Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 722 PM EDT Mon Aug 09 2021 Valid 12Z Fri Aug 13 2021 - 12Z Tue Aug 17 2021 ...Heavy rain for parts of southeastern Alaska later this week... ...Overview... Upper ridging near 155E and 140W will favor troughing in between, roughly from the Arctic through the Bering Sea via several systems. This will keep an active pattern over the high latitudes into next week with both the northern and southern stream affecting the 49th state. Lead system in the Gulf Friday will bring a significant heavy precipitation threat to southeastern Alaska out of the short range (Thu) where several inches of rain may occur in favored areas. Indeed some skillful guidance shows over 10 inches for the event maximum around and inland from Yakutat. ...Guidance/Predictability Evaluation... The ensembles remained in good agreement overall with some slight timing/amplitude differences by next week over the Bering as a couple more systems enter the picture. Overall, a blended solution was preferred. This included the 12Z ECMWF which paired best with the ensembles, followed by the 12Z GFS which was a bit removed from, but still close enough to, the ensembles. The Canadian and UKMET offered a bit of detail for the Friday forecast with the Gulf system but otherwise drifted apart from the consensus. Later this weekend into next week, systems south of the Aleutians, containing the remnant moisture of Tropical Storms Lupit and Mirinae, will tend to lift northeastward toward the central/eastern Aleutians and AKPen next week, bringing in some rainfall to the region. Timing differed in the guidance but was within the expected spread for this time range. ...Weather/Threats Highlights... Atmospheric river event for the end of the week could result in hydrologic concerns for some areas especially near Yakutat despite being used to heavy rain events. Ensemble Situational Awareness Table from the GEFS shows "MAX" values over southeastern areas signaling an unusually wet forecast relative to climatology. For reference, Yakutat has had about 40 3-day 10" precipitation events (some overlapping) in its 124-year station record (about 1 every three to four years), usually in the cold season. By the weekend, the rainfall will subside as it pushes southeastward. To the north, deep upper low in the Arctic will slowly drag a cold front out of the area, allowing another northern stream system out of Russia to push eastward. This will bring in an expanding area of precipitation to the northwestern corner of the state across the Brooks Range next week. Temperatures will generally be below normal across most of the state, focused on northern areas. Some slow moderation is possible next week but the continued troughing will likely result in only small changes to the pattern through next Tuesday. Fracasso 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 Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Aug 12-Aug 13. 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