Extended Forecast Discussion NWS Weather Prediction Center College Park MD 230 PM EST Sun Dec 20 2020 Valid 12Z Wed Dec 23 2020 - 12Z Sun Dec 27 2020 ...Threat of widespread heavy precipitation and strong winds increasing from the Deep South to the East Coast during the Christmas holiday... ...Overview and Model Guidance Evaluation/Preferences... A highly amplified and vigorous upper trough will undoubtedly be the main focus during the medium range period as it could bring widespread impactful weather through much of the eastern U.S. during the Christmas holiday. Models and their ensemble means continue to depict the likelihood of this general scenario but remain quite uncertain regarding the details. One key element of uncertainty is regarding the speed of progression of the associated cold front through the entire eastern U.S. on Christmas Eve and Christmas Day as the upper trough deepens. The strong dynamics and the amplitude of the upper trough also make it difficult for the models to precisely time the track and intensity of low pressure waves that could form along the front. Nevertheless, it appears that the vigorous dynamics ahead of the system will likely result in bursts of heavy rain and even thunderstorms to sweep across the eastern U.S. during the Christmas holiday. The presence of convection may favor a faster eastward progression of the cold front within the model spread. The ECMWF has been on the slower side of the spread while the most recent GFS runs (06Z and 12Z) have definitely adopted a faster eastward motion with the cold front. The latest (12Z) ECMWF has also adopted a faster motion with the front but not as fast as the GFS. The CMC is somewhere in between. The WPC medium range charts and grid packages this morning were derived from a blend of the 00Z ECMWF, 00Z EC mean, 06Z GEFS, 00Z NAEFS, and only a small portion from the 06Z GFS. This yielded a faster eastward motion of the cold front than the previous WPC forecast package over the eastern U.S. Little change in continuity was noted along the West Coast. ...Weather/Threats Summary... The general pattern evolution still suggests a strong signal for a heavy rain threat over portions of the South and East during the mid-late week time frame. The front trailing northern U.S./southern Canada low pressure will likely strengthen the flow of Gulf moisture into the eastern U.S. Wed into Thu, with strong Atlantic inflow possibly reaching the Northeast by later Thu into Fri/Christmas Day. This should bring areas/bands of moderate to heavy rainfall from the Lower Mississippi Valley/Gulf Coast region northeastward through New England. Locations in the Northeast will be sensitive to significant rain given recent heavy snowfall. Any heavy rain falling on top of the recent snow pack would be a bad recipe for snow melt and could result in flash flooding. In addition, there is a distinct possibility of strong wind gusts and wild temperature swings across a large portion of the eastern U.S. due to the tight pressure gradient associated with the cold front. Besides the lake effect snow being likely in the cold cyclonic flow on the cold side of the system, sufficiently strong frontal wave development along a favorable path could also produce a band of meaningful cold sector snowfall from the central Appalachians to the interior Northeast following frontal passage. The scale of one or more such waves is small enough to keep confidence low for a while. Consult the days 4-7 Winter Weather Outlook for a probabilistic representation of areas most likely to see significant snowfall. Also note that this overall system is likely to produce a period of brisk to strong winds across a fairly broad area. The West will see mostly dry weather Wed-Thu to be followed on Fri/Christmas into Sat by increasing rain and mountain snow reaching the northern two-thirds of the West Coast with lesser totals extending into the northern half of the Rockies. Highest totals are likely to be over favored terrain from the Pacific Northwest into northern California. Another Pacific system may start to affect the West Coast next Sun. The southern stream upper low tracking near the U.S.-Mexico border as it weakens should produce at most very scattered/light precipitation over the Southwest before possibly producing a little more rainfall near the Gulf Coast next weekend. From Wed into Fri/Christmas the warm sector ahead of the front advancing into/through the East will bring well above normal temperatures with plus 10-20F anomalies fairly common. Some locations may see min temperatures even farther above normal on the warmest day. The trailing colder air will bring readings down to 5-15F below normal over parts of the West/Plains/Mississippi Valley from Wed into Thu and then the East late week/early weekend. Expect temperatures to moderate by Sun. Meanwhile late in the week temperatures will rebound over the Plains with some areas seeing highs up to 10-15F above normal before a modest cooling trend on Sun. The West should see temperatures within a few degrees on either side of normal late week into the weekend. Kong/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 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