Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 700 PM EDT Sat Apr 11 2020 Valid 12Z Wed Apr 15 2020 - 12Z Sun Apr 19 2020 ...Overview and Guidance Evaluation... Most models and ensembles still show a mean pattern aloft featuring a ridge over the Northeast Pacific and extending into mainland Alaska and western Canada, along with troughing farther west. However today there is more spread and variability in the guidance for some of the details. One prominent uncertainty involves the specifics of flow aloft across Siberia and the Arctic into the mainland, with resulting influence on wave/frontal details over the northern-central mainland. The other uncertainty regards flow upstream from the system forecast to reach the Aleutians during the latter half of the week. Differences with this flow ultimately lead to various ideas for low pressure evolution/track across the North Pacific/Aleutians/Bering Sea by next weekend. Even with the lack of confidence for specifics, guidance still shows that it will be a fairly wet period for a large portion of the state. The manual forecast started with a 12Z operational model blend on day 4 Wed. The rest of the period phased out/switched/added various operational solutions as needed while steadily increasing 12Z GEFS/00Z ECMWF ensemble mean input with time. Total ensemble input reached 70 percent by day 8 Sun. The first required adjustment was to trade the 12Z GFS with the 06Z run from day 5 Thu onward. The 12Z version quickly developed deeper upper dynamics over Siberia compared to other guidance and then had additional digging flow from the north--leading to an unsupported surface low reaching northwest of the mainland by 12Z Fri and then northeastward progression of Aleutians/Bering low pressure to an extent not seen in other guidance. The 06Z GFS run compared better to other models/means in this regard. Meanwhile over the northern half of the mainland the UKMET became more suppressed than most other guidance at the surface and aloft by late in the week, which went against ensemble mean trends from the past couple days toward stronger upper ridging. As for the Pacific/Aleutians/Bering stream, there is decent clustering among the models/means into Fri for the system tracking into the Aleutians during the latter half of the week. The 12Z CMC was the one exception as it became deeper than other guidance with dynamics reaching the western Bering. This led to its Aleutians/Bering system being pulled farther northwest than the majority cluster by late Thu-Fri. At the same time to the east there is a relative consensus toward weakening shortwave energy that supports a modest wave/frontal system near the Alaska Peninsula/Gulf Wed-Thu. From about late Fri onward individual solutions increasingly diverge for what happens to the upper energy associated with the Aleutians system (most likely filtering into/through the mean ridge in some fashion) and the character of upstream flow. There is a general signal that a system emerging over the western Pacific mid-late week will come into the picture, with uncertainty over what interaction could take place among this system and the leading one, or perhaps even with separate dynamics that reach the western half of the Bering. The blend/ensemble mean approach used by day 8 Sun led to some degree of interaction and low pressure near the Aleutians at that time. The new 12Z ECMWF mean shows a trend toward latest ECMWF/GFS runs that maintain more separation and a low track to the south of the Aleutians. Among the various possibilities the 12Z CMC seems to be among the lower-confidence ones as its overall pattern becomes out of phase with most other solutions by next Sun. ...Weather/Threats Highlights... Even with the uncertainty in various details, there is decent confidence that the pattern will lead to a fairly wet period for most of the state including the Aleutians. For the five-day period as a whole the best signal for highest totals is along the southern coast of the mainland, centered near the Kenai Peninsula. Somewhat less extreme relative maxima will be possible over other areas across the southern and western mainland. A modest wave/frontal system near the Alaska Peninsula/Gulf will provide one focus for precip early in the period while flow ahead of the leading Aleutians system may bring a period of focused activity into the Peninsula/western mainland by late week/weekend. The majority of the state will see above normal temperatures with min temps tending to be more above normal than daytime highs. The greatest positive anomalies should be across northern areas. The Panhandle and localized pockets of the southern mainland may see below normal highs on one or more days. There is some uncertainty over precise temperatures over the northern/central mainland by Fri-Sun as guidance spread develops for flow aloft and resulting position of a wavy front that should be over the extreme northern part of the mainland at least through Thu. This front may push southward to some degree by the weekend. 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