GFS/CMC runs. Not surprisingly, the 12Z GEFS mean hedges in the GFS direction while the latest CMCens/ECens means lean weaker/slower at the surface. A blended operational model solution that ends up closest to the ensemble means seems to provide the best intermediate solution for now. Guidance behavior thus far leads to lower than average confidence for details of the next system Monday-Wednesday, with differences in upper dynamical details yielding a potential primary low track ranging anywhere between the northeastern Bering Sea and south of Kodiak Island by midweek. Recent ensemble means have been the most consistent source of guidance with a track along the central Bering Sea and reaching the western coast of the mainland close to Nunivak Island. The 12Z GEFS run was rather ill-defined though. Recent operational runs have been spread all over. The 12Z ECMWF reaches close to Norton Sound after the 00Z run was south of Kodiak Island. The 06Z and new 18Z GFS are along/south of the Aleutians/Kodiak Island while the 12Z run track into Bristol Bay. The past couple CMC runs have been slow extremes over the Bering Sea. ML models reflect the dynamical spread as well, but their centroid and the AIFS ensemble mean end up near the dynamical means. Therefore deterministic forecast preference sides with the ensemble means, though with enough of a signal to carry a triple point wave that can be considered a hedge in the event of any southward trends in clustering. As for strength, the 00Z and 12Z GFS runs are notably deep extremes. Other models look more plausible while the average of ML models is in the low-mid 990s mb. Upper flow details over the northern mainland also are diverse with low predictability in details. The 12Z CMC and 00Z ECMWF seem to have relatively less support from guidance as a whole for their persistence of a moderately deep upper low near the northern coast of the mainland. There is a more prevalent theme of a weakness but with slightly higher heights aloft. Based on detail uncertainties in the first half of the forecast and preference for a central Bering Sea primary low track early-mid next week, today's forecast started with a blend of the 12Z ECMWF/GFS/UKMET/CMC (in order of most to least weight) from Day 4 Saturday into early Day 6 Monday and quickly trended toward about half models/half means by Days 7-8 Tuesday-Wednesday. Day 7 operational weight leaned much more toward the ECMWF while Day 8 brought the ECMWF/GFS closer together as they became more equally distant on opposite sides of the means. Only the 12Z CMCens/00Z ECens were used for the means due to the GEFS being questionably weak over the Bering Sea. ...Weather/Hazards Highlights... Before the start of the period, Gulf of Alaska low pressure will support heavy rainfall along parts of the southern coast and Panhandle continuing into Friday. Thus the Day 3-7 Hazards Outlook depicts a heavy rain area from the southeastern Kenai Peninsula to the northern Peninsula. Expect this precipitation to trend much lighter by Saturday as low pressure steadily weakens. Weak shortwave energy over the mainland and possibly some shortwave energy/surface low pressure approaching from Siberia and vicinity will produce areas of light to moderate precipitation from the weekend into early next week. A fast-moving wave brushing the Aleutians may produce a band of enhanced rainfall, but with a fair amount of uncertainty for exactly where this band will ultimately be relative to the Aleutians. Developing upstream low pressure Monday-Wednesday should bring a shield of moisture and brisk to strong winds across some areas, potentially reaching into parts of the western and southern mainland by next Tuesday-Wednesday. Confidence remains low for the precise track with a broad potential envelope thus far, so continue to monitor forecasts updates. The best potential for highest totals exists near the southern coast. Precipitation through the period may be in the form of snow at higher elevations. The forecast pattern will favor greater coverage of below normal daytime high temperatures during the period. Some areas over the eastern Interior and along the northern coast should see moderately above normal highs. Meanwhile the unsettled pattern will favor mostly above normal morning lows, with only portions of the Panhandle and perhaps a few other isolated pockets over the mainland seeing below normal lows. 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