Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sun Dec 20 2020 Valid 12Z Wed Dec 23 2020 - 12Z Sun Dec 27 2020 ...Heavy precipitation threat over parts of the eastern half of the country mid-late week... ...Overview and Model Guidance Evaluation/Preferences... From mid to late week the model/ensemble consensus still displays a very amplified upper pattern with a Plains into southern Rockies trough as of early Wed advancing into the East while a ridge moves into western North America. Then by the weekend expect mean flow aloft over the lower 48 to trend significantly flatter as eastern Pacific shortwave energy moves into the western U.S. The strong dynamics within the upper trough are likely to produce one or more significant low pressure centers most likely to track across an area from the central Plains and/or Upper Midwest through the Great Lakes and southeastern Canada. Also one or more waves may develop along the trailing front that crosses the eastern half of the country, with the wave(s) ultimately gravitating toward the aforementioned dominant low path. Specifics may take a while to resolve but the guidance signal is fairly consistent that this vigorous evolution could pose a widespread threat of heavy rainfall over the eastern U.S. along with the possibility of meaningful snow on the back side of the system depending on wave details. Flooding concerns may exist especially where heavy rain falls over areas with a deep snowpack. Relative to the 18Z GFS, guidance preferences for significant features early-mid period tilted 3/4 toward the recent ECMWF runs through the 12Z cycle and 12Z CMC. The forecast then continued to favor 3/4 total weight of ECMWF runs/12Z ECMWF mean/12Z CMC versus the 18Z GFS/GEFS mean by days 6-7 Sat-Sun. For the dominant central-eastern U.S. evolution the GFS runs have been running on the fast side of guidance from late in the short range time frame and include stronger wrapping up of upper dynamics/low pressure over the Upper Midwest by the start of this forecast. The combination of the amplified pattern, remaining guidance being slower (with a little more energy hanging back on the southwest side of the upper trough), historical GFS biases, and consecutive 6-hourly GFS runs appearing to nudge slower over some areas, favor leaning more toward the ECMWF cluster. That said, detail uncertainty and some continuity of GFS ideas merit a modest account for its solution. Thus far UKMET runs have been the most inconsistent and poorest-comparing to other guidance for details of the initial upper shortwave coming into the West. Confidence is still fairly low for specifics of how an upper low may close off within the eastern trough but operational runs continue to suggest perhaps an even chance of such a feature. For the surface evolution over the East a blend approach helps to tone down uncertain wave details that will take until closer to the valid time to resolve. Meanwhile GFS runs have also been running on the fast side for eastern Pacific into North America features mid-late period. These include the southern stream upper low that should open up as it tracks near the U.S.-Mexico border, weak shortwave energy that moves into the West between Sat and Sun, and an upstream system that evolves over the eastern Pacific by Sun. The new 00Z GFS appears to have made some favorable adjustments especially for the first two features. ...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. In addition to 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 elsewhere. 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. 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