Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Thu Nov 21 2019 Valid 12Z Sun Nov 24 2019 - 12Z Thu Nov 28 2019 ...Guidance/Predictability Assessment and Weather Highlights/Threats... Models over the past day have trended a bit toward a more phased solution aloft, and a surface low track closer to the Northeast U.S. coastline on day 3 (Sun). Solutions still show some variability on the exact track of the surface low, but a multi-model blend approach was favored for this system, including the 12Z ECMWF/UKMET/GFS. These solutions were most closely clustered within the center of the current ensemble spread. Given the westward adjustment in the track of this system, potential effects on the Northeast have increased somewhat. Increased QPF amounts a bit for eastern/southern New England during the Sun-Mon time period, and continue to indicate low end probabilities in the Winter Weather Outlook. The surface low should quickly move northeast into the Canadian Maritime Provinces by Mon morning, bringing an end to any potential precipitation across the Northeast. A shortwave is forecast to cross the northern Rockies/northern Plains Mon-Tue, with an associated cold front, and a surface low tracking east across the Canadian prairies. Models have continued to struggle with the details of this feature, both with respect to timing and intensity. The 12Z GFS/UKMET were the closest solutions to an ensemble-based consensus with this system. The 12Z ECMWF was on the fast side of the spread, and has also shown the greatest run-to-run variability with this feature, thus something more along the lines of the GFS or 12Z UKMET was preferred. This system will be relatively moisture starved, so any associated precipitation is expected to be rather sparse and relatively light. Warm advection ahead of the system will bring a period of well above average temperatures to the Central/Northern Plains and Upper Midwest Sun-Mon, with high temperatures forecast to be 5 to as much as 15 deg F above average (especially on Sun). Additional shortwave energy is forecast by all guidance to move onshore in the Pacific Northwest Sun night/early Mon, bringing a round of potentially heavy snow to the Washington/Oregon Cascades and coastal ranges. While models agree on the existence of this feature, solutions diverge significantly on the eventual evolution as the system moves east toward the central U.S. As the shortwave digs into the Rockies/Four Corners by late Mon, the ECMWF has trended toward a much less amplified solution over the past few runs. The GFS on the other hand, has backed off slightly on the amplitude of the wave, but maintains a more amplified shortwave exiting from the Four Corners into the Southern/Central Plains by early Tue. The GFS has been highly consistent in developing a wave of low pressure along the surface front across the Southern/Central Plains Mon night/Tue, which then deepens substantially as it moves northeastward, reaching the Great Lakes/Upper Midwest by Tue night/Wed morning. The ECMWF/CMC, on the other hand have both shown a large degree of run-to-run variability, ranging from solutions similar to the GFS idea to a much more suppressed system that does not begin deepening until it reaches the Northeast on Wed. The ECENS/GEFS/NAEFS ensemble means maintain a solution much more similar to the GFS than the ECMWF. Further, a look at scatter plots of ensemble member surface lows indicates a significant cluster of members from all three ensemble systems, for at least 3-4 ensemble cycles, around the GFS idea. Thus, despite what remains high forecast uncertainty, will maintain a forecast along the lines of the GFS (and WPC continuity). This scenario would bring the potential for a round of significant winter weather from portions of the Central Plains to the Upper Midwest Tue into early Wed. Depending on the eventual intensity of the surface low, strong gusty winds would also be possible, but confidence in the details of the forecast is too low for such specifics at this point in time. In the wake of this system, expect a colder air mass to overspread a wide area from the Great Basin/Rockies east into the High Plains, with high temperatures 5 to 15 deg F below average. Lastly, models show another potentially significant shortwave following quickly on the heels of the previous one, reaching the West Coast Tue night/Wed. Models agree that this wave should become quite amplified by Wed-Thu as it cross the Great Basin/Rockies. Solutions differ on the timing, however, with the ECMWF a bit faster than than most other guidance. The ECMWF has also shown a high degree of run-to-run variability with this feature. The GFS solution was closest to the ensemble means in terms of the timing of this system, and has shown a higher degree of run-to-run consistency relative to the ECMWF. Thus, favored a blend of the GFS with ECENS/NAEFS ensemble means by the middle of next week. This system is forecast to bring a significant increase in rain/mountain snow along much of the West Coast and east into the Great Basin and Rockies next week, along with another round of cold temperatures (highs 10-20 deg F below average). Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml