Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 644 PM EDT Mon Sep 11 2023 Valid 12Z Fri Sep 15 2023 - 12Z Tue Sep 19 2023 ...Long duration heavy rainfall likely over portions of Southcentral to Southeast Alaska into next week... ...Synoptic Overnivew and Guidance Evaluation/Preferences... Guidance generally agrees that one upper low near the Bering Strait to start the period Friday should weaken over the weekend and be replaced by another low moving into the Bering Sea early next week. There is a lot of uncertainty though, even as early as this weekend, on the timing and strength of individual shortwaves rounding the base of the initial low near the Aluetians and into the Gulf. There is some rather significant timing differences regarding the next upper low early next week - with the 12z GFS the quickest and the 12z CMC significantly slower, and the 12z ECMWF falling near the middle of these two. This seems to be related to the strength of a system downstream in the Gulf, with the CMC indicating a second closed low and a more blocky/amplified pattern over the domain late in the period. The individual deterministic models quickly become out of phase with each other due to these differences. As expected, given the spread, the ensemble means are generally quite flat and washed out, but placement wise, generally side closer to the GFS and ECMWF. Coming up with a blend for the period was particularly difficult given the agreement in presence of systems, but significant timing issues. Generally was able to use a fully determinisitic model blend the first half of the period, with not much choice but to trend quickly towards mostly ensemble means by late in the period. The presents a more washed out pattern than is likely realistic, but given the spread it is hard to pick one deterministic model over another. ...Weather/Hazards Highlights... The low pressure system moving into the Gulf on Friday will help direct an atmospheric river/south-southwesterly flow into the Southern Coast/Southeast Alaska resulting in the potential a couple of days of very wet weather and heavy rainfall. Some locations could receive upwards of 3 to 6 inches of rain over the entire forecast period, with the greatest totals expected across the windward terrain of the southern coastal mountain ranges. Gusty winds are possible along the coast as well. Persistant, though weaker, onshore flow beyond the weekend and even more so with the next system tracking through the Aleutians should continue unsettled conditions across the southern and southeast Alaska. Otherwise, much of the remainder of the state looks to remain mostly dry through the period outside of the Aleutians/Alaska Peninsula where the storm systems tracking through should spread some rain chances into the area as well. Some enhanced moisture may move into western Alaska late period with the upper low over the Bering, but still a lot of uncerainty in the details. In terms of temperatures, expect readings to generally be below average across southern portions of the state owing to both persistent upper troughing/waves of cooler air and increased clouds and rainfall coverage. Temperatures will be around to above average further north into the Interior and North Slope. Santorelli 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 Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Thu-Sat, Sep 14-Sep 16. 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