Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 619 PM EDT Sat Sep 16 2023 Valid 12Z Wed Sep 20 2023 - 12Z Sun Sep 24 2023 ...Stormy pattern continues with a heavy rain threat for Southeast Alaska to persist through next week... ...Guidance Evaluation/Preferences... Prefer a solution closest to best clustered guidance from the 12 UTC GFS/ECMWF Wednesday and Thursday. These models have stronger GEFS/ECMWF/Canadian ensemble support versus the 12 UTC UKMET/Canadian and maintain max WPC product continuity across the forecast domain. However, forecast spread and uncertainty has increased since yesterday, especially for later next week onward. This is evident with both the handling of a closed upper trough/low over northwest Alaska and upper trough amplitudes over the Bering Sea and the Gulf of Alaska. Prefer to blend more compatible ensemble means given the lack of strong signal support for a given solution at this time. Manual adjustments were applied as warranted to re-deepen systems over weakened by the blending process as based on individual predictability and support. ...Pattern Overview and Weather/Hazards Highlights... There remains a strong guidance signal in this continued stormy pattern in support of additional deep low development and track into/across the southern Bering Sea and Aleutians out from early next week along with an enhanced maritime risk and wrapping heavy rainfall/winds. Steady downstream deep energy transfer should lead to the potential storm, with aforementioned risks spreading past southwest Alaska, the Alaskan Peninsula and Kodiak Island Tuesday into Wednesday, and then Southcentral to Southeast Alaska in earnest by Wednesday into next Thursday, lingering into Friday. This system will likely tap a deep layered and long fetched atmospheric river with low latitude origins to fuel rainfall. Farther north, a closed upper low will settled over northwest Alaska into mid next week may slowly weaken and eject into the Arctic Ocean later next week or alternately linger over the North Slope/Interior. This system seems amplified enough aloft along with an organized surface system to support unsettled conditions, especially early-mid next week, to include broadly scattered precipitation in proximity of the system and with the added terrain effect for some Brooks Range snows. Upstream, another deep low in this powerhouse series may meanwhile work into the Bering Sea next Thursday/Friday that is likely to spread threatening weather to the western mainland heading into next weekend, likely with a farther northward track/effect than the previous stream of systems given favorable guidance trends. In addition to a maritime hazard, this system may offer enhanced precipitation and winds along with the potential for amplified coastal impacts. Schichtel 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