Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 527 PM EDT Tue Aug 27 2024 Valid 12Z Sat Aug 31 2024 - 12Z Wed Sep 04 2024 ...Heavy Rainfall/Runoff Threat for Southern to Southeast Alaska into Friday/Saturday... ...Overview... The 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. Meanwhile, another deep low will spin far enough north in the Arctic Ocean to limit North Slope impacts. Additional systems at longer time frames across the domain and vicinity remain uncertain, but will be monitored in the coming days as stronger guidance signals emerge. The weather pattern for rest of Alaska should be relatively benign. ...Guidance Evaluation/Preferences... Guidance still shows highly energetic and progressive flow in a stormy pattern with below normal forecast spread and uncertainty into Sunday. A composite of best clustered guidance from the 12 UTC GFS/ECMWF/Canadian and NAEFS/ECMWF ensemble mean offers good detail as consistent with above normal system predictability and WPC product continuity. Shifted WPC blend emphasis increasingly toward the 12 UTC NAEFS/ECMWF ensemble means 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 an 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 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 low. 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