Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 756 PM EDT Tue Sep 24 2024 Valid 12Z Sat Sep 28 2024 - 12Z Wed Oct 02 2024 ...Heavy precipitation likely across favored portions of Southcentral this weekend... ...Overview... A trough axis is forecast to be in place across the eastern Bering Sea and western Mainland to Alaska Peninsula as the medium range period begins Saturday. Shortwaves within the western trough are forecast to combine and create an upper low centered over Bristol Bay or so early next week, directing moisture into Southcentral for some heavy precipitation over the weekend. While details of Pacific surface lows are still uncertain, recent models suggest a midlatitude low may turn northward into the eastern Pacific near Southeast Alaska and spread some moderate to heavy precipitation there early next week. Troughing should move eastward as next week progresses, and will have to monitor Arctic energy moving toward northwestern Alaska near the middle of next week. ...Guidance Evaluation/Preferences... Recent model runs agree that troughing over the western Mainland will be in place Saturday, and energies within it should combine to form a broader upper low centered atop Bristol Bay/Kuskokwim/Alaska Peninsula areas over the weekend. Among the 12Z guidance, models initially have pretty minor spread with the details aloft and with associated surface lows. The 12Z CMC was the strongest with an early week secondary low near the eastern Aleutians/western Alaska Peninsula, but a model blend handled the systems reasonably well. The dominant forecast issue continues to be the track/depth/existence of a potentially deep low pressure system initially tracking across the midlatitude Pacific and then lifting into the Northeast Pacific into early next week. Some model runs over the past couple of days have shown a deep low turning north more quickly toward Southcentral Alaska, including the 06Z GFS with a central pressure into the 950s. However, recent 12Z deterministic guidance (ECMWF, GFS, and CMC) happens to agree fairly well on a track of this low farther east before turning northward into the northeast Pacific Monday-Tuesday (nearing Southeast Alaska), with central pressures that vary somewhat but certainly stay in the 970s or higher. The 18Z GFS came in differently again, with a low that is slower to turn north, but weaker. Ensemble member low plots indicate that any of these solutions are plausible as they show ample spread with the track and central pressure of the low. The means continue to try to combine the two lows, for more uncertainty. Models generally show troughing moving eastward into Tuesday and beyond, while there is some signal for Arctic troughing to move toward northwestern Alaska. The details will have to be determined in future forecast cycles. The WPC forecast began with a blend of the 12Z deterministic guidance favoring the GFS and ECMWF, but reduced the proportion of deterministic runs gradually in favor of the ensemble means, reaching over half means by the end of the period as spread increased especially with the Pacific low. There is support for heavy precipitation over the weekend in parts of Southcentral with the initial low features, which does not rely on the track of the uncertain Pacific low. However, there is certainly spread in how much QPF may reach the Panhandle early next week and with winds, depending on the Pacific low's strength and track. ...Weather/Hazards Highlights... Light to locally moderate precipitation is likely in the Aleutians to Alaska Peninsula late week with upper and surface lows in the vicinity. As these pivot east and tap into more Pacific moisture, precipitation totals will increase over the weekend, especially 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. Beyond that time, the forecast gets quite uncertain depending on the evolution of surface lows in the northern Pacific. Recent forecasts show that Southeast Alaska may see notable precipitation as the low track may be closer to that area, so precipitation amounts trended up in this forecast, but without high confidence. There may be an uptick in precipitation across western parts of the state by the middle of next week ahead of possible Arctic energy. In a broad sense, cooler than normal temperatures are likely over western Alaska with warmer than normal temperatures over northern and eastern Mainland Alaska under the trough to ridge pattern respectively. 40s to low 50s are forecast for highs in most lower elevation areas. Expect greater coverage of above normal morning lows, with most of the below normal readings confined to the southwest and the Panhandle. Cooler than average temperatures are expected for 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