Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 643 PM EDT Mon Aug 26 2024 Valid 12Z Fri Aug 30 2024 - 12Z Tue Sep 03 2024 ...Heavy Rainfall/Runoff Threat for Southern to Southeast Alaska into Friday/Saturday... ...Overview... A primary main storm to impact Alaska during this forecast period will be a deep low pressure system developing and tracking eastward into and across the northern Gulf of Alaska Friday into the weekend. A strongly supporting upper troughing/closed low will be maintained through the weekend as several weaker shortwaves reinforce the pattern and upper riding builds downstream over northwest Canada. This sets up the potential for a multi-day heavy rainfall event across Kodiak Island and Southcentral to the Panhandle. Additional systems in this pattern are uncertain in this energetic flow, but will be monitored in the coming days as stronger guidance signals emerge. The rest of Alaska should be relatively as another deep low should spin far enough north in the Arctic Ocean. ...Guidance Evaluation/Preferences... Models and ensembles show highly energetic and progressive flow in a stormy pattern with below normal forecast spread and uncertainty Friday into Sunday. A composite of reasonably well clustered guidance from the 12 UTC GFS/ECMWF/UKMET/Canadian offers good detail as consistent with above normal system predictability and WPC product continuity. Gravitated to a 12 UTC NAEFS/ECMWF ensemble means composite through longer time frames amid increasingly growing flow embedded system uncertainties. ...Weather/Hazards Highlights... Heavy rain will progress eastward with a likely deepened storm track eastward into and across the northern Gulf of Alaska Friday into Sunday. This sets the stage for a possible atmospheric river to take aim across the Southern Coast into the Panhandle to fuel a multi-day heavy rainfall event. Precursor rains over the next few days and the aforementioned late week/weekend heavy rainfall threat across this region may combine to increase the threat for runoff/flash flooding over time. Gusty winds/enhanced waves may support maritime/coastal hazards that may accompany this well organized system. 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