Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Sat Sep 9 2023 Valid 12Z Wed Sep 13 2023 - 12Z Sun Sep 17 2023 ***Long duration heavy rainfall likely over portions of South-central to Southeastern Alaska into next week*** ...Synoptic Overview... The main feature of interest going into the middle of next week will be a large upper low situated over the Alaska Peninsula that should slowly settle over the northern Gulf by Thursday, with an associated surface low moving inland and advecting copious moisture inland across southeast Alaska to end the week. By Friday, a second storm system approaches the western Bering Sea and likely develops a closed low aloft along with a strong surface low potentially affecting the southern coastal areas by next weekend with renewed chances for additional rainfall. ...Guidance Evaluation/Preferences... The 12Z model guidance suite is in decent overall agreement on the synoptic scale for Wednesday, although mesoscale differences are already apparent with shortwave perturbations across the Gulf that affect the timing of the surface lows. Timing differences are still quite big across the Bering regarding the arrival of the next organized storm system, with the CMC likely a fast outlier, and the ECMWF situated generally slower than the consensus. The GFS is closer to a middle ground solution here, but it is stronger with the surface low. Taking these factors into account, the updated WPC forecast begins with a deterministic guidance blend with contributions from the GEFS/ECENS ensemble means gradually increased as the period progresses and uncertainty in timing/strength of embedded systems within the mean troughing increases. ...Weather/Hazards Highlights... The southeast portion of the mainland and the southeast Panhandle region will remain rather unsettled for the middle to end of the week. Onshore flow associated with low pressure systems traversing the northern Gulf of Alaska will advect multiple rounds of enhanced moisture inland with the potential for an atmospheric river event or two. Some locations could receive upwards of 3 to 5 inches of rain over the entire forecast period, with the greatest totals expected across the windward terrain of the southern coastal mountain ranges. Otherwise, much of the remainder of the state looks to remain mostly dry through the period. An upstream system moving in from the Bering Sea late next week will likely increase precipitation chances for the Aleutians/southwest Mainland, though uncertainty with the evolution of this system remains higher. In terms of temperatures, expect readings to generally be slightly below average across southern portions of the state and slightly above for the Interior and North Slope due to noted ridging over the north from western Canada, and slightly below average for the southern coastal areas owing to increased clouds and rainfall coverage. 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