Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 736 PM EDT Thu Aug 22 2024 Valid 12Z Mon Aug 26 2024 - 12Z Fri Aug 30 2024 ...A couple of upper/surface lows will spread moderate to heavy rain across southern Alaska next week... ...Overview... Early next week, an upper low centered over the northeast Pacific will promote cool temperatures and moderate to heavy rain for Southeast Alaska. A brief period of upper ridging is likely behind this feature, while upstream a notable upper trough/low and surface low will track across the Bering Sea/Aleutians early week eastward into the southwestern Mainland by Tuesday-Wednesday. Precipitation chances with modest to heavy rain amounts are possible near and ahead of the low track, i.e. for the aforementioned areas east into parts of Southcentral Alaska into the southeastern Mainland. Yet another upper trough is forecast to set up over the eastern Bering Sea again late week. ...Guidance Evaluation/Preferences... Model guidance is rather agreeable with the large scale pattern for the early part of the forecast period. An upper trough/low is set to be located atop the northeast Pacific and southeastern Alaska into Monday before it quickly rotates out, with a brief period of upper ridging behind. Upstream, model guidance has shown good agreement on the existence of the upper and surface lows tracking across the southern half of the Bering Sea for several cycles now. Today's 12Z runs have even shown improvement in the timing/movement of the upper and surface lows through Tuesday-Wednesday. Thus a blend of deterministic models was able to be used for the early period. Greater uncertainty arises by the latter part of the week. The upper trough and surface low should be weakening as they track east into the Mainland, but their track and how fast they weaken are uncertain. GFS runs have the trough shearing out quickly while the non-NCEP models maintain energy around Southcentral into early Thursday. AI models (AIFS and GFS Graphcast at least) do show some remaining troughing into Thursday but farther east. The evolution of this trough is dependent on additional energy/troughing coming in quickly from upstream. The GFS shows a stronger upper low moving faster into the Bering Sea by Thursday, likely one reason that the previous trough moves out. The ECMWF and CMC are slower to bring this energy in, but the CMC becomes an outlier in diving the energy farther south across the Aleutians. The WPC forecast trended toward a blend heavily favoring the ensemble means by Days 7-8 given the increasing spread in the deterministic models. ...Weather/Hazards Highlights... The upper low over the northeastern Pacific and southeastern Alaska will provide support for precipitation as it directs a weak to moderate atmospheric river into the Panhandle early next week. Locally heavy rain is not expected to be hazardous particularly considering their antecedent dry conditions. Rain there should be tapering off later Monday. The upstream system tracking west to east across the western to central part of the state will increase precipitation chances and winds. The Aleutians can expect modest rain amounts into Monday, while maritime and Aleutians winds are expected to be in the 30-40kt range lasting behind the low system into Tuesday. Wave heights may be enhanced near the Aleutians into Bristol Bay and near the Alaska Peninsula as well. Enhanced rain amounts are forecast to move into favored areas of the southwestern Mainland, Alaska Peninsula, and Kodiak Island late Monday into Tuesday, and into the southern Kenai Peninsula/Prince William Sound and the southeastern Mainland to the Yakutat area Tuesday-Thursday. Most precipitation will be along the southern coast, but modest amounts could spread especially into the Alaska Range, with some snow in high elevations. Lighter amounts are possible farther north into western and central parts of the Mainland. Then by later next week, another round of precipitation is likely to spread across the Aleutians to Southcentral, but placement and amounts remain uncertain. Colder than average temperatures over the weekend should last across much of the state into Monday, but are forecast to gradually moderate closer to normal during the workweek. Temperatures should even reach a bit above normal for lows as warmer southerly flow comes in ahead of the upper troughs--but precipitation chances could hold high temperatures down below normal. 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