Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 657 PM EDT Sat Apr 04 2020 Valid 12Z Wed Apr 08 2020 - 12Z Sun Apr 12 2020 ...Overview... The early part of the period will focus on a concentrated storm system likely to reach the Gulf of Alaska by day 5 Wed, with accompanying heavy precipitation and strong winds. After this system departs expect upper ridging to progress across the North Pacific/Bering Sea, and then build into the mainland once an Arctic upper low and associated trough pass by. The ultimate evolution/path of an upper low that starts the period over the east-central mid-latitude Pacific continues to be somewhat of a wild card for the latter half of the period. Farther west there is general agreement that a system reaching the western Aleutians mid-late week will be replaced by another one by next weekend. There is some uncertainty over the eastward extent of the first system though. ...Guidance/Predictability Assessment... Over the past day the guidance has made some progress in narrowing the spread for the strength and track of the system expected to reach the Gulf of Alaska by Wed. Using an approach similar to yesterday, a 12Z ECMWF/GFS blend tilted two-thirds toward the ECMWF, maintains continuity for strength and yields perhaps a slightly westward/slower adjustment for track. Latest ensemble means support a similar path, between the farther west 12Z CMC and southeast UKMET (which was the weakest/southeastward 24 hours ago). Today's clustering appears better for the compact upper low that crosses the Arctic and trough that extends southward to brush northern parts of the mainland. Ensemble means have trended noticeably more amplified with the trough over the past day or so, adding support to an operational model blend. Considerable differences arise with low pressure reaching the western Aleutians mid-late week. The most prominent discrepancy involves the latest GFS runs tracking fairly robust low pressure all the way into the northern mainland, whereas nearly all other guidance keeps the system(s) well westward. The 12Z GEFS mean has a hint of the GFS idea but enough individual members hold it back for the mean to bring a defined surface low center no farther east than the west-central Bering Sea with only a front extending northeastward. Preference remains with a non-GFS consensus for this aspect of the forecast. During the past 24 hours the ensemble means have trended stronger for the system expected to reach the western Aleutians next weekend. Operational runs generally support the existence of this system but with typical differences for the details. This supports an ensemble mean/consensus approach. There is still a low-probability potential for what eventually remains of the upper low and associated surface system over the east-central Pacific as of Wed to have some influence on the forecast later in the period. The majority of guidance maintains the idea that it should remain fairly suppressed as mean ridging aloft becomes more prevalent over the Northeast Pacific. Recent ECMWF runs have been inconsistent though, with today's 12Z run like yesterday bringing mid level energy and a surface reflection well northward of most other solutions later in the period--while 00Z runs have been more suppressed. As a result prefer to lean away from the 12Z ECMWF by next weekend. Today's considerations led to starting day 4 Wed with the aforementioned ECMWF/GFS blend for the Gulf system and other features of note. Lack of confidence in the GFS upstream after Wed led to quick phasing down of its weight (while adding some GEFS), followed by further increases of GEFS/ECMWF mean input along with transitioning 12Z ECMWF input toward the 00Z run instead. By next weekend this led to 60-80 percent total ensemble mean weight and a lingering ECMWF component of mostly just the 00Z run. ...Weather/Threats Highlights... The storm system reaching the Gulf of Alaska around midweek will bring a brief period of heavy precipitation and strong winds. Best confidence in highest precip totals is over the Panhandle and possibly extreme southeast corner of the mainland. There is still enough guidance spread in the track to support potential for a farther northwest extent, or on the other hand confinement to the Panhandle. Late week/weekend progression of Aleutians-Bering Sea low pressure in the GFS would bring an area of meaningful precipitation to the western/northern mainland but remaining guidance is not enthusiastic about this scenario, keeping precip lighter and more scattered. Agreement is better on moisture over the western half of the Aleutians. Expect the North Slope region to see above normal temperatures during the period while most remaining areas should be below normal. Chilly surface high pressure may drift far enough east by next weekend to support some moderation in temperatures over the western part of the state. 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