Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 651 PM EDT Wed Aug 28 2024 Valid 12Z Sun Sep 01 2024 - 12Z Thu Sep 05 2024 ...Heavy Rainfall/Runoff Threat for Southern through Southeast Alaska into Saturday, and possibly again in about a week... ...Guidance Evaluation/Preferences... Models and ensembles show highly energetic and progressive flow in a stormy pattern with below normal forecast spread and uncertainty into Monday. A composite of best clustered guidance from the 12 UTC GFS/ECMWF/Canadian enjoys good ensemble support while offering good detail as consistent with above normal system predictability and WPC product continuity. Switched preference to compatible GEFS/ECMWF/Canadian ensemble means through longer time frames amid growing forecast spread. However, the guidance signal for another possible impactful Gulf of Alaska storm in about a week has increased. Uncertainties remain, but the pattern is quite favorable. Accordingly, WPC applied manual edits to ensure sufficient low depth in WPC progs given signal and upper trough support aloft. ...Weather/Hazards Highlights... A leading storm in this forecast period to impact Alaska will be a deep low pressure system developing and tracking eastward into and across the northern Gulf of Alaska late week, along with secondary development slated to exit this weekend. A strongly supporting upper trough/closed low will be maintained through the weekend as several weaker shortwaves reinforce the pattern and upper riding builds downstream over northwest Canada. A multi-day heavy rainfall event will affect/linger from Kodiak Island and Southcentral through vulnerable Southeast Alaska as fueled by an slowly shifting atmospheric river. Meanwhile, another deep low will spin far enough north in the Arctic Ocean to limit North Slope impacts. Additional deep low develolpment and focus has grown increasingly likely over the Gulf of Alaska into next midweek. While details at these longer time frames remain uncertain, guidance signals for low developments and longer fetch moisture influx are emerging for support of enhanced martitime gales/waves and southern to southeast Alaska coastal/terrain focusing heavy rainfall and winds. Precursor rains over the next few days across this region may combine with activity over the next week to increase the threat for runoff/flash flooding over time. Gusty winds and enhanced waves may support maritime gales/coastal hazards that may accompany this well organized system. This may include some locally enhanced gap/channel flows, especially on the backside of the main lows. The weather pattern for much of the rest of Alaska should remain relatively benign for late summer, especially across the Interior. 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