Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 647 PM EDT Sun Jun 21 2020 Valid 12Z Thu Jun 25 2020 - 12Z Mon Jun 29 2020 ...Overview and Guidance/Predictability Assessment... In contrast to operational runs as described below, the latest ensemble means provide a fairly agreeable and consistent depiction of the expected large scale evolution during the period. They show a deep Arctic upper low to the northwest of the mainland anchoring a broad area of cyclonic flow covering portions of the mainland, the Bering Sea, and Siberia. With some separation from the Arctic flow, an upper trough aligned over the eastern Bering Sea late this week should eject eastward after Fri. Meanwhile a progressive North Pacific system will track just south of the Alaska Peninsula and into the Gulf Thu into Fri and then an evolving central Pacific upper trough with decent potential for an embedded low will likely bring a system into the Aleutians by around Sun-Mon. From Thu into Fri a composite of the 12Z operational model guidance offered a reasonable starting point with more definition than the means. Note that the 12Z ECMWF/UKMET were closest to yesterday's continuity for the system tracking into the Gulf late this week and the initial blend was adjusted slightly to reflect their depth for the system. By Sat-Mon nearly all of the 12Z model runs significantly diverged from each other, their previous runs, and the means for one or more significant features. This required a rapid transition to 50-60 percent ensemble mean weight (12Z GEFS/00Z ECens) while reverting operational model input to the 06Z GFS and 00Z ECMWF. For the Arctic upper low and surrounding flow the operational guidance spread has become more extreme compared to previous days. The 12Z ECMWF tracked the low well northward of previous run and the means while the 12Z GFS pulled the low northward as well but in contrast to other guidance also dropped a significant bundle of energy into the Bering Sea--ultimately interacting with the late-period central Pacific/Aleutians system. The 12Z CMC offered yet another questionable idea, bringing the upper low center to the northwestern coast of the mainland by the end of the period. The ensemble means have drifted a bit westward versus 24 hours ago and the 12Z ECMWF mean adjusted somewhat northward versus the 12Z run but overall the means have thus far provided the most stable forecast for the upper low and its broader circulation. GFS/ECMWF runs have continued to show decent potential for the evolving central Pacific upper trough to contain an embedded low and support a well-defined surface system that tracks into the Aleutians by days 7-8 Sun-Mon. CMC runs have tended to differ for some details while seemingly underplaying the surface reflection relative to depth of dynamics aloft at some valid times. Multi-day trends for this system have generally been slower but with some oscillation. Given the signal for a fair amount of separation from flow to the north for a meaningful period of time, it is a question mark whether the faster adjustment in the 12Z ECMWF/ECMWF mean will hold. The 12Z GFS evolution over the Bering differs enough from other guidance to question its specifics as well. Still there is considerable uncertainty over the degree of stream interaction toward the end of the period. ...Weather/Threats Highlights... The system tracking just south of the Alaska Peninsula and into the Gulf late this week should provide a brief period of precipitation to the southern coast and likely with greater focus to the Panhandle. Convection/rainfall over portions of the mainland should be most common late this week when deepest upper troughing aligns a little west of the mainland and before a front draped across the North Slope retreats northward/dissipates. Ejection of the initial Bering trough energy and rising heights aloft in general should reduce precipitation coverage somewhat during the weekend and early next week. Expect the system forecast to track into the Aleutians late in the period to bring a shield of moisture into/east of the region, though with ongoing uncertainty over coverage/intensity/timing. The temperature forecast is consistent in showing below normal highs over a majority of the state with greatest negative anomalies over the south/southeast/Panhandle as well as the northwest. Expect more moderate anomalies for lows, including some areas of above normal lows between the northeast and southwest. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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