Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 645 PM EDT Sat Jul 25 2020 Valid 12Z Wed Jul 29 2020 - 12Z Sun Aug 02 2020 ...Overview and Guidance Evaluation... The majority of latest guidance shows upper troughing with one or more embedded lows over the Bering Sea while an elongating east-west trough eventually reaches into the northeastern Pacific/Gulf of Alaska. A downstream ridge should build into the mainland and northwestern Canada, with this ridge gradually helping to dislodge an initially deep upper low over the Arctic. There are various uncertainties that persist for some specifics but overall the models/means appear to be somewhat better clustered than yesterday. A blend of 12Z operational models (ECMWF/GFS/CMC/UKMET) represents consensus well for significant features during the first half of the period, from Wed into early Fri. Before this cycle there had been considerable spread and variability for the deep Arctic upper low, ranging between rapid east/northeast departure and slight retrogression, so there are some reservations about whether the current model/ensemble mean cluster (northeastward departure after Wed-Thu) will be able to maintain continuity in successive runs. Meanwhile the initial upper low moving northeast across the Bering will support a surface low and associated frontal system. A wavy front will likely persist over the northern/north-central mainland, possibly becoming a little more diffuse after departure of the Arctic upper low. By the latter half of the period the primary uncertainty involves details within and around the Bering Sea mean trough/low. Specifically the models diverge regarding the evolution and timing of energy that drops from Siberia across or near Kamchatka and into the Bering, possibly interacting with the low already in place there. Increasing model differences and fairly good agreement among the means for the general pattern recommend a transition toward 50-60 percent total weight of the 12Z GEFS/00Z ECMWF means by days 7-8 Sat-Sun. By mid-late period the operational ECMWF component of the blend splits its weight between the 12Z run and prior 00Z run, as the new run strays a bit faster than consensus to bring troughing toward the mainland while the old run was a tad slow due to being stronger than consensus with the mainland upper ridge. The new 12Z ECMWF mean provides added support for a blend/intermediate approach. Farther west the guidance signals potential for some degree of surface development over and/or south of the Bering next weekend, from a combination of initial Siberia energy and western Pacific dynamics/surface wave that may become embedded within the larger scale flow. At this time confidence is fairly low for the specifics of this evolution. ...Weather Highlights/Threats... The moisture shield ahead of midweek Bering Sea low pressure and associated wavy front should bring some focused precipitation into the Alaska Peninsula/southwestern mainland during the latter half of this week. Specifics will depend in part on frontal wave details. Some activity may persist over the southern mainland for a time as the upper pattern evolves toward more elongated east-west troughing over the Northeast Pacific/Gulf of Alaska. The wavy front forecast to be draped across the northern mainland may provide a focus for meaningful precip as well. Possible surface development over/south of the Bering may bring another area of moisture into the Aleutians next weekend. The North Slope and Panhandle/extreme southeast, as well as more of the southern mainland after midweek, have the best potential for below normal high temperatures during the period. Locations in-between should generally experience above normal highs. A broader area will likely see above average low temperatures, with negative anomalies more narrowly confined to the extreme north and over/near the Panhandle. 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