Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Sat Aug 03 2024 Valid 12Z Wed Aug 07 2024 - 12Z Sun Aug 11 2024 ...Atmospheric river will produce heavy rainfall across favored areas of Southcentral Alaska around mid-next week... ...Overview... An highly amplified upper-level pattern will be in place across Alaska as the medium range period begins Wednesday, with a trough axis atop the Bering Sea to Aleutians and ridging to its east over Southeast Alaska into western Canada. This will promote deep layer moist southerly flow into parts of the southern coast, with an atmospheric river causing heavy rainfall over southern parts of the state, focusing along Kodiak Island and favored areas of the coastal Southcentral Mainland into Wednesday and shifting east into Prince William Sound into Thursday. As this upper trough lifts as it moves east into later week, another trough is forecast to move into the Bering Sea and lead to rain chances for western parts of Alaska into next weekend. ...Guidance Evaluation/Preferences... Model guidance continues to show good agreement with the trough to ridge pattern described above, with above average predictability and good cycle to cycle continuity. 12Z guidance was also reasonably agreeable that the trough should track eastward into later week but lift and weaken while it does so. A multi-model deterministic blend was able to be used for the early part of the forecast period, with the 12Z ECMWF, GFS, CMC, and UKMET in decreasing percentage amounts. Meanwhile, the next trough of note seems to show better agreement than yesterday with its track and timing, but with remaining detail differences. This trough/low looks to sweep southeast from eastern Siberia late week into the Bering Sea over the weekend, with ridging building ahead of it again in Southeast. The 12Z GFS was a bit west compared to other deterministic models with its upper and surface low track, but ensemble means were generally in between the west GFS and the farther east EC/CMC. The WPC forecast blend used the GEFS and EC ensemble means as half the model blend by Days 7-8 to account for individual model differences. ...Weather/Hazards Highlights... An atmospheric river will be directed into parts of southern Alaska into Wednesday-Thursday and produce rather heavy rainfall of potentially 5-10 inches, particularly for favored coastal parts of the Mainland (Lake Clark National Park and Preserve) and the southern Kenai Peninsula slowly shifting into Prince William Sound. Some of this moisture will extend inland through the Alaska Range for a few inches of rain there, and light to moderate rain amounts are possible farther north as well, locally enhanced in the Brooks Range. Some rain is forecast to push into the southeastern Mainland and Yakutat region into Friday, but with lessening amounts as upper-level and surface support begins to dwindle. Then the next upper trough and surface low coming into the Bering is likely to spread precipitation chances ahead of it, affecting the Aleutians Friday-Saturday and the western Mainland into Southcentral into the weekend. Details of this system will have to be refined in future cycles, but generally precipitation totals do not look as heavy in this next round given a more southwesterly than southerly mean flow direction. Temperatures will start out warm on Wednesday for eastern Alaska given the southerly flow, with highs even reaching the 80s in the eastern Interior. But the first trough aloft shifting east should gradually spread modestly below normal temperatures across the bulk of the state. Parts of Southeast Alaska may be the exception though, staying warm with highs in the 60s to potentially over 70 in some locations. 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