Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Wed Sep 25 2024 Valid 12Z Sun Sep 29 2024 - 12Z Thu Oct 03 2024 ...Heavy precipitation likely across favored portions of Southcentral this weekend... ...Wind and heavy rain forecast for Southeast Alaska early next week... ...Overview... An upper low will track from Bristol Bay and surrounding areas eastward through the Gulf of Alaska early next week, which will support surface lows including a possibly strong midlatitude low turning northward into the northeastern Pacific near Southeast Alaska. These features should direct moisture into Southcentral for possibly heavy precipitation over the weekend, and bring heavy precipitation and strong winds into parts of the Panhandle early next week. Behind these upper/surface lows, a couple of troughs and surface lows may approach Alaska from the Bering Sea around Tuesday-Thursday. ...Guidance Evaluation/Preferences... Recent model runs agree that an upper low should be developing as the medium range period begins Sunday, centered atop Bristol Bay/Kuskokwim/Alaska Peninsula areas. There is a general consensus that this upper low could drop southeast and then east across the Gulf Monday-Tuesday, with the energy aloft as well as the associated surface lows combining with the possibly deep low pressure system initially tracking across the midlatitude Pacific and then lifting into the northeastern Pacific. While there is still notable spread with these forecast features, at least models are more agreeable today in the track of the surface low compared to the past couple of days. Namely, model guidance is slower to turn the surface low northward in the Pacific, leading to an eastward track toward Southeast Alaska. Within the broad consensus, the 12Z ECMWF had a more southern track toward Haida Gwaii while the 12Z CMC and especially UKMET were slower with the low. Since the timing of the low already trended slower since yesterday, tended toward the 12Z GFS that was a bit faster with a middle ground position. Thus this forecast brings heavier QPF into the Panhandle as the low approaches. Behind these systems, guidance generally shows a round of troughing spilling southeast from eastern Siberia into the Bering Sea around Tuesday, perhaps moving into western Alaska. The 12Z GFS was weaker with this feature while the 12 CMC is stronger, but dives energy well south into the Pacific. The 12Z ECMWF seems to match the ensemble means best with this trough. Then another, potentially stronger, trough along with a deep surface low looks to come into the Bering Sea later next week as well. The WPC forecast began with a blend of the 12Z GFS, ECMWF, and CMC deterministic guidance, and quickly reduced the proportion of deterministic runs in favor of the ensemble means, reaching over half means by the end of the period as spread increased. ...Weather/Hazards Highlights... Precipitation chances are forecast along the Southern Coast with low pressure in the northeast Pacific. Precipitation totals have come down somewhat in Southcentral in favor of higher totals in Southeast, but heavy precipitation is still possible in favored areas like the terrain on the west side of Cook Inlet and the southern terrain of the Kenai Peninsula to Prince William Sound, with rain and/or snow depending on timing and elevation. On Monday and Tuesday, heavy precipitation is forecast for the Panhandle with the low and a weak atmospheric river approaching, but not blockbuster amounts for this typically wet time of year. Additionally, high winds are a potential threat as the surface low approaches. The depth and exact track of the low make impacts uncertain, but the far southern Panhandle may have the best chance for impactful winds. High winds are likely over marine areas. Then by Tuesday-Thursday, there may be an uptick in precipitation across western parts of the state associated with systems coming from Siberia into the Bering Sea toward Alaska. In a broad sense, cooler than normal high temperatures are likely over southern parts of Alaska and warmer than normal temperatures are likely for the northern two-thirds of the Mainland during the period. 40s to low 50s are forecast for highs in most lower elevation areas, with a slight cooling trend as next week progresses. Most areas should see greater coverage of above normal morning lows though, with most of the below normal readings confined to the southwest and the Panhandle. Tate 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