Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 625 PM EDT Sun Aug 8 2021 Valid 12Z Thu Aug 12 2021 - 12Z Mon Aug 16 2021 ...Atmospheric river expected to hammer parts of southeastern Alaska with very heavy rain late in the week... ...Overview and Guidance/Predictability Evaluation... A well developed Arctic low and trailing upper level trough over the Bering and Aleutians is expected to be in place at the beginning of the forecast period on Thursday, along with a rather strong upper ridge over British Columbia and the Pacific Northwest. Shortwave energy exiting the Aleutians trough should emerge over the northern Gulf and attempt to displace the ridge axis to the east, and this will tend to focus strong onshore flow over southeastern Alaska and the panhandle region. Farther upstream will be the eventual remnants of Tropical Storm Lupit, which should pass south of the Aleutians. Decent synoptic scale agreement exists among the 12Z guidance suite on Thursday, with differences becoming more apparent by Friday with the progression of the trough across the Gulf region. The 12Z becomes a faster and much stronger solution by Saturday over the Gulf, whereas the CMC is depicting nearly zonal flow across this same general area. There is better support for the ECMWF/UKMET idea through Saturday, along with the 12Z GEFS mean. By later in the forecast period, the CMC is stronger with building a ridge axis across the state and this does not have much in the way of ensemble support, so more of a ECMWF/GEFS/ECENS approach seems prudent by the end of the forecast period. ...Weather/Threats Highlights... The main weather headline will be the potential for extremely heavy rainfall for portions of southeast Alaska, particularly for windward terrain from near Mount St. Elias to Juneau. There is now a good signal in the CMC/ECMWF/UKMET/GFS for an enhanced atmospheric river event for the end of the week that will be directly nearly perpendicular to the coast, and rainfall totals on the order of 4 to 8 inches are within the realm of possibility for these areas. This could result in mudslides and serious rises on creeks and rivers. The axis of the atmospheric river then moves eastward and gradually weakens going into the weekend across remaining portions of the southeast panhandle region, with 2 to 5 inches of rainfall possible for the greater Juneau area and points southeastward. Elsewhere, upper level troughing over much of Alaska will tend to keep temperatures generally below normal across most of the state for this forecast period. These anomalies will likely be even greater for the northern third of the states as a cold front and a polar upper low govern the weather pattern. The closed Arctic low may eject eastward next weekend in response to an approaching upstream upper trough from eastern Siberia. Hamrick 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