Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Mon Sep 10 2018 Valid 12Z Fri Sep 14 2018 - 12Z Tue Sep 18 2018 ...Guidance/Uncertainty Assessment... A strong upper ridge which has remained a fixture in the forecast across Alaska during much of the month will finally collapse during the period. This ultimately leads to a fairly chaotic period given the cascade of shortwaves traversing from the Bering Sea and Aleutians downstream into mainland Alaska. On Friday morning, a strong band of height falls will be tracking toward the Aleutians with the guidance trending toward a much more amplified solution. Some remnants of the upper ridge may be present over northern Alaska although this will be short lived as a deep low near the Arctic further lowers heights. The resultant pattern is rather unpredictable given the free for all of shortwaves sweeping through Alaska once the ridge erodes. Consequently, operational models could not be utilized much beyond Day 5/Saturday which led to a more ensemble based forecast approach. Before then, kept modest contributions of the 12Z GFS/00Z ECMWF in the mix given some level of certainty to the forecast. ...Weather Highlights/Hazards... Temperatures will be on the decrease throughout the period given the loss of the mean upper ridge. Generally speaking, numbers drop around 5 degrees from Friday into early next week with the coolest conditions prevailing across northern Alaska as highs stay in the upper 30s while lows plummet to below freezing. Mixed precipitation types will also return to the picture given the intrusion of colder air. The state as a whole can expect increasing precipitation chances from west to east as shortwaves advect through the region. Difficult to say where the heaviest amounts will reside but the GFS/ECMWF solutions favor higher QPF across south-central Alaska. Meanwhile, the southeastern portion of Alaska should stay dry for much of the period until perhaps next Monday. Rubin-Oster WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html