Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Mon Sep 23 2024 Valid 12Z Fri Sep 27 2024 - 12Z Tue Oct 01 2024 ...Heavy precipitation likely across favored portions of Southcentral this weekend... ...Overview... As the medium range period begins Friday, initial upper troughing and a surface low causing precipitation over Southeast Alaska will move away, while a trough axis sets up over western Alaska. Shortwaves within the western trough are forecast to combine and create an upper low centered over Bristol Bay or so and direct moisture into Southcentral for some heavy precipitation over the weekend. Details of Pacific surface lows supported by the low aloft are still uncertain. East of the trough, upper ridging will generally prevail until the trough starts to move eastward into Tuesday. ...Guidance Evaluation/Preferences... Recent model runs agree that low pressure over southern parts of the Panhandle will be weakening by Friday and then moving away from the state. Troughing over the western Mainland will be in place, and on the southern side a shortwave or compact upper low is forecast to shift eastward across the southern Bering Sea Friday, which possibly combined with other energies (like a shortwave diving south through the western side of the trough like the 12Z ECMWF showed) should form a broader upper low centered atop Bristol Bay/Kuskokwim/Alaska Peninsula areas over the weekend. Models have some variety in the details aloft and with associated surface lows for typical spread that a model/mean blend handled reasonably well. The dominant forecast issue continues to be the track/depth/existence of a potentially deep low pressure system initially tracking across the mid-latitude Pacific and then lifting into the Northeast Pacific into early next week. Many models were quite strong with this surface low yesterday and with today's 00Z/06Z runs. However, 12Z operational runs greatly backed off on the potential for a strong low, or at least delayed it from reaching near Alaska until Tuesday or Wednesday. The 12Z ECMWF did not strengthen the low much at all in the midlatitudes and keeps it weak as it turns north early next week, possibly directed east by the other low that it has south of Kodiak Island. Meanwhile the 12Z GFS shows a couple of different surface lows turning north into the northeast Pacific that end up combining by Tuesday for a strong low (low 970s). The CMC indicates one low that is deep, but slower than the GFS where it does not even get near Alaska by the end of the period next Tuesday. Ensemble member low plots show ample spread with the track and central pressure of the low, as the ensemble means generally end up combining the two lows. At least with the ensemble means there is some agreement with each other despite the disagreements overall. 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. The WPC forecast began with a blend of the deterministic guidance favoring the 12Z 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... The initial northeast Pacific/near Panhandle or Haida Gwaii system may bring increased precipitation continuing into early Friday before moving away. Farther west, 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. Will have to await better model agreement before determining if Southeast Alaska will see significant precipitation or not. The central and northwestern mainland may see some light and scattered precipitation late this week, followed by a drier trend over the weekend, but then perhaps expanding in coverage early next week. In a broad sense, cooler than normal temperatures are likely over western Alaska with warmer than normal temperatures over 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