Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 532 PM EDT Fri Sep 15 2023 Valid 12Z Tue Sep 19 2023 - 12Z Sat Sep 23 2023 ...Stormy pattern continues with a heavy rain threat for Southeast Alaska to persist through next week... ...Guidance Evaluation/Preferences... Prefer the now very well clustered 12 UTC GFS/ECMWF through next midweek. These models have stronger GEFS/ECMWF/Canadian ensemble support versus the 12 UTC UKMET/Canadian. This is evident over the Arctic with a NW AK closed low ejection that seems more in line with downstream upper ridging as well as farther south within the strong storm series pattern from the Pacific through the Bering Sea and Gulf of Alaska. Predictability seems above normal next week, but prefer to blend in the ensemble means with the still reasonably overall compatible GFS/ECMWF into later next week amid growing forecast spread. The main differences by then relate to the extent of upper trough digging into the eastern Gulf of Alaska and Southeast Alaska where the 12 UTC GFS/ECMWF are much more ambitious than the ensemble means. ...Pattern Overview and Weather/Hazards Highlights... There is still high confidence that a very stormy weather pattern will continue next week on the heels of several potent systems this week. The next storm in this series will cross the northern Gulf of Alaska to the Southeast into Tuesday and again feature enhanced winds/waves and heavy rainfall both as maritime risk and inland with focus from southern coastal areas into especially Southeast Alaska. This system has origins from a strong east Asian jet and may also entrain long plume tropical moisture to further fuel activity in an already wet pattern, so runoff issues are possible. Farther north, predictability is closer to average that a closed upper low settled over northwest/western Alaska through mid next week may slowly weaken and eject northward into the Arctic Ocean later next week, now with an alternate more eastward track less likely given downstream upper ridging. This system seems amplified enough aloft and with a now deeper/organized surface system to support unsettled conditions, especially early next week, to include broadly scattered precipitation in proximity of the system and with the added terrain effect for Brooks Range snows. West of Alaska, there remains a strong guidance signal supporting additional deep low development and track into/across the southern Bering Sea/Aleutians 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 and associated risks spreading into southwest Alaska/AKpen/Kodiak by Tuesday then Southcentral to Southeast Alaska by Wednesday into next Thursday. Yet another deep low in this powerhouse series may meanwhile work back upstream into the Bering Sea next Thursday/Friday that could again spread threatening weather to the the mainly the western mainland heaidng into next weekend, likley with a farther northward track/effect than the previous stream of systems given favorable guidance trends. Schichtel Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Mon, Sep 18 and Wed-Thu, Sep 20 -Sep 21. 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