Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Fri Aug 23 2024 Valid 12Z Tue Aug 27 2024 - 12Z Sat Aug 31 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. The low near the Southeast will weaken, brief period of weak ridging moves in over the southern Mainland, followed by the next notable upper trough/low and surface low advances across the Bering Sea/Aleutians early week eastward into the southwestern Mainland by midweek. 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... The latest run of guidance overall has a decent handle on the large scale pattern for the early part of the forecast period before the typical model timing/depth variances arise by the later periods. A low pressure system in proximity to the northeast Pacific and Southeast Alaska into Monday will quickly rotate out, with a brief period of upper ridging behind. Upstream, model guidance continues to show run to run agreement on the existence of the upper and surface lows tracking across the southern half of the Bering Sea. The first couple of days utilized a combination of the ECWMF/UKMET/GFS/CMC and components of the NBM. Although there have been incremental improvements the past few cycles there still are some timing/placement differences for the end of the forecast period that does lower confidence a bit. The evolution of this trough is dependent on additional energy/troughing coming in quickly from upstream. The GFS has been persistent with a stronger upper low moving faster into the Bering Sea by Thursday which the ECMWF and CMC have maintained the slower trend to bring this energy in. 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... A weak to moderate atmospheric river aimed toward the Alaska Panhandle early next week will support locally heavy rain, however it is not expected to be hazardous particularly considering their antecedent dry conditions. Rain there should be tapering off later Monday. Rain, accompanied with gusty winds, will be increasing from west to east across the southern Mainland as the upstream system tracks across the western to central part of the state. 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. Given the multi-day modest/moderate rain with locally heavy rain, portions of the southern coast and northern extent of the Panhandle may see local impacts from run off or instances of flash flooding. 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. Campbell/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