Alaska Extended Forecast Discussion National Weather Service College Park MD 711 PM EDT Wed Aug 6 2025 Valid 12Z Sun 10 Aug 2025 - 12Z Thu 14 Aug 2025 ...Heavy rain could cause flooding concerns in Southeast Alaska into early next week... ...Overview... An upper trough is forecast to extend from the Arctic Ocean southward across the western Mainland as the forecast period begins Sunday, with perhaps an embedded upper low. Energies along the base of the trough and a Gulf surface low will promote persistent moist inflow in the form of an atmospheric river into Southeast Alaska this weekend into Monday, leading to heavy rain and flooding concerns. Moderate precipitation is also forecast across much of the Mainland, along with cool temperatures. Into Tuesday, the upper trough should track east and flatten, while another round of energy upstream will bring precipitation to the Aleutians, reaching the Alaska Peninsula and western Mainland by mid-next week. ...Guidance Evaluation/Preferences... Even early in the forecast period Sunday-Monday, models disagree with the details of shortwave energy within the relatively more predictable trough that extends from an Arctic Ocean upper low. Most models show an embedded upper low somewhere along the western Mainland Sunday, but the 12Z ECMWF is a primary exception. The handling of this upper low leads to differences in the trough moving eastward by Monday. The 12Z EC and UKMET and AIFS were among the fastest to pivot the trough northeastward, while the 12Z GFS and the GFS Graphcast are slow. The 06Z GFS and 12Z CMC were in between. Prefer a solution faster than the 12Z GFS and GFS Graphcast, as most AI/ML models including the EC version of the Graphcast were much faster. Tried to go generally in between the faster EC/UK and middle ground CMC/06Z GFS, which yields a quicker trough track east compared to the forecast from a day ago. Farther south, shortwaves and weak surface lows rounding the base of the trough in the vicinity of the southern coast/Gulf early next week are not very predictable into the extended range. Used a general model blend but with low confidence in any one solution. Regardless of the exact strength and timing of the features, heavy rain will be a concern. Another shortwave could come in by Tuesday, but the 12Z ECMWF was alone in closing off an upper low near the Gulf/Kenai, so did not favor it. The older 00Z ECMWF looked more reasonable. Meanwhile upstream, a couple of rounds of energy are likely to track east from Japan and the Kamchatka Peninsula into the Bering Sea next week, which show some differences in timing and their interactions with each other as well as possible interactions with an upper low meandering farther north over the Arctic Ocean. The mean troughing should support at least one surface low/frontal system that gradually tracks east. But once again the details will need to be refined in future forecasts, as models do not show consensus on timing/position/depth of these features. The WPC forecast used a multi-model blend early in the period. As the forecast progressed, quickly ramped up the proportion of GEFS and EC ensemble means to a bit more than half Day 7 and more Day 8 as spread with the individual models continued to increase. Included some older models (06Z GFS and 00Z ECMWF) in the model blend at times, given the aforementioned issues with their 12Z counterparts. ...Weather/Hazards Highlights... The primary hazard of concern through the period will be heavy rainfall across Southeast Alaska this weekend into Monday. Rounds of moderate to heavy rain are likely for multiple days, with total rain accumulations of multiple inches, due to a reloading moderate to strong atmospheric river at the base of the Mainland upper trough. A Heavy Rain area is delineated on the Day 3-7 Hazards map across Southeast Alaska for the peak of the rainfall this weekend into early next week. Southern parts of the Panhandle are likely to see the heaviest rain, but areas farther north could be more sensitive to heavy rain, especially areas near the Mendenhall Glacier that could see a worsening of potential glacial outburst flooding. By Tuesday-Wednesday, lighter showers are forecast across Southeast. Farther north, just ahead of the main upper trough axis and a cold front sweeping through, generally moderate precipitation is likely to track from central parts of the Mainland Sunday and across the eastern Mainland by Monday. Higher totals are possible in upslope flow areas over mountains, and the highest peaks of the Brooks and Alaska Ranges could see some snow especially after the cold front passes. This precipitation will also help extinguish remaining fires in the Interior. With the upper trough expected to track more quickly eastward today, precipitation coverage should lessen by late Monday. Meanwhile, upper level energy and a surface low pressure/frontal system are forecast to move across the Aleutians early next week and into Bristol Bay by Tuesday-Wednesday, potentially reaching the western Mainland by next Thursday. This system will spread light to moderate precipitation to and ahead of these areas, with another round of precipitation affecting much of the Mainland by the middle of next week. Model guidance varies with the depth of the low and thus the strength of the winds, so will continue to monitor if wind speeds will become impactful in the Bering Sea and western Alaska. The upper trough will promote cooling conditions across much of the state into early next week. Temperatures are expected to be around 5-15F below average for highs and 5-10F below average for lows. Most areas can expect highs in the 50s and 60s. Temperatures should moderate pretty close to normal by Tuesday-Thursday behind the initial trough. 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