Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 646 PM EST Mon Nov 25 2019 Valid 12Z Fri Nov 29 2019 - 12Z Tue Dec 03 2019 Today's guidance continues to display some important detail discrepancies within a large scale evolution that has been fairly agreeable and consistent. The greatest uncertainties revolve around important specifics near the southern coast during the first half of the period and one or more low pressure centers tracking northeast from the mid-latitude Pacific Sat onward. During the late short-range into early extended time frame, recent trends have been toward the western GFS with respect to western mainland shortwave energy aloft and associated surface wave/boundary that are likely to produce a very concentrated area of heavy precipitation along and near the Kenai Peninsula. Some of the latest guidance also shows more of a hint of some interaction with the strong system initially south of the Alaska Peninsula--which would further enhance precip totals. While consensus has gravitated more toward the GFS in principle over the past couple days, nearly all other guidance is weaker with the mainland shortwave energy. Thus confidence remains low in the heavy interior southwestern mainland activity seen in the GFS runs even though trends are increasing the potential for a longer duration of significant precip near the Kenai Peninsula. Upstream expect a fast-moving front to cross the Bering Sea/Aleutians Fri into Sat, anchored by strong low pressure which consensus has been tracking over or near the extreme western-northern Bering. Guidance clustering has been gradually improving toward the idea of triple point development (possibly incorporating a leading wave near Kodiak Island) reaching the Gulf of Alaska by day 6 Sun. This area of low pressure will eventually extend precipitation eastward along the southern coast and into the Panhandle. Along the trailing front models and ensemble members continue to show diverse ideas for timing and track of developing low pressure lifting northeast from the central Pacific from Sat onward. In particular the past couple days of ECMWF runs have been persistent with a progressive system tracking into the Gulf while GFS runs (and at least the past couple CMC runs) have tended to be well to the west/northwest. At least during most of the weekend latest GEFS/ECMWF ensemble means offer support for some variation of the faster ECMWF wave while the slow GFS/CMC are on the western side of the full ensemble spread. However after day 6 Sun the GEFS mean shows lower surface pressures over the eastern Bering in a nod to the GFS while the 00Z ECMWF mean supports a Gulf wave to the north of operational ECMWF runs. Note that eastern Bering low pressure by Mon could be supported by some combination of the upper trough/embedded low crossing the Bering and/or the trough crossing the western-central Pacific. Overall it seems appropriate to reflect a blend of these model/ensemble mean ideas to yield the best flexibility in the future--a leading wave that is similar to but weaker than the ECMWF but also an upstream system that may be forced by Bering/Pacific energy aloft. For the moment this intermediate approach seems to be validated by the 12Z ECMWF mean trending toward the GEFS mean by Sun-Mon. The latter system may reach the southern coast/northern Gulf by day 8 Tue as the overall upper trough approaches the western mainland. Coverage and intensity of precipitation along the Peninsula/southern coast and areas to the north will be very sensitive to specifics of surface evolution. Based on the preferred strategy, the initial forecast blend incorporated ideas from the 12Z GFS/ECMWF and 12Z GEFS/00Z ECMWF means, with 12Z/06Z runs combined for GFS input mid-late period (to tone down the strong 12Z GFS) and ensemble mean weight increasing from 50 percent early to 70 percent late. Mass fields/wind grids included some manual adjustment due to the greater than usual weight of the means for the initial blend. 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 Hazard: - Heavy precipitation across portions of mainland Alaska, Thu-Sat, Nov 28-Nov 30. 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