Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Thu Nov 26 2020 Valid 12Z Sun Nov 29 2020 - 12Z Thu Dec 03 2020 ...Weekend heavy rainfall threat for the South lifts up the East Coast early next week with major storm development... ...Deep low offers heavy snow/ice/wind risk from the Great Lakes/Ohio Valley/Appalachians through interior Northeast next week... ...Guidance Evaluation/Preferences... Models and ensembles overall offer reasonably similar larger scale and stormy pattern developments days 3-7, bolstering forecast confidence. However, important mid-smaller scale and stream phasing variances lingering in guidance cloud the local focus of significant weather features over time. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of the overall seemingly best clustered guidance from the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/ECMWF ensemble mean and 01 UTC National Blend of Models through medium range time scales. This forecast strategy maintains good WPC forecast continuity. Newer 00 UTC GFS/GEFS and ECMWF/ECMWF ensemble mean runs remain roughly in line. ...Sensible Weather and Hazards... Deep Gulf of Mexico moisture streaming in ahead of the southern stream closed low/surface system will lead to enhanced rainfall/strong convection for the Gulf Coast states through the weekend, transitioning north and eastward early next week with continued storm genesis. Widespread heavy rain is forecast for the Eastern Seaboard Sun-Tue, lingering in New England midweek. Locally higher amounts may cause flooding/flash flooding issues. On the western side of the precipitation shield, precipitation could fall as significant snow/ice across the Great Lakes region, Ohio Valley, the Appalachians, and interior portions of the Northeast. While significant low development and a wind event seems increasingly likely, the placement and amounts of notable snowfall remain uncertain at this time beyond lake effect areas. Below to much below normal temperatures can be expected across southern portions of the Plains and Mississippi Valley toward the Tennessee Valley/Southeast for the first half of next week given anomalously low upper-level heights and post-cold frontal flow. The Northwest will see pretty quiet weather for late Nov/early Dec, albeit with a round of modest precipitation Sun/Mon with passage of an amplified Pacific upper trough and surface front. Subsequent upper trough/low digging through the West Tue/Wed may produce unsettled conditions including some terrain enhanced/deformation snows for the south-central Great Basin/Rockies. System ejection to the south-central U.S. in a week may favor southern tier system genesis whose lead Gulf of Mexico moisture inflow would re-enhance rainfall back across the Gulf Coast region. 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