Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 742 PM EDT Sun Aug 04 2024 Valid 12Z Thu Aug 08 2024 - 12Z Mon Aug 12 2024 ...Atmospheric river will produce heavy rainfall across favored areas of Southcentral Alaska into Thursday... ...Overview... An highly amplified upper-level pattern will be in place across Alaska as the medium range period begins Thursday, with a trough axis atop the eastern Bering Sea to eastern Aleutians/Alaska Peninsula and ridging to its east over Southeast Alaska into western Canada. This will promote deep layer moist southerly flow forming an atmospheric river and causing very heavy rainfall over favored areas of Southcentral Alaska and more modest rainfall farther north. 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 at the start of the period, with above average predictability and good cycle to cycle continuity with the trough to ridge pattern described above. Models generally show that the first trough should track eastward into later week but lift and weaken while it does so, but vary with the timing. Early Saturday for example, the 12Z GFS and CMC were among the slower solutions, lingering troughing into Southcentral Alaska, while the 06Z GFS and 12Z ECMWF and UKMET were faster. With models split, it was difficult to choose a cluster, but since the ECMWF has been steady from run to run and GFS runs have been bouncing around with timing, preferred a faster solution more like the ECMWF. Thus the early part of the WPC forecast blend was heavy on the 12Z ECMWF but also included the 06Z and 12Z GFS and 12Z UKMET/CMC. Then the next trough of note is forecast to approach from eastern Siberia and move atop the Bering Sea into the weekend, along with a surface low. The primary model differences were with the depth of the trough--GFS runs were more shallow while the ECMWF/CMC showed an embedded upper low and thus the accompanying surface low farther south. Complicating matters is that additional energy looks to come in from the west and interact with the trough. GFS runs considerably amplified the secondary energy to make that the primary trough by next Monday, while the ECMWF/CMC maintain the original trough with the secondary energy as just a shortwave. This puts the models out of phase over much of the state. Overall preferred the ECMWF/CMC type solutions over the GFS, but blended in over half GEFS/EC ensemble means by Day 8 to reduce these individual model differences. The new 18Z GFS is fortunately not as strong with that secondary energy. ...Weather/Hazards Highlights... An atmospheric river will be directed into parts of southern Alaska this week and produce ample rainfall. While the heaviest rainfall amounts have phased into the short range for areas like Kodiak Island, some heavy rain should continue into Thursday across favored coastal parts of the Mainland into the Alaska Range (Lake Clark National Park and Preserve) and the southern Kenai Peninsula even after several inches of rain are forecast to fall Wednesday in those areas. Prince William Sound could see 3 to 7 inches of rain Thursday-Friday, and some rain is forecast farther eastward and north into the Alaska Range through Friday, but with lessening amounts as upper-level and surface support begins to dwindle. Some moisture should spread farther north across the Mainland for more modest rainfall amounts, but locally enhanced in the Brooks Range. Then the next upper trough and surface low coming into the Bering is likely to spread precipitation chances into the state from west to east, affecting the Aleutians Friday-Saturday and the western Mainland into Southcentral into the weekend and early next week. 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 may continue to be warmer than normal for eastern Alaska into Thursday given the southerly flow, with highs in the 70s and 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. Near to below average temperatures are forecast to last into early next week. Parts of Southeast Alaska may be the exception though, staying warm with highs in the 60s to even 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