Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 616 PM EDT Sun Aug 25 2024 Valid 12Z Thu Aug 29 2024 - 12Z Mon Sep 02 2024 ...Heavy rainfall to impact Southern/Southeast Alaska into this weekend... ...Overview... The main weather player across Alaska will be a deep low pressure system sliding across the Aleutians late this week into the Gulf by the weekend. Troughing will be maintained through the weekend for the Gulf as a couple of more weaker shortwaves reinforce the pattern and upper riding builds downstream over northwest Canada. This does set up the potential for a multi-day heavy rainfall event across the Southern Coast and into parts of the northern Panhandle. The rest of Alaska should be relatively quiet as another deep low should be far enough north in the Arctic. ...Guidance Evaluation/Preferences... The latest guidance continues to have a good handle on the large scale pattern during the extended range, but plenty of uncertainty in the details, especially pertaining to the placement/speed of the strong surface low as it moves into the Gulf this weekend. Overall, compared to yesterday, models have trended faster and with a less blocky downstream pattern which should help to weaken the initial low late weekend near the southern Coast. The 12z UKMET was a bit slower than the better consensus. Lots of uncertainty late period as a couple of shortwaves round the parent low/trough through the Bering and into the Gulf, helping to reinforce mean troughing. Favored a non-UKMET blend for Days 4-6, increasing the ensemble means by late period to 60 percent of the overall blend (with 40 percent combined of the 12z ECMWF and GFS). ...Weather/Hazards Highlights... A cold front into the Panhandle associated with an initial weakening surface low over southern Alaska will maintain some moderate to heavy rainfall on Thursday. Some rainfall may be locally heavy, especially as enhanced by the Alaska range (where snow may fall in the highest elevations). Precipitation will progress west to east across the Aleutians late this week before the next surface low swings eastward into the Gulf by the weekend. This sets the stage for a possible atmospheric river to take aim at portions of the Southern Coast and northern Panhandle region with a multi-day heavy rainfall event. There is model support for upward of 6 inches of rainfall, but the exact location and extent east or westward along the coast is highly dependent on exactly where the upper low tracks. Regardless, given multiple rounds/days of heavy rainfall across this region, this may increase the threat for run off or flash flooding. Some gusty winds may accompany this system as well especially across the Aleutians and Alaska Peninsula. Temperatures across Southern Alaska and the Panhandle should remain below normal the entire period with general unsettled weather and precipitation. The North Slope will be below normal as well, but trend slightly warmer by the weekend, as upper troughing over this region pulls northward. The best chance for near to above normal temperatures are across the Central Interior Late this week underneath weak upper ridging. But as mean troughing moves over the state this weekend, temperatures should begin to trend cooler. Southerly flow ahead of upper systems should generally keep morning low temperatures above normal across most of the state. Santorelli 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