Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 626 PM EST Sat Nov 23 2019 Valid 12Z Wed Nov 27 2019 - 12Z Sun Dec 01 2019 The majority of today's guidance agrees fairly well for the large scale pattern evolution but with some important detail uncertainties yet to be resolved. The forecast also reflects a combination of continuity and some adjustments from yesterday. A strong and impactful short range storm crossing the western/northern Bering Sea will track across the Arctic from day 4 Wed onward, with latest model runs noticeably farther northward. The next storm in the series is most likely to track over or near the western Bering Sea late week into the weekend--with a leading front that may contain one or more waves whose development and track are very uncertain. Farther east expect a wave to travel near the Alaska Peninsula/Kodiak Island into the northern Gulf of Alaska during the mid-late week time frame. Along with the changes in track for the Arctic system, the other most notable continuity change involves the mid-latitude Pacific system well south of the Peninsula. Best consensus now shows this system remaining more separated from flow to the north, keeping it well south of the northeastern Pacific/Gulf. As for some of the forecast specifics, there is decent agreement on existence of the mid-late week wave that may track into the northern Gulf. However most guidance suggests that the wave will be somewhat better defined and make more eastward progress than latest GFS runs. Leaning more in the direction of the majority, the potentially significant precipitation with this feature should taper off over the Peninsula/Kodiak Island earlier than the GFS and extend farther eastward. The 12Z CMC is suspect in its amplitude of shortwave energy aloft crossing the Gulf though. Meanwhile an average of ECMWF/GFS runs and their means will reflect the most likely scenario for the system tracking near the western coast of the Bering Sea. The 12Z CMC is farther south than this cluster. Individual model runs and ensemble members offer a variety of possibilities for one or more waves that may develop along the front anchored by the primary low. The best signal for any particular wave exists from the mid-latitude Pacific just west of 180 degrees longitude early day 7 Sat and then northeastward from there, as upper troughing moves into/across the western Pacific. Model runs still vary considerably on this possible late-period wave (but within a fairly broad ensemble envelope), favoring a model/ensemble mean blend as the best way to reflect some wave potential but without going as deep as the 12Z GFS and much deeper 00Z ECMWF. On the other hand the 12Z ECMWF switched to a weak/suppressed wave. Based on today's forecast considerations the initial blend used input from the 00Z-12Z ECMWF and 12Z GFS/UKMET early in the period followed by a gradual increase of the 12Z GEFS/00Z ECMWF means so that total ensemble weight reached 60 percent by day 8 Sun. Cold frontal progression across the mainland Wed-Thu will bring a drier trend from west to east while the wave near the southern coast should produce a period of focused precipitation in its vicinity. The wavy frontal system approaching from the west will likely bring precipitation first across the Aleutians and then into portions of the mainland--but with considerable uncertainty over coverage as well as intensity at any particular location. 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