Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 728 PM EDT Tue Aug 20 2024 Valid 12Z Sat Aug 24 2024 - 12Z Wed Aug 28 2024 ...Overview... Broad troughing will be in place atop most of Alaska over the weekend, promoting cool temperatures and precipitation chances that move from the Mainland Saturday into Southeast Alaska early next week. A brief period of upper ridging is possible 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, so for these areas into parts of Southcentral Alaska into the southeastern Mainland, and enhanced winds are also possible. ...Guidance Evaluation/Preferences... Models are reasonably agreeable with the large scale pattern and less agreeable on the medium and smaller scales this weekend into next week. First, models remain somewhat varied with a possible shortwave trough over Southeast Alaska and exiting on Saturday, but show the broader trough to the northwest across the Mainland with better consensus. Additional shortwave energy on the southern side of this trough shows some differences in its timing by Sunday-Monday. The 00Z CMC and 06Z GFS were somewhat faster in moving this shortwave from the Gulf and across Southeast AK, but the 12Z guidance settled on a slower track. These variations do lead to some QPF and surface low differences. Then upstream, model guidance has shown pretty good agreement on the existence of the upper and surface lows tracking across the southern half of the Bering Sea for several cycles now. But there are some differences on the timing of the lows pivoting east. After the 06Z GFS was in good alignment with other guidance, the 12Z GFS slowed notably and became out of tolerance with the ECMWF/CMC. In terms of AI/ML models, the GFS Graphcast was even slower, but the EC AIFS was faster than the ECMWF. The GEFS mean was slightly slower than the EC mean but still faster than the operational GFS. Since the ECMWF/CMC have stayed steady and show a middle ground position along with reasonable agreement from the ensemble means, preferred a solution more like the ECMWF (and 06Z GFS and CMC) as the low and trough move east. The WPC forecast used a blend favoring the 06Z/12Z GFS and 12Z ECMWF initially, removing the 12Z GFS and gradually transitioning to more GEFS and EC ensemble means in the blend, reaching half means by the end of the period. ...Weather/Hazards Highlights... The upper trough pattern by the weekend will provide support for fairly broad precipitation coverage across the Mainland, with some areas of localized focus especially along favored terrain. Precipitation should fall as snow in the higher elevations of the Brooks Range and farther south over portions of the Alaska Range. Shortwave energy within this first trough and a surface low pressure/frontal system moving over the Gulf of Alaska will likely support increasing precipitation from Prince William Sound to Southeast Alaska later weekend into early next week. Rain totals are uncertain based on the details of the energy and how much moisture reaches the area, but some moderate amounts are likely. The upstream system tracking west to east across the western to central part of the state will also increase precipitation chances and winds. The Aleutians can expect modest rain amounts Sunday-Monday, while maritime and Aleutians winds are expected to be in the 30-40kt range lasting behind the low system into Tuesday. 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 Tuesday-Wednesday. Most precipitation will be along the southern coast but light to modest amounts could spread especially into the western part of the Mainland and the Alaska Range, with some snow in high elevations once again. Some locally higher waves are possible due to the low's flow, into the Copper River Basin and Prince William Sound. The troughing aloft to start the period this weekend will promote cooler than normal temperatures across the state, particularly for highs. Temperatures are likely to gradually moderate closer to normal and even reaching a bit above normal for lows as next week progresses and warmer southerly flow comes in ahead of the second upper trough--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