Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 AM EST Tue Nov 24 2020 Valid 12Z Fri Nov 27 2020 - 12Z Tue Dec 01 2020 ...Widespread heavy rainfall threat across the South Friday-Monday... ...Overview... A larger-scale trough should move through the western and central CONUS. Its northern and southern streams are likely to separate by the weekend; northern stream troughing moves quickly toward the Northeast while a potentially closed upper low in the southern stream lingers over the south-central CONUS, causing possibly heavy rain centered over eastern Texas to the Lower Mississippi Valley and the Southeast. The separate streams may combine again around Mon/Tue as additional northern stream energy enters the north-central CONUS, and that trough axis shifts east toward the Midwest. This pattern may also support some wintry weather from the Great Lakes through the north-central Appalachians and interior Northeast as we usher in December. 'Tis the season. ...Guidance Evaluation/Preferences... Guidance was in generally good agreement with the pattern described above through the weekend. The main differences that develop have to do with the degree of separation that develops in the northern and southern streams over the weekend and the timing for joining the streams back up, as well as regarding the potency of the southern stream upper low. Recent GFS/GEFS runs have been on the weaker/progressive side of the full envelope of solutions. The WPC forecast for fronts/pressures used a blend of the most consistent and well clustered guidance of the 12 UTC ECMWF/ECMWF ensemble mean Fri-Sun. This maintained good continuity with the previous WPC forecasts. Opted to quickly transition to just the ECMWF ensemble mean for days 6/7 amid rapidly growing forecast spread and uncertainty. ...Sensible Weather and Hazards... A lead coastal low will exit New England on Black Friday along with associated showers. The energetic trough/closing low tracking across the West should lead to snow chances in the Central/Southern Rockies late in the week, with some potential for light snow to spread into southern/central parts of the High Plains toward the weekend. However, the likely bigger threat with this potent upper trough/low will be multiple days of rainfall as moisture from the Gulf of Mexico streams in ahead of the low. Areas from the eastern half of Texas to the Lower Mississippi Valley can expect persistent enhanced rainfall/convection Fri into the weekend. A surface low/frontal system is forecast to develop and strengthen in the Mississippi Valley over the weekend as the upper low pivots eastward. Precipitation should then continue along and ahead of this system, with heavy rains spreading across the Southeast and the Southern Appalachians/Carolinas. Moderate precipitation may meanwhile spread northward through the Ohio Valley/Mid-Atlantic/Northeast early next week, including some wintry threats on the cooled northwest periphery of the main precipitation shield, but with more uncertainty due to model spread. Elsewhere, generally light precipitation is forecast for the Northwest for this time of year, but precipitation chances could increase by Mon as renewed troughing approaches behind an amplifying/warming ridge slated to shift from the Rockies to the Plains. Schichtel 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